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Where I am in California, there's a $30+/mo charge to connect to the grid, and the largest savings from a battery was being able to disconnect from the grid. There's lots of time I have excess power generation when I could give to the power grid, if I were connected, but I would have to pay extra to do so, so the potential goes unused.

Why do people still go for tesla powerwalls when you can get BYD batteries with 70%+ more capacity for cheaper ?

You can buy a BYD HVM 22.1 kWh for 6000 euros now (£5200) vs powerwall 2 13.5kwh for 7000 euros.

It's wild how overpriced Tesla Powerwalls are.

16 kWh battery with all of the UL supported listings etc = $3300 [0]

13.5 kWh Tesla Powerwall is $12k~$15k

You would get your return way back quicker.

[0] - https://www.ruixubattery.com/product-page/lithi2-16-battery-...

EDIT: As others have pointed out, powerwalls have inverters built in so it's not totally apples to apples. You can get a beefy inverter for $5k and it's still cheaper and you wouldn't need an additional inverter every time you add a battery.

Battery prices are getting really low, if you're willing to do some DIY. Just received a 15kWh battery from China. A 'Humsienk'. Combined it with a GroWatt SPA3000TL-BL inverter.

Total price, 1600 euros. So close to the magical 100 euros per kWh. Driving it with some interesting combinations of Raspberry PI's and serial interfaces and custom written Go code, but it works... :)

Good analysis. And kudos to the author for saving money. But still 21.6MWh per year excluding solar production seems too high for a household. I use electric heating and drive an electric vehicle, and my household annual energy consumption is about one fifth of that.

9-11 year payback isn't bad based on the projections. You could probably goose it a bit with inflation of electrical prices (depends on how the electrical policies change and what they pass through).

I'll also add theres some O&M coming down the line. Inverters @ year 10, small maintenance and Im assuming you re-did your roof before you installed. Anyone putting solar up make sure you do it at the same time as a roof because taking it down to redo a roof kills your economic value.

The payback calculation doesn't consider time value of money. They are paying down £40K today but the savings are realized over years in the future. But they implicitly assumed the discount rate on those savings to be 0.

You can add your car to your whole-house AC bus using the Enphase bi-di EVSE, releasing this year:

https://enphase.com/ev-chargers/bidirectional

How does a household use 1,500+ kWh per month? At my home, we sometimes get above 200, in summer with AC we may score 300 kWh, but how do you consume 5x of that? Is it heating?

Heat Pump water heater running in heat pump only mode is a way better ROI if you’re looking to save some money on electricity.

It's crazy how power hungry UK homes are, or maybe it's UK power consumption habits in general.

I use about ~300 kWh/month. A little bit more with AC some times of the year. What are you even powering with 15000 kWh?

I've been following a story where Elon Musk's xAI is building an 88 acre solar farm next to its Colossus data center near Memphis TN after public outrage due to running 35 methane gas turbines without a permit, which increased NOx emissions enough to allegedly impact health:

https://techcrunch.com/2026/01/12/trumps-epa-plans-to-ignore...

  88 acres = 356,124 m2
  4.56 kWh/m2 per day solar insolation (4.5 is typical for much of the US)
  4.56 kWh/m2 per day \* 356,124 m2 = 1,623,924 kWh/day = 67,664 kW = 67.66 MW average
  1000 W/m2 \* 356,124 m2 = 356 MW peak

They're estimating that they'll get 30 MW on average from that, but I'd estimate more like 15 MW at a solar panel efficiency just over 20%. Still, the total cost for that power should be less than for turbines, since solar is now the cheapest electricity other than hypothetical nuclear (assuming an ideal breeder or waste-consuming reactor and excluding mining/waste externalities/insurance).

30 MW is still only 10% of the the 300 MW used by the data center. But there's lots of land out there, so roughly 1000 acres per data center doesn't seem that extreme to me. That's a 4 km2 or 1.5 mile2 lot, or about 2 km or 1.25 miles on a side.

