How big a solar battery do I need to store all my home's electricity?
(shkspr.mobi)280 points by FromTheArchives 16 hours ago
280 points by FromTheArchives 16 hours ago
If you also consider half the Earth is experiencing day, while the other half is experiencing night, there's another benefit to transferring the power East/West as well as North/Sourth. Perhaps by doing both, we could create some kind of "power grid"...
That's just me being snarky, but we've been scaling towards this for decades, we just haven't fully gotten there. We can probably solve the technical problems, it seems the main issue to building a fully-connected worldwide power grid is that the cost of scaling that much isn't worth it (yet).
It would be extremely expensive and also risky.
One of the problems with our reliance on oil is that so much comes from an unstable part of the world (although the oil itself contributes to the instability).
Cables under the ocean can be cut by anyone who can get to them with a submarine.
You would be look at cable literally going around the world - at least a good proportion of half way round to be useful. They will be vulnerable one way or another at some point.
Then there is reliability. There have been some fairly bad failures of national power grids. A failure in a global grid would be a lot worse.
Good thing that could never be used as a "death ray".
They currently most efficent method is called HVDC and it's not really efficient enough to be anyhing resembling economic at those distances. Ohm's law is a thing.
Edit: I again made the mistake to comment on a thread dealing with energy x politics. Sorry, I'll try not to do that again. I'm out. It's feral.
HVDC is actually incredibly efficient over long distances. The conversion losses typically dominate.
The trick is the "HV" part. China is already running 1100kv on some of their HVDC lines. Transmission losses decrease with the square of voltage, so any increment from that point would be very substantial.
> They currently most efficent method is called HVDC and it's not really efficient enough to be anyhing resembling economic at those distances. Ohm's law is a thing. > > Edit: I again made the mistake to comment on a thread dealing with energy x politics. Sorry, I'll try not to do that again. I'm out. It's feral.
I didn't see any feral responses? Did you not like the ones that pointed out that losses over 800km are <3%, and so your assertion that Ohmic losses are the issue is essentially wrong?
What is "not efficient enough"?
As a first guess, one would think it makes more sense to eat 30% loss (so you need 1/0.7=143% installed capacity) than to need 200% capacity plus batteries since it's night about half the time on average. And afaik HVDC is more on the order of ~15% loss
Aside from the physics, HVDC doesn't compete successfully on cost. It's cheaper to overbuild PV and use batteries.
I'm considering buying enough batteries for my day usage, then recharging off the grid during off-peak hours. I can add solar later on.
The panels are the cheapest components. If you're incurring all the costs to retrofit your electrical panel with an inverter + batteries, you may as well implement on-site generation from the start to recoup more of the costs.
An odd thing about this article is that it ignores the deeper question: what balance of solar over-provisonioning + battery would most cost-effectively cover anticipated yearly needs?
I suspect that something like 3x'ing the solar (under 100k) would then let the author get away with much, much less battery, and result in a net cost savings.
Yeah seems like a relatively simple maths/econ problem to solve for, given some parameters like local solar power per m2 in the various seasons, electricity use in the various seasons and time of the day, and LCOE of solar and battery storage.
My guess is the differences in either choice aren't huge, as both solar and battery storage keeps getting cheaper.
Having an electric vehicle can really help, also. It basically soaks up excess solar power of an outsized installation during much of the year (making the payback time on the outsized installation very good), and can be charged away from the house during a few low-chance bad winter days when the outsized installation is enough to power the house but not the car. Electric cars are charged fully about 3 times per month on average in the US, so working around that with smart charging is not a complex challenge in the next decade.
I think he can't imagine 3x'ing because he already has his house covered and only the shed roof remains empty.
But that is a super interesting question that immediately comes to mind.
I am pretty sceptical about batteries and see overbuilding renewables plus bitcoin mining to monetize excess as a more viable solution.
The HTW Berlin has a autarchy calculator. Unfortunately only in German language: https://solar.htw-berlin.de/rechner/unabhaengigkeitsrechner/
They also test and publish yearly the latest battery combos.
In case you didn't realize he is looking to store ALL of the summer generation into a battery and generate zero power in winter.. so rely entirely off of a battery during winter.. which is absolutely no feasible for a normal person and nobody would ever do.
I once did a related calculation on "How much of my garden do I need to dedicate to coppiced willow to heat my house for a week per year?"
I concluded that we're all going to need much bigger gardens.
> It is possible that, not too long in the future, every home could also have a 1 MegaWatt-hour battery. They would be able to capture all the excess solar power generated in a year.
Seems like we'll get to a point where adding extra panels "makes economic sense" long before this kind of storage makes sense.
