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Comment by dchichkov | Hacker News Reader

There is plenty of opportunity in being more efficient. To give an example, houses in the Bay Area has gray/black roofs. In the summer such roof receives about 10kW of energy and readily converts it into heat. Heat, which we are trying to evacuate from the house with our super-efficient heat pumps in the air-conditioning units.

If only we'd painted these roofs with white reflective roof paint (~92% reflection). We would have removed ~8kW of heating from such roofs! And then, maybe, we wouldn't have to pump all that energy into air-conditioning. So the surrounding air would be cooler, less noise would be heard from air conditioning, less energy would be spent. And the cost? 4 gallons of white reflective paint and a couple of hours of work, painting the roof...

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> To give an example, houses in the Bay Area has gray/black roofs.

Those are no longer legal. Title 24 requires a “cool roof” *

When I had my roof re-covered a couple of years ago the roofer apologized and said he had to use a light color. Which wasn’t a problem for me so no apology was needed.

* https://coolcalifornia.arb.ca.gov/cool-roofs-codes-and-stand...

That's strange. We just built a house in the Bay Area with a black roof, and have never heard of this. It passed planning/inspection, and the thing you linked says we are in a title 24 zone.

We hadn't thought about it, and would likely have opted for a lighter roof, had someone pointed it out to us.

Well, I assume you covered it with solar panels, so it doesn't really matter.

My understanding is that you can insulate your way to Title 24 compliance. You just have to have a total ingress performance over some level.

Now that said, it would still be better for our communities to reflect this energy back out into space to reduce environmental heating. Houses in my area at solar noon have almost half a megawatt of heating potential. By municipal code they are not allowed a roof reflectivity of more than 40%.

I guess having mirrors on all the houses could cause problems for other people, even for flat roofs. But I agree: reflecting the incident beams is important: when it emerges as heat (IR) it can be trapped by GHGs.

I had been thinking of the reflectivity benefit rather than the reduction in air comditioning.

Specular reflection (a mirror) would be ideal for sending energy back out into space most of the time but would occasionally death-ray your neighbors. White paint isn't quite as good (some of the diffuse reflection will hit non-reflective things) but it also won't incinerate anything. Feels like a good compromise.

Yeah this was figured out in the Mediterranean probably a few hundred years ago: https://www.santorini-view.com/white-houses-of-santorini

Unfortunately, we are losing some of this cumulative knowledge due to it not being documented, and due to the looks of alternatives (although the grey buildings in the US are as ugly as it can get)

Well also asphalt tiles being cheaper material and cheaper installation on sloped roofs (snow!) over wood frame construction.

They're only cheaper in the short term. Long term energy costs and replacement costs factor in strongly, and chances are that once you make that calculation you'll end up with old fashioned terracotta roofs being cheaper over the life of the house.

I don't really like how the old fashioned terracota roofs behave during the summer. They heat up during the day and release that heat in night time, driving the inabitants crazy with heat. Maybe with some kind of ceramic coating to increase the albedo they'd be okay.

We're not losing the information that light colors heat less in the sun. Neither could anyone who spent five minutes on a black roof in the sun forget that tidbit. It's just that other factors than the energy required to cool a building took precedence in the past.

Anecdotally it seems like we are. The rad thing to do in my area of california is to paint your white washed spanish style home mud brown or ozzy osbourne black. Then you have to install a much more powerful AC system when in the past maybe just opening windows and getting crossbreeze was enough. All these houses used to have awnings over sun facing windows when they were build in the 1920s but at some point that must have been seen as ugly and its now pretty rare to see.

That rad thing to do is an example of what they meant by other things taking precedence.

  > The rad thing to do in my area of california is to paint your white washed spanish style home mud brown or ozzy osbourne black.

It seems that the Ames Research Center is rather rad hardened, then. Fitting. JPL too, for that matter.

If the roof assembly is decently insulated, then relatively little of that 10kW heats the house. So, while improvement is available, it’s not nearly that large.

Still, though, I have to wonder what the equivalent "CO2 reduction" would be considering that a white roof is likely to radiate most of the incoming sunlight back into space, where if it is radiating in infrared then that heat is more likely to be trapped on Earth.

For now, everyone is free to pump out kilowatts of heat with their roofs, parking lots, air conditioners into the surrounding air. It is not considered pollution, even when it is already 100 °F outside.

On the days when there is an inversion in the atmosphere, the hot air stays trapped. People have to run ACs in their houses, stores and offices. And that results in even more heat routed to the place, with ACs units, literally pumping energy from the solar arrays somewhere in the desert into the city.

My intuition says that the volume of inside vs outside air is so big that it has little effect.

You can look at how much space roofs cover in the suburb in the Bay Area. It'd be roughly 25% streets, 40% roofs, 30% backyard, 5% front-yard. Adding white reflective coating increases "Solar Reflectance" and decreases "Thermal Emittance" and absorption. Most dark roof materials reflect 5 to 20% of incoming sunlight, while light-colored roof materials typically reflect 55 to 90%. A white roof coating that you can purchase at your regular home improvement store would do that 90%.

So effectively, in the suburban area there is an opportunity to change our average solar reflectance by ~32% [(90% - 10%) * 40%]. If we just abandon the idea that a good house should look like a house in Normandy and have a black roof.