Basically every GPU server uses 1 kW (about 1 space heater), which puts into perspective just how much computing power is available at these data centers. Running a GPU continuously at home would need 24 kWh/day, so with > 20% efficiency panels that's 4.5*.2 = 0.9 kWh/m2 per day, so 26.67 m2, so at 2 m2 per commercial solar panel and assuming that my math is right: that's about 14 panels considering nights and seasons.

It's interesting to think just how many panels it takes to run a GPU or space heater continuously, even when they put out 500 W or 250 W/m2 peak. And how cheap that electricity really is when it's sold for on the order of $0.15 per kWh, or $3.60 per day.

I've found that the very best way to save on your electric bill is to have a few south-facing slider doors and windows, which is like running a space heater every square meter of window. There's just no way that any other form of power generation can compete with that. Also, I feel that we're doing it wrong with solar. This analysis shows just how much better alternatives like trough solar and concentrated solar (mirrors towards solar panels) might be cost-wise. On an ironic note, solar panels now cost less than windows by area, and probably mirrors.

> The batteries can fill up on the off-peak rate overnight at £0.07/kWh, and then export it during the peak rate for £0.15/kWh, meaning any excess solar production or battery capacity can be exported for a reasonable amount.

Honestly I didn't know this was allowed.

I recently got a heat pump and am on a time-of-use tariff (https://octopus.energy/smart/cosy-octopus/) and have been thinking about pulling the plug on battery storage for a similar purpose (charge during the cheap hours; run the house off battery during the day). I am currently using between 40-50kWh per day - anyone have similar usage to this and can recommend batteries for this?

So what I take away is that he is using approx 3x electricity, that I do and that is including my electric car. I use an additional 5-7MWh of heat but on a heat pump that would still only be a max of 2MWh which doesn’t even bring me to half of his usage, for a family of 4.

The US clean energy tax credit is only available for equipment installed on or before Dec 31, 2025 https://www.irs.gov/credits-deductions/residential-clean-ene...

As a result, more used solar should become available on ebay. I'm excited to see what I can do on a shoe string budget.

£3,632.86/year for electricity seems wild. 17,000kWh/year is about 46kWH/day. We consume less than 3kWh/day on average.

I have 5kwp panels one tesla powerwall (bought before musk had a midlife crisis) and a single electric car.

In 2025 we consumed 6Mwhr, imported 2.7 & produced from solar 5.1

I assume that OP must have electric heating to account for the extra power use, or just does huge amounts of miles. its about 54kwhr a day consumption.

The rooftop solar game in Texas is strongly into scam territory. Most homes I see with panels on the roof are two story homes where you have a negligible amount of area to work with relative to interior space. There was a point where you'd have to deal with a door-to-door salesman approximately every 48h for an entire summer.

The most realistic residential installation I've seen was firmly on the ground at a ~2 acre property. The panels were much larger and heavier (i.e., capable) than what you'd typically find on a roof. It's much easier to build and maintain a solar array when you don't need a ladder/crane to move things around.

I think that it's great that we want to participate in making things better, but not every situation makes sense. When you factor in all of the downstream consequences of sub-optimal, fly-by-night installs, it starts to look like a net negative on the environment. I'm not trying to claim that all rooftop solar projects are bad, but most of the residential ones I've seen make absolutely zero economic sense.

Large scale wind and solar projects are the best way forward. You get so much more bang for buck. I'd consider investing in these projects or their upstream suppliers and owners if you want to get involved financially in making the environment a better place.

Recently switched to octopus too because they have a proper api with 30min consumption updates

My electric bill was up 35% year over year from 2024 to 2025 - only two of us live here and there were no infra changes or new appliances.

I really need a solar solution but I feel so far out of my wheelhouse.

> Having our full costs returned in ~11 years is definitely something we're happy with

Except that after 11 years the equipment will have broken down or become obsolete, at which point you have to start over.

> we've also had protection against several power outages in our area along the way, which is a very nice bonus.

This seems to be the real benefit of the setup.

This setup only really works because of a very specific combination of smart tariffs, EVs, and aggressive automation. Without those, the math would look very different

7.8kw on the roof here in Canada.