I wonder if compressed air energy storage systems can be built for homes. It can be a tank part of the foundation. Too small CAES doesn't work, too large may get messy, but a house size tank might be best.
Crashing panel prices, output degradation, local and state laws, and most importantly, bidirectional charging should all play into the long term calculations.
I always thought about this myself in terms of personal sized long term, high density energy storage. Compressed hydrogen with a fuel cell is the obvious solution. Excess electric is used in a electrolysis cell and a matched compressor fills a bank of storage cylinders. More cylinders = more storage. Though likely very inefficient with a risk of fire or explosion.
Are there any other long term high density electric storage technologies that can fit in someones basement, garage, or even apartment closet?
> Compressed hydrogen with a fuel cell is the obvious solution.
To achieve volumetric energy density of hydrogen at room temperature that's on par with batteries (and that's charitably assuming you're using inefficient resistance heating with batteries) you need to store it at a pressure in the order of 100 bar.
You're better off with batteries realistically speaking.
Compressed hydrogen is no joke. It can escape most containers, actively degrades many grades of steel, has a very low ignition energy, and will explode over an enormous range of air/fuel ratios. Definitely not something to keep anywhere where you care about the roof :)
https://archive.is/wOP3A (the site seems to be struggling under the load, e.g. not showing images)
Let me guess why this is coming on, its top-up time on our solar energy bills. Those transport fees on externally sourced electricity sucks.
Relevant video from USCSB, from a propane explosion in 2007 that killed four people.
It would make me nervous, although that's only due to my engineering background.
In any case, it all depends on what you want to stand next to. A large explosion, or a multi-day metal fire releasing clouds of hydrogen flouride.
If the metal fire is over multiple days, you can walk away from it. The large explosion can kill you before you know there's a problem.
> What I want to do is find out what the maximum size battery I would need in order to store all of summer's electricity for use in winter.
This is ridiculous. It's like asking how much propane do I need to store in summer to go the whole winter? You would need a tank so big it could fill up propane tankers. Thus why we get regular deliveries of propane throughout the year on an as-needed basis... and why you should only have enough battery to store the peak energy use for a couple days at most. If you have an extended period of no sun, first you reduce your energy use, and then you top up with a generator. This is far cheaper and more flexible than having a gigantic battery array for an edge case.
22kWh in battery (self-contained units, BMS, heating element, etc) costs <$4400 in the US, shipped from China, for a reputable (but not pricey-brand) battery provider. More than enough to power most homes (in Northeast US, cold winters without much sun) for a couple days. Add in everything else you need (wiring, solar panels, power inverter(s), MPPT(s), generator, etc) and you get to $6.5k-$9k
If you want 100% GHG reduction, for every 1 watt nameplate solar rating, 1 x 0.2 capacity factor x 730 hours a month x 3 months a season = 438 Watt-hour of battery or other energy storage. 80% reduction needs 100 hour vs a whole season, so 20 watt-hour required storage. Dr. Chu of Stanford is my source.
Another way to look at this: how much solar do you need to synthesize 1 MWh of methanol ("e-methanol") from water, which is only ~54 gallons (200 liters)? Actually you need 135 gallons since the generator is likely 40% efficient (or less..). This is not much fuel, I used to have two 275 gallon oil tanks in my basement.
I think e-methanol synthesis is ~%50% efficient, so double the solar. Doesn't sound so bad.
Now if you could synthesize methane you could push it into the gas grid and run the meters backwards, thereby avoiding the need for storage... actually methane synthesis is even more efficient, >70%.
Heating oil in a tank is quite safe. It doesn't evaporate rapidly, and will not burn unless atomized or spread out. It won't explode unless provided with a much richer source of oxygen than normal atmospheric pressure air.
The cars in people's garages are far bigger fire risks. For example it's not uncommon to have a 70kWh+ EV battery, and the chemistries used to get the extra energy density for cars are far more unstable.
LFP (rarely used for cars) is fairly stable. And sodium batteries are even more stable.
Pretty quickly. There's also a point where it becomes a serious explosion risk too.
Every other fire you can stop if you're right there and you catch it. If a battery pack starts to go, you might have a few seconds before the local environment is incompatible with life.
None of those release hydrogen flouride when they burn (among other things).
I'm in the middle of a renovation now, that will include batteries and solar.
One thing that I never see mentioned is how to support on-demand instant hot water heaters.
The 6.5 gal/min heater (what it takes to fill a tub) that I'm installing uses 100A at 220v when operating!
I haven't found any battery system that can support that.
Do "totally off-grid houses" all use a typical electric storage tank hot water heater? For my solo occupation, that's a lot of hot water storage over times when almost none is being used.
I do have a smaller water heater under the kitchen sink, so that the giant one doesn't have to run for that usage.