[edit: I've opened a section of sat map (in Sunnyvale, near Reed Ave/Sequoia Dr.) and estimated roof/street/backyard/front-yard by area, for a lot of 2 houses and adjacent street. The numbers above are from that estimate. Heat and noise of ACs is somewhat local (urban heat island effect, etc), so that local estimate is what counts. It is not surprising that we can have 40% roofs. Land is expensive in the suburb like that.]

As per: https://en.m.wikipedia.org/wiki/Reflective_surfaces_(climate... - "If all urban, flat roofs in warm climates were whitened, the resulting 10% increase in global reflectivity would offset the warming effect of 24 gigatonnes of greenhouse gas emissions, or equivalent to taking 300 million cars off the road for 20 years. So it seems, there is not only local/urban heat island effect, but even some global effect. To put this into perspective, Tesla sold 2 million vehicles. No idea, if the Wiki numbers are correct...

I suspect that there are good stats somewhere, but I think that 40% roofs might be a pretty high over estimate in say, Sunnyvale, or even San Jose.

I'm not aware of any stats, but I've experienced the difference between hot neighborhoods and cooler neighborhoods, right next to each other in Houston. It would be great to have a quantitative measure for this effect, similar to a walk score (which isn't perfect, but can be occasionally useful).

Are you sure it’s because of AC and not the amount of concrete pathways compared to green areas, vegetation and shade from trees? Also various thermal properties of the ground?

Even if houses were 50% of the land area, they take up a miniscule amount of air volume. Also, hot air floats up, so there is little chance you could feel the heat from an AC outside. On the other hand, if the earth is hot because you have few trees, you will feel it everywhere when walking on a sidewalk.

In Europe at least it’s common knowledge that you need to have trees and greenery everywhere to make heat bearable outside. If you replace trees and grass with concrete you get an island of heat that will be unbearable during summer to walk through.

edit: thermal camera image of a walkway with trees in summer https://images.app.goo.gl/m2XN7rqodavDcwoE9

At this time last year, this region was subjected to a 'heat dome'. Luckily this building has a 'cool dome' in the basement. Spent a few days listening, reading and solving down there (12-16°C cooler).

Much cooler temperatures (13°C) are found just a few feet below ground. (Memorable to those who've visited 'root cellars.) Before long those mysterious 'underground cities' in Anatolia may not seem so mysterious. It's a low-cost, low-tech, zero-energy solution, used by native communities in many regions of the world.

This and geo-thermal. The crazy megalomaniacs pushing solar are as bad as the rest of oil/coal kingpins, unsustainable and another ecological train-wreck in progress.

> Much cooler temperatures (13°C) are found just a few feet below ground. (Memorable to those who've visited 'root cellars.) Before long those mysterious 'underground cities' in Anatolia may not seem so mysterious.

When the London metro was built (over a century ago), the clay was at 14 degrees C. But over time, it heated up from all the power dissipated by the metro system. Now it is around 20 to 25 degrees C and doesn't really act as a heat sink anymore. The same happened in the NY subway.

So soil can act as a great heat sink for a while, but not forever. I suspect the same would happen if you'd put a modern city underground and relied on the temperature of the soil instead of pumping heat out somehow.

Indeed.

https://en.m.wikipedia.org/wiki/Urban_heat_island

https://en.m.wikipedia.org/wiki/Reflective_surfaces_(climate...

Yes this. It helps but it isn't a magic bullet because the roof is already airgapped from the roof and the house insulated from the attic. If the roof was directly insulated shingles would curl up and start failing within one or two summers due to the heat. That is why there are roof vents, gable vents, and soffit vents, to let air freely flow into and out of the attic/roof cavity.

> If the roof was directly insulated shingles would curl up and start failing within one or two summers due to the heat.

This is an old school carpentry trope and yet I've never actually seen any statistical evidence indicating that roofs over vaulted ceilings which are directly insulated must be replaced more often than non-vaulted.

Anecdotally we have about 1/3 of our roof with a vaulted ceiling and the other 2/3 with an attic. The side with an attic is the side that would be suitable for solar panels (more sun at the right angle) or adjacent to the vaulted ceilings. The leaks that came up were all on the insulated vaulted ceiling side; starting about 25 years old, and we had another every couple years for about five years until replacing the roof. The attic parts never had any problems.

We have a vaulted ceiling and there is still an air gap under the roof. The ceiling "vault" is separately framed; there is still a foot or two of air space between the top of the vault and the roof deck.

In every vaulted ceiling ive ever seen or worked on it still has had a significant air gap between the insulation and the roof and it was still vented with roof and soffit vents. Also the areas that tend to leak before any other part of the roof because it is hard to vent enough air through such a small space.

And nearly every roof ive worked on that had curled shingles lacked proper roof ventilation. That isn't direct evidence, but ive never seen curled shingles on a well ventilated roof either and the only common factor in curled shingles is heat buildup in the attic/roof cavity.

As far as I know, it’s important to have outdoor air that flows under the roof in cold climates to avoid ice dams. Otherwise insulating the roof deck is just fine and possibly even advantageous.

An unconditioned attic is not so great because it’s difficult or impossible to do a good job of avoiding leakage into the living space. Also, any mechanical equipment in the attic would like to be in conditioned space.

Black / dark tile roofs across the south are completely ridiculous. I doubt people understood this when shingles were first developed but we do now and we should adjust accordingly

Pretty sure they were recently banned in Sydney, Australia.

Not quite unfortunately - the new Planning Minister caved to the property developer lobby: https://www.theguardian.com/australia-news/2022/apr/09/plan-...

What exactly are the challenges with producing lighter colored roofing?

