Hacker News Reader

Comment by bagels | Hacker News Reader

Battery backup seems like a great way to divorce yourself from the grid and solve this duck problem, except for the cost.

Whole house backup batteries are, for some reason much, much more expensive than electric vehicles despite not needing any of the car parts (motor, wheels, seats, chassis, etc.)

Comparing the Tesla Powerwall $9,200/13.5kWh ($681/kWh) vs Model 3 Battery: $14,000/75kWh ($186/kWh)

Why is there such a premium for house batteries, when there are even less engineering constraints on them?

Obviously there are inverters to pay for, but that can't explain a 4x difference in price?

View on HN

Not sure why the prices are so much higher when compared to EV batteries, but now that Na-ion batteries are in production we should see a significant reduction in prices for house backup battery systems over the next few years - 1/2 to 1/3 the cost of Li-ion. And while Na-ion has less energy density it's probably better suited for home power backup (less likely to catch fire, works better in low temps, for example) where you don't care about the extra weight.

Sodium ion batteries don't have the same longevity as LFP in their current form (2-3k cycles vs 4-8k cycles), so they are not really an economical replacement at this point.

Wouldn't batteries used in a home power backup situation be rarely cycling? (unless you completely go off grid as the earlier poster was suggesting)

If the battery is being used to address the duck curve, it cycles approximately every day.

If you can buy twice the capacity for the same price a cell cycles every second day, except on the rare days where you would have had a problem with the old solution.

>Whole house backup batteries are, for some reason much, much more expensive than electric vehicles

It could be the invisible hand of the market. Cynically or un-cynically.

Not being cynical I mean that is the market could be smaller, so they're pricing in the additional overhead per-unit due to having to spin up additional production lines for everything but the storage medium itself.

More cynically, even if the price difference driven by differing, smaller product delivery workflows isn't enough to explain the markup, their market analysis may have shown that the list price that sector can bear is higher. And there could be rebates, tax incentives, etc that they're pricing in so they can "offer" while maintaining their desired profit margin even after these.

It also bears mention that the construction sector is less competition driven. The "payoff" price isn't necessarily the price that a consumer will balk at and find a competitor but the price that the consumer will bear before "value engineering" it out. When you're dealing with home construction prices a relatively large line item like this can slip through unnoticed.

Doesn't the power wall include an inverter that allows for hot switching between battery and main line supply? That would explain a large part of the price disrepancy.

An inverter is a single speed motor controller. Every EV has at least one.

I suspect the better availability of V2L setups recently and in the next years will make those prices closer. Unless you drive lots, unexpectedly or every day, you essentially have a huge battery sitting idle in your garage. Why bother getting a separate one then? (Or maybe just a tiny one integrated with hot water)

And with the vehicle battery, if you need extra energy you can drive to a fast charger somewhere and bring it back to your home.

It seems so inconvenient. My wife has to shut off the air conditioning if I want to drive the car? I can't store my solar when I'm at work during the day? I can't go for a drive at night because the house used all the power that was stored in the car?

Yes, the grid connection disappears the moment you install a battery at your home, and exactly as I wrote in the comment there's no possibility to have a smaller battery as well. Everyone drives their own and only car to work for a whole day in 2024. And no car has a "charge/discharge to threshold X" setting. It's a terrible solution that won't satisfy anyone. /s

LiFe02 batteries for home grid-tie systems run about $150/kwh (10kw grade A 200ah cells) at the consumer level. Inverter capacity runs about 3k$ for an 18kw grid tie inverter. So, in order-able parts it costs about 5000 total to build a 10kw “power wall”.

It seems like there should be (and probably are) comparable integrated solutions around the 5k price point, which would be something like 500 per KWh.

At any rate, at sizes under 20kwh the battery is likely to be the smaller part of the cost of the unit….which also makes it possible to build an 80kwh system with 18kw capacity for around 18000 dollars, so there’s that.

So in short, yes, it’s probably the inverters and the branding that make up the vast majority of the powerwall price.

I'm interested in a 50-100kWh system, and (I think) I'd only need one inverter? Maybe I have that wrong.

It just depends on how much energy you will need to use at any given time and how quickly you want to be able to charge your batteries. If you are somewhat careful, 5-10kw is enough for many homes, depending upon whether appliances are electric or gas, as well as heating/cooling loads.

My guess is higher manufacturing cost.

Assumption: manufacturing batteries in a different form factor is a significant cost driver - a big slice of the manufacturing cost pie (the other being common raw materials). Powerwall is lower volume, therefore doesn’t leverage manufacturing economies of scale as effectively as the Model 3, making it significantly more expensive to manufacture (hypothesis; I haven’t done the math).

Idk why is this being downvoted, looks like a very probably explanation to me

This company sells rack-mounted LI batteries and inverters that are sufficient to build a Powerwall-like system (in nice enclosures) at a significantly lower cost [1]. I think Tesla is banking on wealthy customers and installers wanting the premium product. The problem is that (unlike cars) it’s not likely that brand name is going to win here, which means long term this product category is going to be a race to the bottom.

[1] https://www.currentconnected.com/product/sok-sk48v100-ncbt-u...

I expect the answer is some combination of economy of scale (they sell lots more cars than walls) and subsidies....

Warranty is probably a big factor as I believe Tesla powerwalls came/come with a 10 year warranty and considering these batteries are basically fully drained daily in normal usage that’s a bit of wear.

Still doesn’t justify 4x however!

I don't know the particular reasons for the battery discrepancy, but going off-grid is pretty complicated (assuming you want to connect to the grid sometimes). Generally, solar and/or battery installations stop working when the grid goes out, so as not to be feeding power into a potentially dangerous downed circuit. I'm not sure of every scenario, but the most common grid+off-grid usage lets you wire a seperate set of outlets for off-grid and on-grid usage. This adds a fair bit to the cost, as well as decreasing convenience.

Hybrid inverters have auto shutoff (islanding I think?) so you can merge the grid power with your solar power and there is no difference when the grid is down. Off the shelf, installed, and magically works. It acts as a glorified whole house UPS.

Solar is one of those areas where there is a lot of bad info floating around promoted by installers and certain manufacturers (ahem Victron).

Interesting; it might be different in different jurisdictions, or my information might be out of date.

Going off grid is easier than keeping the grid around when you have batteries. You need an automatic cutoff that prevents backfeeding to the grid during outages for safety. Off grid you do not have such a constraint.

Reply on news.ycombinator.com