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Comment by csense | Hacker News Reader

Interesting to see a post about this from someone else who I'm assuming is in a northern climate (they say New England).

One of my biggest questions about solar is, what happens when it snows? Do you have to get up on the roof and clean them off by hand? Or do they have built-in defrosters?

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> One of my biggest questions about solar is, what happens when it snows? Do you have to get up on the roof and clean them off by hand? Or do they have built-in defrosters?

Nothing.

In northern climates rooftops usually have more of a pitch to them so snow doesn't accumulate (because you don't want the roof to collapse). The panels, because they are black, will generally collect enough heat to melt the snow in a day or 2 after a snowstorm.

If you want to clean the panels, you risk damaging them. So the common advice is don't do anything and let the sun do the work.

Northern climates is relative. I live in the north of Sweden and the solar panels here will be completely covered from December to February or similar.

It's true though that the snow will melt or fall off quickly when it's not that cold anymore.

In the yukon we brush them off. Still plenty of power generated at -40C.. though the days are not very long!

>The panels, because they are black

Doesn't seem like that would matter once covered by a few inches of snow. But otherwise your points seem solid.

All it takes is a little snow to melt or blow off and the exposed panel will heat and cause the rest of the snow to melt and slide off. The panels are relatively slippery so a little melt at a pitch will clear them in no time.

my partner has panels on her house around boston. They work well. The snow slides off, usually with a great "wump" as it drops 2 stories into the yard. Its acually a little startling at first.

They work better in the summer, but provide most of the house power. Doesn't help much with the EV she got since except in the summer. She got a electric water heater as well.

If her water heater is in the basement, when it comes time to replace look at the heat pump water heaters. As a bonus, they dehumidify as they work - which is usually a very good thing for basements!

It somehow does - dark asphalt melts snow faster than grass even when not shoveled.

But once it’s deep enough it stops.

The dark asphalt has stored heat, and is in good thermal contact with the bottom of the snow. Grass insulates the snow from the ground.

You don't ever get a few inches of snow on the top of the roof. The point of the aggressive pitch is so the weight of the snow pulls itself off the roof so your roof/home doesn't have to support 5 tons of water weight.

IDK if you live in a place that sees significant snow, if so, just drive around a neighborhood and notice what the snow looks like on the rooftops. Generally none on the top and some on the bottom. That's by design.

While the bottom of the panels aren't doing anything to melt the snow, the top portions are, and that accelerates as the snow melts.

>You don't ever get a few inches of snow on the top of the roof. The point of the aggressive pitch is so the weight of the snow pulls itself off the roof so your roof/home doesn't have to support 5 tons of water weight.

This is definitely not how snow on roofs work. Here is an example with a roof with lots of snow on it:

https://millworksconstructionservices.com/wp-content/uploads...

Roofs in cold climates are designed to hold the weight of snow on top of them.

ETA:

The no snow on top but some on the bottom is because their attic isn't insulated well enough. Ideally you lose no heat through the ceiling into the attic. But without enough insulation you will lose a significant amount and the hotter air will rise to the highest point and melt the peak first. You don't want this because the melted snow can run down your roof and freeze again on the colder parts.

That's absolutely how snow on top of the roof works. Most building codes in snow heavy areas have pitch requirements.

https://www.atozroofingdenver.com/how-does-roof-pitch-affect...

>That's absolutely how snow on top of the roof works.

How do you explain that picture with ~1 foot of snow on the roof's peak?

It's a low pitch roof which allows for a lot more buildup.

Also, if you read the article I linked, you'll know that pitch isn't perfect (pun intended), but it does go a long way in preventing snow buildup. A major snow storm with wind blowing snow in the right direction can twart a high pitched roof. But that sort of condition is a lot more rare.

How north do you mean? Solar panels are at a high angle, slippery and black. Snow generally does not stick for long. They'll self clear.

My brother's panels in Saskatchewan don't self-clear. But that's for an average temperature of -20C.

You don't produce much in the winter anyway, you can brush it off the panels with an extendable broom but it will naturally melt and slide off. Unless you have snow cover all winter you'd likely spend more on the extendable broom than you'd produce from the panels.

My brother-in-law's panels produce just as much electricity at noon in the winter time as they do in the summer time. Winter sun in the Canadian prairies is quite intense, and the panels are more efficient at -20C than they are at +20C.

But they'll produce for less time each day in the winter.

What the GP is saying is that with the losses from higher angle and shorter duration of insolation, the decreased temperatures increased efficiency such to match the summer's output. The heat in the summer reduces a panel's efficiency.

sure but the sun is going down at 3pm, canada gets 1.5 hours of peak sunlight at best in the winter… on a lot of houses the cost of removing the snow is more than what’s gained during the short daily window of direct sunlight… eventually the sun solves the problem for free :)

Keeping those panels clear is saving him almost $100/month.

According to the folks on this YouTube channel that have an installation in a very snowy climate if you have two sided panels they melt any accumulating snow off much quicker.

https://www.youtube.com/c/ambitionstrikes

My issue with New England (I live there) is there that only 50 to 60% of the days are sunny - we get a lot of overcast skies. That, combined with the lack of available roof space on our Four Square style house, has kept me from thinking about installing panels.

https://www.currentresults.com/Weather/Massachusetts/sunshin...

Take most of the sibling replies here with a grain of salt. In areas where there are large snowfall events (eg Tahoe) the story is very different; panels can and do cause ice dams, dangerous snow slides, and even the panels can get ripped right off the roof.

I've lived in very snowy areas. Snow mostly falls off the roof because of the pitch, or it melts off in the sun. Its pretty much only in the absolute worst storms AND roofs with low pitches you need to get up there and clean them.

Usually if any significant amount of the panel is exposed, they will start to heat up from the sun and defrost the rest. Its a significant reduction in performance, of course, but still worthwhile if utility power is expensive.

The actual performance of the cells increases as they get colder. The attenuation due to ice or snow will, of course, reduce power generated.

I had them when I lived in Pennsylvania. The panels self-clear in a literal avalanche that destroyed all the bushes where it landed. It sounds like a freight train in your attic when it happens.

Had someone been standing underneath when it happens they would have been seriously injured.

Had the same experience in Canada - woke up thinking the house was collapsing. Found a mountain of snow next to the house the next morning.

Edit: There are "reverse" shovels called roof rakes[1] that can help remove snow before it builds up.

[1] https://www.thespruce.com/best-snow-roof-rakes-6830889

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