People who are happy to let the world burn so they can have their roof look the way British roofs look.

Asphalt is black, and a high albedo is hard for anything to maintain with dirt, dust, and age.

I had assumed that the white color would come from a white granular material impressed into the asphalt base

That’s not it in Australia - we don’t use asphalt roofing. Normal roofs are tile, steel or (more rarely) zinc here.

I think the US is fairly unique in that regard. I’m so used to our houses here that the idea of using shingles made of asphalt just seems bizarre to me.

The main reason they are dark is because they are made with oil.

Asphalt tiles are fairly cheap, and that's a good thing. It's the labor that makes up most of the cost of a roof.

I'm trying to get up the nerve to shingle my four bedroom home, and it's not the cost of materials I'm worried about. It's my body.

My fear with global warming measures is the poor, and middle class, will be required to bear the brunt of the preventative measures.

I guess the race is on to develop a 50 year life expectancy, but reasonably priced, white shingle? They might be out there? I really haven't researched them.

If anyone knows a cheap roofing contractor in the Bay Area, please divulge?

Twenty years ago I had a estimate of $25,000. I think that's too much now. I'm a ex General Contractor.

> My fear with global warming measures is the poor, and middle class, will be required to bear the brunt of the preventative measures.

They'll also bear the brunt of the effects of global warming, and those will be worse.

We should try to make environmental programs as equitable as possible. But first and foremost we need to do them!

Yea, but trying to squeeze more out of the lower middle class, and poor, dosen't seem kind at this point in history.

I see a lot of virtue signaling out there.

Most of us can't afford a electric vechicle, and those federal credits seemed to just go to people whom could afford any vechicle.

I can't even afford the smog check, and registration, on my 4 banger toyota. A Truck I've keep going 20 years. It uses very little gas. (My vechicle has always been tuned up properly. I thought by now we would have had free smog checks for the poor? In CA we have a low income deal that was enacted when CA decided to require smog checks. It's so complicated most people don't use the "gift". You need to bring your car to a mechanic in order to get any money back. Shade Tree mechanics are out of luck.

My point is the corporations/government will pass any Global Warning measures down to us.

We are already tapped out.

If we are really serious about Global Warming lets go after the wealthy boys first. Take away their private jets.

And enough with their Globalization. If they can make a product that dosen't need to be shipped long distances, have them make it here.

I'm a Democrat, and winced when Biden pretty much gave a FY to the oil companies in order to sound Progressive. I knew at the time, the poor can't afford to pay more for gas, and natural gas.

Now--I don't blame the president for this spike in gas prices, but I would have rather have him say this is a big problem, and we can't just do away with oil.

Be it my PG&E bill that just seems to go up, and the $6.69 gas at my Chevron station, the poor are already paying to much of their income on a complex problem. (My gas station raised their prices the minute they started talking about inflation, and the Russia sanctions. It is pure greed. They made a record profits in the past few years.

Sorry about my rant. I just don't like being squeezed so much, while my wealthy neighbors buy crap from Amazon, and think they are doing their part by driving a Tesla.

When I left California I was driving a ‘67 ford galaxy which, being over 25 years old at the time, wasn’t required to be smogged anymore and was fully surprised when they made me do it when I registered it in Arizona (where it is anything post-66 has to have smog). Even my motorcycle had to be smogged (which, I believe, California doesn’t require) and that thing couldn’t help but get really good gas mileage.

So, fear not, a couple more years and you will be smog exempt.

My daily driver is a 1955 and I'm exempt from safety inspections at this point. I also have a one-time(!) $175 registration fee in my state. If you can keep up with the basic maintenance (grease certain joints every couple thousand miles, etc)the lack of regulatory oversight on these older vehicles can sometimes push them back into being more affordable than driving a 10-15 year old car. Just don't get in a wreck in them, crumple zones weren't really invented until the 90s.

No--in CA it's 1975, or earlier. It's a rediculious law because the old cars do not put that many miles on the road. I wish it was 30 years.

While I'm on my soap box; Neusome (a guy I went to high school with (Redwood, and voted most Fashionable, a liberal Marin county boy) has a bill on his desk that would do away with fees associated with Parking Tickets. As of today, he has not signed off on it. The only reason I can think of is he's getting more middle of the road for a run at the big office?

These extra fees and fines are not fair to the poor. A parking ticket in SF is running $80 bucks without fines. $5-6 an hour if you find the right spot. Neusome knows this personally.

He barely graduated from Redwood High School, and dropped out of college due to some nebulous learning disability. He opened some coke dens in SF, and a few Bootjack liquor stores with family money. The SF bars went up in flames because they were poorly managed. Oh yea, his cousin Brendan has a DUI that disappeared? I might write a book on this guy one day. While I seem pissed, he's better than the alternatives.

If he wasn't born wealthy, he would be in the Embarcadero in SF with a needle stuck in his arm.

You need to vote for people who have policies to increase tax on the rich.

Government will pass climate change measures on to those that squeal the quietest.

Globalization is not the cause of climate change, it's a natural effect of economics as our technology and capabilities improve.

No one is saying to do away with oil, but that also means that oil companies don't have a social license to extract excessive profit. So tax them.

We recently paid thirty for asphalt, simple design, under US average size.

I once knew a handyman that swore by painting over black membrane roofs with some sort of white rubber patch paint. It was probably a terrible idea, but I wonder if such a product exists for asphalt roofs.