How do off-grid homes deal with the high instantaneous current consumption of on-demand water heaters?
They don't. You either have a solarthermal system to heat the water, or you use a tank to store hot water. I've previously installed an "iBoost" which diverted excess solar into an immersion heater.
Heating slow(er) and storing is going to be easier than suddenly ramping up for a shower.
In a cold climate you have to add heating and an insulated enclosure to keep them alive.
Bury them below the frost line in a concrete box with a sealed access port, add in some water cooling for when the battery heat goes above a cutoff. They'd never get too cold that way, stay around 60 Fahrenheit by default.
It's a nice back of the envelope calculation. I think the conclusions are correct for the stretch goal but it does not make economical sense. Yet (those sodium ion batteries could change that).
There are several things you might want to consider:
- wind, there are smallish turbines that you can put on your roof that generate a few kwh. Also when the sun doesn't shine. Extended periods without any wind at all are rare. 2-3 weeks would be a lot. That probably drops the amount of battery you actually need quite a lot.
- Second hand EVs are relatively cheap and come with some affordable batteries that are probably larger and cheaper per kwh than most commercial domestic storage solutions. Not for everyone but if you can wire things together, that might not be a bad option. Especially if you can get ,a good deal on some well used EV with a half decent battery. Relatively low loads might increase the life that battery has if you just use the car for storage.
- You don't have to generate the power next to the battery. Some cars can provide power to your house; when your house battery runs out, you can just use public chargers and drive back and forth to top up your house batteries. A bit of a chore but probably better than investing in batteries you don't need most of the year. Not a bad option if you live off grid. Batteries on wheels in general are a thing. Electrical semi trucks come with > 500-600kwh typically. That's a lot of power that you can move between your home and your charger. Container sized batteries are a thing. If you want to, you can get about 3-4mwh on your property. It's not going to be cheap. But it's doable. The point here is not that you can have a huge amount but that you could stretch a modest amount quite far by simply driving to and from the charger. Of course if you have a grid connection, using that is more convenient and cheaper.
- The capacity factor of your batteries is going to be a function of how often you cycle them. If you rarely cycle them fully, they are going to be relatively expensive. So, while hoarding batteries might make you feel nice and comfortable, it's not a great economical choice to make until batteries become a lot cheaper.
- The money you save on not paying for grid power needs to be balanced with the cost of a battery and how long it will last you (10-20 years?). If your monthly bill is 100, you might spend 1200$ per year and 12000$ for 10 years. So, that's your budget for a huge battery. If you factor in that it will have a low capacity factor, it might last quite long. Twenty or even more years. I have a lithium ion battery screwdriver that's nearly 20 years old; still fine. Because I rarely use it. So your budget could be 20-30K$ Adjust as needed based on grid prices and usage.
- As others mention, generators are relatively cheap and they do work if you can stand the noise and exhaust fumes. Not clean. But relatively cheap.
It's a valid thought experiment to repeat until the cost adds up. Your opportunity cost while you don't invest in this stuff is basically what you will continue to spend on the grid. Which is probably not horrible for most people. Until those cost curves cross, you are better off waiting. Or compromising and buying a battery that won't solve the whole problem but is cheap enough that it will earn itself back in a reasonable time.
It's trade off between need and cost. If you absolutely need to be off grid, it's doable if you have the space and resources. But it's not going to be cheap. Until then, some hybrid solution is probably more optimal.
In a manner of speaking, the grid is already the storage mechanism. In summer you sell the excess to the grid; in winter you buy it back. Obviously you pay more to buy than you get for selling but that's the premium for using someone else's infrastructure. You'd spend a load more buying a battery the size of a small house.
Snark aside, there are examples of community-scale energy infrastructure below grid-scale: see "district heating" and "co-gen plants". Sand battery people have been experimenting with neighborhood-scale infrastructure (though industrial heat uses is a better return on that tech right now)
Of course then you have the collective action problem, and convincing your neighbors that grid storage is actually a real thing that exists. And that grid storage is not of the wrong political partisanship. The box of what's considered "politically incorrect" is getting fairly large these days:
https://www.theguardian.com/environment/2025/sep/10/south-da...
EV battery capacity is expected to grow to 100kWh.
People will park them at home every night, and probably somewhere with a charging point during the day.
Smart house energy management should be able to pick up on that usage pattern and use the car battery for the house while making sure the car is kept ready for use.
In the same way that wifi/mobile/satellite comms can keep us "always connected", the changes in power generation and storage are going to keep us "fully charged".
A house might use 20 to 30 kWh each day. Modern EVs have enough battery capacity to power some appliances for many days.
Vehicle-to-load ("V2L") is currently offered in vehicles made by Hyundai, Ford, GM, Volkswagen, Volvo, Mitsubishi and Nissan (the new LEAF).