You can easily do it yourself, I did it first on a chicken coop to practice and once I got the hang of it I did a 145 square meter roof on top of a two story building in relatively short time. The clock was ticking because we were about to have frost and that makes working on the North side of such a roof next to impossible. If you have to remove the old stuff first use a flat shovel, that works the fastest compared to more fancy tooling.

Make sure you invest in a proper safety harness, they're not that expensive and even if I really hope you won't need it you'll be very happy you did that in case you do...

I think people understood. It's just that tar/asphalt is cheap and makes a very good roofing material.

Seems like a better idea to cover the roofs with PV panels, to be honest. This would reflect heat and provide power for the cooling / travel / regular house electricity.

Why not both? If you have a light colour roof, you can use bifacial panels and get additional generation for the same surface area!

Imagine instead of paint if we could come up with some sort of material that generates electricity.

P.S. - If you look into the white paint thing more carefully you'll be rather disappointed.

Wouldn't a reflective coating work better?

Given it's a once off cost it seems like this should be doable.

Neighbors and pilots may complain

I dislike people and planes are noisy.

I take your point, I’m doing it anyway.

Actually, I was looking into installing solar, to offset $400/month of air-conditioning costs, during the summer. I could have had around 1.5kW of solar. But the cost of this is north of $25k, plus all the hassle to get the permits, plus all the extra complexity. While the cost of painting the roof is $100 + work. And now it is very rare that I have to run the AC.

Do you mean 15kw? 1.5 is only like 4 panels, it wouldn’t make $400 per month and wouldn’t cost anywhere near $25k.

I was looking at installing a very small solar array, just to offset the AC costs. The quote for the solar kit that could produce ~1.5kW (2170W Solar Kit: 7xURE NSP 310W - Mono 60 Cell - All Black Panels + 7xEnphase IQ7 60-Cell Microinverters w/QCables + 1x Enphase IQ Combiner 3 + Wireless Monitoring) was $6.5k.

But, I was also considering having a 10kW battery to store that energy and to have a backup power, like Tesla Powerwall - $10k. And then there was work to install, permits, my time. Overall my estimate was, realistically it'd be ~$25k. Maybe I was wrong.

On my house I have two sections of roof that are flat/horizontal. Roof is pretty thin there and although there is insulation, on hot days ceiling was getting worm in the 2 rooms under these sections. I also was hoping that an extra layer of solar would insulate my roof a bit more. So I wouldn't have to run AC that much. So the hope was to stop wasting annoying $400 on AC in the hottest months. But the cost of installing solar felt prohibitive. So I've made a quick calculation and spent $100 on the reflective roof paint instead...

Sounds wrong, check this out: https://www.youtube.com/watch?v=QWJpafOHoD0

5kWh battery thing for $1,500: https://signaturesolar.com/eg4-lifepower4-lithium-battery-48...

Growatt 3kW Stackable Off-Grid Inverter for $700: https://signaturesolar.com/growatt-48v-3kw-150vdc-stackable-...

Solar panels are $2-$3 per watt depending where you live. 4x300watt panels is just the ticket, depending where you are and whether you can get a 30% tax credit that's about $2000.

Whole thing all together would be about $5000. It would generate about $1 of electricity a day. Would eventually pay for itself over its lifetime if you don't mind faffing about with it to get started.

> On my house I have two sections of roof that are flat/horizontal. Roof is pretty thin there and although there is insulation, on hot days ceiling was getting worm in the 2 rooms under these sections. I also was hoping that an extra layer of solar would insulate my roof a bit more.

It will be slightly cooler underneath it.

That’s insanely cost prohibitive, tbh. No wonder more people aren’t jumping at the opportunity. You would need 14 years just to pay back the cost of installation, and that’s assuming zero maintenance simultaneous with magically sustained like-new performance.

After those 14 years, you “make” an annual non-compounded 7% ROI (again in 2022 dollars).

Spending some of that money on insulation and sticking the rest in a savings account somewhere is a much wiser bet.

I’ve never understood the calculations people make to justify that solar roof panels save them money.

Every time I do the calculations, it’s money loss.

If you want to install them for environmental reasons, great. But don’t kid yourself that’s is a great investment. It’s not. Putting the money into an S&P 500 index fund for 14 years is a better investment. It will double about twice in that time.

If everyone does that, all your stock won’t be worse much when the world will be either at war or starving. People have kinda forgot to think long term clearly. Also, you must live in a country where the politics are very anti climate. In my country it’s a good investment to install solar power.

The batteries are often rated for 20 years, after which you're down to 80% capacity, they don't simply stop working. Solar panels are rated for 30 years. So yeah, pretty hands off.

As for why do it, well, a right sized system allows you to be independent from the utility company so if monthly financing for it falls to the same ball park you're paying for electricity already you've simply locked in a rate and hedged against further price hikes.

If I was paying on average $99/mo to PG&E or whomever I'd eyeball a $12,000 system financed over 10 years. That would get you about 20kWh storage and enough panels to keep it full which may or may not be enough for your house.

That crossover point obviously varies from house to house and what's available to you locally. I think it makes a lot of sense for new houses, that way they might avoid a potentially very costly hookup to the grid which in some locations is tens of thousands of dollars in itself.

> The quote for the solar kit that could produce ~1.5kW (2170W Solar Kit: 7xURE NSP 310W - Mono 60 Cell - All Black Panels + 7xEnphase IQ7 60-Cell Microinverters w/QCables + 1x Enphase IQ Combiner 3 + Wireless Monitoring) was $6.5k.

Remember to take 23% off for the federal tax credit.

> But, I was also considering having a 10kW battery to store that energy and to have a backup power, like Tesla Powerwall - $10k.

That's a not-so-small detail, accounting for $18.5k of the bid! A battery is also pretty unhelpful for reducing the A/C portion of your electric bill, especially since most traditional A/Cs require more than the 20A that the Enphase 10kWh battery can output and will pretty quickly drain any home backup battery that could support that current demand.

> So I've made a quick calculation and spent $100 on the reflective roof paint instead...

$100 in reflective roof paint, while a great measure to take, isn't going to reduce A/C electricity usage more than PV, which has the added benefit of offsetting your other electricity usage when you are not in the cooling season.

The enphase batteries are stackable to get more amperage. However, we were definitely amperage limited when we sized ours. We have a few high amp / low duty cycle devices.

It can definitely run our 60+ amp HVAC system, but not overnight. (We didn't buy it for that, but it's nice that we don't even notice our frequent, short outages until enphase and pg&e email us...)

It shouldn’t cost you more than $2/Watt. Obviously if you only do 5 panels, you still have all of the interconnect cost and it’s not nearly as efficient.

$2/watt has been way below average for a long time in CA, especially with a battery.

I read the national average recently spiked up to $3.10, thanks to the Biden administration's mishandling of Trump's China Tariffs.

We have an enphase system with a solar array that's a bit under 10x that size. We paid about $60k (ignoring tax credit) in the bay area, and I think we overpaid.

We have about 2.5 watt hours of battery per watt of solar panel, which seems about right around here. (The battery is fully charged by the end of day about 95% of the time.)

Your quote has 6.6 Wh/W, and the PV bit is small, so fixed costs are also biting you.

Edit: we have electric heat, and you probably don't. Our worst case days are cold and cloudy, so perhaps the battery sizing makes sense, though if you want AC at night, you'll want enough solar panels to charge the battery and run the AC.

I didn’t get the battery but paid 28.5K (without rebates) for a 12.8kW array. I think that’s very reasonable. Heating, cooling and car charging are covered with around 6mWH capacity left a year. Thinking of converting my water heater now.

I like our Ruud/Rheem hybrid heat pump water heater. (The two brands are the same company. The only difference is whether you pay someone to install it.)

However, it was not trouble free:

The time of day scheduling feature is useless. Once or twice a month it gets stuck in "lower temp" mode. The app reports kWh used and displays alerts, which is nice. Otherwise, there's no reason to give it WiFi access.

Pay for the leak detector option. It's relatively cheap, but not included for some reason.

You're likely to have condensation issues unless you use a good installer, or do it yourself and follow directions.

The power lead on ours wore through due to vibration too, so pick someone that's experienced/conscientious enough to route wires carefully.

You get about 6 hours of direct sunlight on the panels so his sizing is probably about right?

My dad gets about 6 kWh daily during the summer from an 1140 Wp PV array. 45° latitude, 1250 W/m² yearly insolation.

1.5 kW of solar for $25k ? Somethings way off, closer to $1 per watt.

This is more reasonable. I have 2x400W solar panels sitting in my kitchen right now (they’re going on a boat) that cost me £215 each.

I think your math is off my an order of magnitude somewhere. Your $25k can buy a 40Kw kit according to this¹ South African retailer (excl installation, but my point still stands.

Also, don't unferestimate painting a roof. I recently had my ~400m^2 roof painted, and the final bill was 50k ZAR (~3k USD), and this is in a country with dirt cheap manual labour costs. It's a very labour intensive operation if done properly.

1 - https://www.sustainable.co.za/collections/grid-tied-solar-po...

Installation is most of the cost. So "excl installation" isn't a useful number.

I have photovoltaics on my roof. I won't be doing it again.

I'm almost certain that putting the money that they cost into making my home more efficient in the first place, by doing things like sealing leaks, upgrading the insulation, and, yes, making the roof a lighter color, would have reduced our energy consumption by more than the amount that the PVs are generating.

The PVs absorb a lot of thermal energy, too, more than the shingles themselves do. So they make the roof hot. You can actually feel the difference inside the house - the rooms that are underneath the solar panels get warmer in the summer than the ones that aren't. So some amount of that electricity we're generating would seem to just be undoing the effects of the PVs on the house's thermal situation. There's still a net benefit, but probably because we're people who are inclined to accept that summer is hot in the first place. Trying to keep those rooms at or below 80F/26C might be an expensive endeavor.

Also, I've discovered that one does not simply have a roof leak fixed when there are solar panels on the roof, because they are in the way. So maintenance has been rather more expensive than I expected.

In general, I'm sanguine about solar as a renewable option. I'm just increasingly inclined to believe it's better implemented as an industrial technology, and that homeowners probably have better options for greening their houses.

> The PVs absorb a lot of thermal energy, too, more than the shingles themselves do.

I've found this to be interesting going back as far as middle school, where we're taught renewable energy sources like solar panels can help reduce global warming by replacing greenhouse gas emitting alternatives. Yet fundamentally, they are designed to turn the only external source of energy our planet receives (sun light) into heat and other forms of energy that will eventually become heat rather than reflecting it back out into space (albeit through the atmosphere). Seems so backwards to me.

What do you think normally happens to the 1000w per square meter which the sun radiates?

More of it is reflect back into space from many natural surfaces than from solar panels. Not scientific but look how black this solar farm appears compared to the surrounding land

https://eoimages.gsfc.nasa.gov/images/imagerecords/85000/854...