Vehicle-to-grid (V2G) is more ambitious.
Not for the thought experiment of "I want my summer excess to power my winter usage" posed by the author.
In the UK you'd need a class D "loi-sonce" to be able to pull the shipping container sized battery trailer for that to work.
However, if you were wanting to use pure lead acid batteries for your house, because you'd be doing slow charge/discharge you'd probably be able to get away with just 1100 130ah lead acid car batteries.
I mean you'd be optimising for peak current, which isn't what you'd want. However it could be interesting to see what happens when you have ~500mega Amps at 48v. (24Mw would heat your radiators up pretty quick. )
for lithium, then you'd need 12-14 secondhand tesla/polstar batteries, which if they caught fire, might be a challenge to contain.
Lead-acid car batteries are designed for high short-term current supply, for starting engines, not longevity. You can buy deep-cycle lead-acid batteries that last much longer, on the order of over ten years. Moreover, lead-acid batteries wear out when you discharge them too much more than by time elapsed, so taking good care of them can make them last even longer. Lead-acid batteries are great for standby storage, where you normally only discharge them a small amount between charging, but then need to use the whole capacity every now and again (for instance if you have a few dull days).
LiFePO3 batteries don't take as much wear from cycling, so they usually wear out from time elapsed instead of over-use. It's economically sensible to cycle LiFePO3 batteries as frequently as possible to get as much "benefit" out of the investment. They're great for time-shifting energy production by charging them at a cheap time of day and discharging them when you need the energy at an expensive time of day.
"loi-sonce", would be better "loi-sunse". That 'o' is very jarring.
Not even close. A typical electric split will take 12a to maintain and that is just the heating/cooling system. Car batteries are meant for starts, not maintenance flow.
going forward, solar pv and batteries will continue to get cheaper, but each house will vary quite considerably in how to maximise and optimise things for what will always be an exercise in averages that is modified by whatever contingencies are included, and after living off grid for decades I could care less and recomend careing less about the math, start now with something modest, iterate,learn, end then install the largest system that you can. If you are going to build new, start with site selection, plan for car charging, and use all of the best practices for integrating passive solar into the homes structure.ie : homes built with double walls and a thermal break require NO home heat source as normal domestc activities will generate excess heat that must be vented. Anywhere north or south of 30° design should be for bad weather in the winter, and the rest of the year will almost certainly be gravy. For all refits and renovations to solar, the math will always be ,the most that will physicaly fit. Regulatory limitations will most certainly restrict batteries to a day or two backup in most situations.In most jurisdictions, a large battery would require a seperate building with significant set backs to any structure, untill we get to solid state non flamable batteries, which may be on the horison now.
You might want to factor latitude into your calculations.
> Remember, this is just a bit of fun. There's no practical way to build domestic batteries with this capacity using the technology of 2025.
Huh? A single Tesla Powerwall 3 stores just about the same 13.5 kWh the author describes as being the battery size they need [1]. And they are by far not the only ones offering ready-to-install battery packs.
Fully electric vehicles with vehicle-to-grid wallboxes enable even larger systems.
So this basically just shows that his solar panels are subsidized by the grid at whatever the depreciation rate is on 1000kWh of batteries.
which being very approximate is 15k gbp/year
If you don't live at some combination of high altitude, low latitude and low cloud cover then don't do this. You will never pay off the CO2 produced in making your installation. So it does more harm than good. This isn't true for all sites, but the large majority of where humans (especially English speaking humans) don't live somewhere where this type of setup doesn't do more harm than good.
Mexico City, Santa Fe, places like this are great for your proposed setup. Wherever you likely live, probably not.
That is a trick question designed to make people argue and feel like there is some science or math to it. There is not. Nobody here can accurately predict weather far out enough to be a factor in this decision. The truth will vary by demand by family which may have variability throughout the year or decade. Another variable is the number of cloudy days which will vary as climate changes.
The answer is somewhere in the neighborhood of as much as one can safely store and afford accepting that batteries have a short life. Much like wells in cold climates the batteries should be in an underground insulated vault made from higher quality concrete as to keep fire hazards away from the home. That is also where whole-home generators and fuel belong, in their own vault so they can be easily maintained without having to rent an excavator to dig out the tank.
> "excess" of PV panels ... aligns with as much as one can afford.
does it? Panels are not the most expensive part of the system any more. Overcapacity of panels isn't the bottleneck any more. Battery capacity or roof space might be instead.
If you consider the fact that only half of Earth is experiencing summer while the other half is experiencing winter, there's an obvious madlad solution to instead of storing power, transfer it from the summer hemisphere solar panels to the winter hemisphere electric heaters, somehow.