Right and so to follow this logic through, how do you think we get energy out of fossil fuels? Or nuclear reactors? Or a hypothetical fusion power plant?

Steam turbines? What are you trying to say?

> they are designed to turn the only external source of energy our planet receives (sun light) into heat and other forms of energy that will eventually become heat

How does a steam turbine work? By heat: we burn things to get heat, and use steam to turn it into work which..turns it into heat.

Your own statement is arguing against using energy in the first place, yet trying to frame it as a "renewable energy" problem as though the 60% of energy in gasoline isn't being thrown off into the environment, and 30-50% of energy in coal isn't the same, or as if literally every last bit of that - all of it because of the laws of thermodynamics - isn't eventually turned into heat.

Total estimate energy consumption per year [1] is about 14420 TWh as of 2021. The total solar energy we would've received in that same year [2] is 1,515,480,000 TWh. Or 0.00095%.

The amount of thermal emission from human activity on Earth is negligible. The idea that the amount of solar panels to power human civilization would have any non-local effect on the temperature of the planet is absurd.

[1] https://en.wikipedia.org/wiki/World_energy_supply_and_consum...

[2] https://news.mit.edu/2011/energy-scale-part3-1026

I wasn't arguing against the use of solar panels, just pointing out that they don't reflect light as much as the natural environment that would have been in their place. This whole discussion is purely academic because the proportion of earth covered by solar panels is negligible.

I doubt that picture is recording most of the wavelengths of light responsible for getting heat back into space.

It's mainly the visible wavelengths that can get through the atmosphere. You should see the absorption spectrum for water, it's remarkably transparent in the visible! That's how the greenhouse effect works - longer wavelength thermal radiation is blocked by greenhouse gasses.

Visible light is also the ones turned into electricity, so it would make sense for panels to not be highly reflective at them.

It also heats up dark surfaces. One way to remove that heat would be to have some kind of combined PV + solar thermal panel. The other lower tech option is to cover your roof with soil and plants, which release the heat back during the night. Vegetation is esentially a heat buffer.

Yes, that's why they're dark! But it doesn't change the fact that they're dark.

One could argue that the only energy sources that aren't doing this somehow are geothermal and nuclear.

That’s not correct - nuclear power plants only have efficiencies of about 33-37%, so 63 to 67% of the energy that nuclear reaction is producing is dumped into the atmosphere which wouldn’t have otherwise (obviously there would have otherwise still been some radioactive decay of the material in the ground if it wasn’t used as nuclear fuel, but nothing like refining it and bringing it to a critical mass in a reactor).

Meanwhile, 100% of the energy that a solar panel receives (and radiates some back as heat) would have hit the earth anyway.

The problem we're trying, broadly, to solve right now isn't "we're directly heating up the planet" but "we're trying to stop trapping heat in the atmosphere through a potentially self-propagating greenhouse process".

This is why carbon neutrality is the focus and not just purely energy usage.

It appears to be a different (and more direct) way of framing the same problem: https://www.nasa.gov/feature/langley/joint-nasa-noaa-study-f...

A better argument for solar panels is that per kwh of useful energy, a solar panel traps less solar energy (in kwh) than the CO2 produced by coal or gas or w/e. But I've never see this particular argument made and therefore couldn't even guess at HOW much better a solar panel is than coal and HOW much worse it is than nuclear or geothermal (which could actually be net energy loss for earth, by dispersing some thermal energy locked deep within the earth out to space).

> A better argument for solar panels is that per kwh of useful energy, a solar panel traps less solar energy (in kwh) than the CO2 produced by coal or gas or w/e.

It's not a better argument because the solar panel doesn't "trap" the energy. At most it borrows it, eventually it will radiate back out if allowed to do so, just as it would if it were absorbed by a rock. Humans do not occupy enough of the surface of this planet to make a dent in that.

The thing that will keep it from radiating back out is greenhouse gases in the atmosphere. And unlike some brief capture of energy, the greenhouse cycle is essentially a chain reaction.

But it is radiating it 'back out' less efficiently than a white roof. Thus black solar panels may indeed be a net negative if every building otherwise would have had a white roof.

In a land of mostly dark roofs it's probably a net positive.

Unless you pump the energy deep in to the ground it will still eventually find its way to radiate back out in the form of infrared blackbody radiation. The timescale we're talking about isn't very long in the grand scheme of things, and the entire surface of the planet is doing this to some extent or another.

Even if we did one of those megasolar projects people like to talk about like paneling the whole Sahara or Mojave, we would still only be absorbing a tiny amount of the petawatts per second the sun is constantly bombarding the planet with, much of it being absorbed by plants, the ocean, darker rocks, etc.

Our solar panels just don't hit the scale that the melting ice caps do, and the only thing we can do to stop that is reduce the flow of new ghgs into the atmosphere.

What benefit is there in installing solar panels over a white roof, if it is in fact slightly net negative?

I can't think of a single one.

Yeah, it’s a little counterintuitive, I’d just keep in mind that the amount of energy we use is tiny compared to the amount that the sun bombards the earth with. The absorption and generation is going to be insignificant compared to the amount trapped by the addutional greenhouse gasses.

That doesn’t follow. Consider the alternatives - thermal generation (including nuclear) must reject massive amounts of waste heat into the atmosphere, before the electricity is used and heat is generated from that use. That’s what cooling towers are designed to do.

That’s even before thinking about the effect from CO₂ or methane from gas fugitive emissions.

The earth has the ability to radiate away quite considerable amounts of heat luckily, and solar produces considerably less waste heat than thermal. So solar and wind can only make things better.

Minimising waste heat is of course good, which is why energy efficiency is so important, and heat pumps are doubly so - because they are extremely efficient, but also because they don’t produce as much heat but transfer most of it!

>> The PVs absorb a lot of thermal energy, too, more than the shingles themselves do. So they make the roof hot. You can actually feel the difference inside the house - the rooms that are underneath the solar panels get warmer in the summer than the ones that aren't.

I've actually read the opposite: most PVs don't sit directly on the roof, but rather a few inches above it. And that air gap further limits the amount of solar energy that makes it onto the roof and into the attic.

That claim doesn't really add up. I have solar panels as well and yes - not only is their an air gap, but pretty obviously the rooms they're over are north facing (I'm in Australia) with north facing windows and obviously get hotter anyway due to that.

Is this not something simple foam insulation would fix?

Interesting. My PVs have made the house substantially cooler. There is an air gap between the panels and the roof itself - they effectively shade the roof.

Aha. Well, I would not be at all surprised to discover that we ended up with a sub-par installation.

Are these solar tiles (solar shingles) or a traditional air-gaped panel?

Same. This statement is bizarre to me. You can’t bolt solar panels directly to a roof. There is an air gap from the rails and also the edges of the panels.

You actually can if you get the stick on variety. They don't last as long, etc, etc. But they do exist and have their uses.

I came across a forum thread for Baja California, where thay have and discuss off-grid air conditioning in a very hot area of Mexico. Interesting discussion.

Air Conditioning Off The Grid: http://forums.bajanomad.com/viewthread.php?tid=97584

Are you in a mobile home or something?? I have solar too and I absolutely love it. I’m not sure how anyone could feel warmer in a room with panels above as i’ve never seen a stick house without attic spaces and i’ve used my IR camera to look for hot spots and i see nothing out of the normal where any panels are… also, panels tend to cover the entire surface area where the sun is shining the most too not just rooms so it makes no sense to me read what you’re saying.

To make solar worthwhile we did an energy audit so we also replaced windows, got new siding and installed new doors. Energy efficiency doesn’t have to be mutually exclusive. I got a bigger rebate from the city with proof of energy audit.

as far as roof repairs… why would solar matter and why so many roof repairs? I’ve been through tropical storms, down pours and many a hail storm and not a single problem and my roof is 23 years old..

I could never go back. I charge my Tesla at home and my electric bill averages about 70 a month over a year after solar. That’s less than a weeks worth of gas for my Jeep.

year 6 on panels and 0 maintenance with a 20 year guarantee on efficiency and install warranty if i had problems i’d call my installer for service.

Solar panels are placed on best places to receive solar emission, so the room also be hot, isn't it?

There’s actually a $500 tax credit to install reflective shingles for this very reason. https://www.energystar.gov/about/federal_tax_credits/roofs_m...

Changing how roofs are approached (color, materials) may also have a (possibly beneficial) hydrological effect, even better would be some form of vegetation grown on roofs (likely called 'green roofs' to some) to help with evapotranspiration for facilitating a small water cycle (helping water vapor travel inland) and reduce the 'urban heat dome' effect.

(see Millan Millan's papers like https://academic.oup.com/bioscience/article/69/2/143/5254231 , can link to some other newer stuff if desired)

This is one of the reasons metal roofs a nice, they reflect heat and you don't need to paint them white, they make special paints for metal roofs that help reflect the heat and also look nice.

Couldn't agree more.

I'm in the UK and in the process of installing underfloor insulation in a house built around 1890. It is a flooboards-up job involving a windproof one way moisture permeable membrane above the joists, with hygroscopic insulation on top, and then a vapour control barrier on top of that, with all edges of layers taped to the joists where possible, all holes for wires, pipes etc. sealed with tape, and finally membranes brought up to wall behind the skirting boards. It's going to cost around £1k for a room of about 25m2. We have double glazing, the walls aren't particularly cold in winter, and the roof is insulated. I'm convinced most of our cold is coming through the floor and I reckon this job (along with the rest of the house in time, ugh) will make a huge difference.

But I'm having to do it myself because there are literally no specialist fitters where I live, and there are no government incentives that I am aware of. Why aren't the government going hell-for-leather for insulation etc.?

What happens to the "heat" energy that gets reflected by white/reflective roofs? It seems like it'd stay in the relative vicinity of the house, heating up the outside air further (even by a small amount), right? If that's the case, are there diminishing returns to worry about with effectively just moving heat from one place to another, assuming the (hotter) outside air will probably continue to warm up the house anyway? Do we just bank on something else absorbing the energy instead? Does it leave the area/atmosphere if nothing absorbs it?

(Please don't take this as a rhetorical "this is why X solution doesn't work" -- I'm no expert here and am just curious where all that reflected heat/energy goes!)

I would assume those light rays reflect off the roof instead of getting absorbed…in that case the heat is absorbed into the atmosphere, I have a hard time believing it sits around the house and lingers there for any significant amount of time (doesn’t heat rise?)

Heat rises is short for hot air rises. The light would not be completely absorbed by the atmosphere more than it did on the way down.

With a light or reflective surface, a lot of the energy is reflected as light so it's not converted to heat. Relatively little will be absorbed by the air near ground level, heating the air only a little.

“Heat” from the sun that we feel is mostly IR/Visible light. Mix of stuff happens if we reflect it with white surfaces. Some of it does go back into space. Some of it will get reflected by the atmosphere back towards the ground. IR light is also directly absorbed by the air molecules, but very little. Air is mostly transparent at visible wavelengths so it absorbs very little energy. Solids are much better at converting IR to thermal energy. Realistically air conditioning is just moving heat from one side of a wall to another. Overall painting buildings white won’t change the temperature of Earth, but it does help the building in question, which is all air conditioning cares about anyways.

All that energy started at the sun, we call it "light" when it's visible and "radiation" when it's outside our visual range, but it's the same stuff.

The fact that it reached the roof in the first place means the atmosphere is transparent to it. So with a white roof, the wavelengths being reflected are the same ones being received, it'll reflect right back through the atmosphere and back into space. Effectively it just increases the Earth's albedo a tiny bit.

Any wavelengths which were gonna heat the air by absorption, were absorbed on the way in and didn't reach the roof in the first place.

HOWEVER.

In the case of dark/black roofs, the heat DOES stay in the vicinity of the house, at least for a while. The radiation was absorbed by the roof, and now two main things happen to it:

One, the hot roof heats the nearby air by convection. This is the main contributor to the "urban heat island" effect. Eventually this warmed air mass will affect weather patterns, but even before that, it just makes the place feel hot.

Two, the hot roof emits its own radiation, in a whole range of wavelengths determined by its absolute temperature, according to Planck's law. This includes infrared wavelengths that the atmosphere is not transparent to (the absorption region between 5 and 8 microns), so that radiation IS absorbed by the atmosphere and heats it as well. And just like convective transfer above, the warmed air contributes to heat-island and weather effects, etc.

(The phenomenon of a hot object emitting its own radiation is why thermal cameras work, by the way. And if you get the object even hotter, the Planckian locus shifts into our visible range and you get an object that is "red hot", or even hotter still and it's "white hot", etc. This is why the "whiteness" of lightbulbs is expressed in Kelvin, that's the temperature of an object that would glow the same color as the bulb emits. The "warm white" and "cool white" are misnomers; the so-called "cool" color represents a much hotter blackbody source.)

> Any wavelengths which were gonna heat the air by absorption, were absorbed on the way in and didn't reach the roof in the first place.

What is the basis for this? Intuitively (I am no expert) it seems more likely that the amount of thermal energy produced would be a function of the distance light travels through a medium and the amount of energy carried by the light in the first place. So I would expect that the "inbound" light would produce more thermal absorption energy than the reflected "outbound" light, but it wouldn't arbitrarily all be absorbed on the way in.

Reality seems to be more complicated: https://www.sciencedirect.com/science/article/abs/pii/096016...

It was a bit of an oversimplification on my part, since for a given wavelength, the atmosphere isn't binary, completely transparent or completely opaque (i.e. the radiation would go a very very short distance before being absorbed), but for _most_ wavelengths it's _very close_ to binary. Either it's absorbed or it isn't, to a first approximation.

Yes, some wavelengths are absorbed very weakly, and some are scattered rather than absorbed. But these represent a tiny fraction of the spectrum, the nuance would've overcomplicated the post, and they don't change the result much.

For wavelengths that are not absorbed, they can be reflected back out just as easily as they came in. For wavelengths that're strongly absorbed, they heat the first parts of the atmosphere they encounter, and this is why the thermosphere is called that.

For those few wavelengths that're weakly absorbed, yes they produce some heating on the way in, and if you reflect them back out, they do produce some heating on the way out. But as you've said, because the intensity of the radiation is decreasing along the whole path, the "outbound" contribution is weak indeed. It turns out to be basically a rounding-error in the thermal contribution, which is overwhelmingly dominated by blackbody absorption and re-emission, which often does produce wavelengths that're strongly absorbed. On a perfectly-white planet (or snow-covered region), you might have to start caring about weakly-absorbed wavelengths, but for most types of ground cover, other effects swamp it.

In northern New England, we see a lot of homes using heating oil and woodstoves. These create a lot of local emissions and can be expensive or use a lot of time/effort (woodstoves). Heat pumps that work especially well in cold climates would be a huge efficiency gain over oil, woodstoves, and electric resistive space heaters which are also commonly used as stop-gaps in the region.

Hell most of the 1950s and 1960s Bay Area housing stock is worse - it’s tar and gravel roofs without any significant insulation thereby turning houses into ovens. The right answer to most of them is a new foam roof which both provides insulation and reflectivity being white but people get quite upset about the foam roof thing, especially with Eichlers.

I always wondered why in such climates we don't simply stick mirrors on the roof (cheap) but solar panels would seem to be the best option in the longer term (much more expensive than either mirrors or white paint). But you are absolutely right: black or dark gray roofs are quite silly in a hot climate.

I have the same question, it would seem a quick win to have optimized house colors for the current temperatures. Many hot countries do it right, but especially northern architecture hasn’t adapted as well. Also, too much unshadowed windows. There’s a lot to win with well thought architecture.

But that 10kw isn’t going directly into your house. The attic is designed to vent the air and let that hot air escape.

For my house the problem is that the sun's energy is converted to radient heat which makes its way hours after the air has cooled. The HVAC people have said no amount of insulation or attic fans will help. They suggest radiant barrier roofing of varoius types. The simplest being reflective paint, which the HOA won't allow.

Agreed, though you still need to account for cold days when you want more heat in the house.

Would that work in terms of all the other stuff? Rain snow etc.

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