7.72MWh for the calendar year produced saving smack on $1000.

$5000 gov grant (free money)

Full remaining install cost covered by interest free loan, so we put that money onto the loan for the next 7 years, then get $1000 a year for the following 20 or more.

Complete no brainer.

Great writeup!

Tldr; their full costs of the system are returned in 11 years.

Whether that's good depends on your perspective and assumptions, you can take a look at opportunity costs.

Imagine you have 100k for say 30 years, and you have three choices: 1. put it in a UK government bond at 4.4% -> 100 * 1.044^30 = 363k 2. put it in the S&P500 (dividend reinvested) at nominal 10% rate -> 1.7 million 3. buy a system that can't be made liquid after 30 years, but returns 11k flat per year = 330k.

1 is very safe and virtually guaranteed. 2 is considered less safe, but over 30 years broad based stock indexes are far less risky than short-term stock investing.

3 is perhaps the most difficult to make assumptions, as its house-tied and operational. Switch houses for any personal reasons, and you'll not be able to fully make your investment liquid and recuperate it. Blow an inverter, see panels degrade and replacement costs must be factored in. This pushes down the final cash position of 330k.

We could be generous and say that the 11k flat savings will increase, as electricity prices rise. Prices grew by 5% yearly in the UK, under that rate so the 11k savings today would grow to 47k annual savings in year 30, and total savings over 30 years would be 870k, pushing up the final cash position, but still not getting close to a long-term stock index investment.

But even that's somewhat generous for two reasons: one is that the 5% inflation was unnaturally high due to the EU's energy crisis from the Russian invasion, and not necessarily indicative of the next 30 years. Various countries in the EU are also curtailing renewable production because there's too much of it (precisely during the moments solar systems were making their biggest profits < 2020, you since see curtailment growing), and with more storage coming online rapidly the profits from their battery system are expected to decline, not increase. -- generally speaking, solar energy producers were more profitable a few years ago, and are becoming less and less profitable over time as competition from cheap panels undercuts them. Many countries have begun to cut the reward from exporting back to the grid from the retail prices of €0.30 to the puny wholesale prices of €0.05 and all countries are expected to go down this road eventually.

On the other hand, AI seems likely to push electricity prices higher for a long time... but it's the newest and biggest question mark compared to the other assumptions we've made above.

Aren’t powerwalls overpriced?

Great read!

I am just wondering would stacking up batteries, charging them off-peak and using/selling back during peak usage be as good as this, or even better? Seems like this shouldn't be a viable scenario, but given the prices and idle capacity, it seems just investing in batteries and charging them at night, to be used/sold to the grid during the day would be as good as a solar installation.

this can be disallowed in the UK, depending on their agreement either their provider. the OP is exporting way more energy than they have ever produced through solar; in effect they’re selling back off-peak energy to the grid, which is making a profit

This is indeed nice for a well-to-do home. But there is a tragedy of the commons issue here.

The grid needs to be up 24/7. And while peak usage is just that, the grid capacity still needs to support peak usage.

This can theoretically be done using batteries but not for an extended amount of time. To say we can have batteries for 2 weeks of normal consumption is highly improbable.

The metals do build those batteries do not exist. Or put in a worse way, the mines do not exist.

An off the cuff calculation of costs and the massive amount of batteries required in the context of Sweden can be found (you need to translate) here: https://www.tn.se/naringsliv/40181/utrakning-60-globen-batte...

In other words, 60 full scale Globen arenas of batteries to replace current Swedish nuclear production.

So for small houses these investments can make sense currently. But from a larger perspective it's not that interesting.

Interesting breakdown for the UK where sunshine isn’t always plenty. If you have more sun this will be different.

Solar tracking trees seem to be an interesting way to get wintertime solar way up.

https://youtu.be/r7HwQdssbas

Solar panel investment has slowed down substantially in Sweden. Basically, when the sun is shining, electricity is close to free. Similar situation with wind power.

Neither technology can move forward until there's a 100x leap in electricity storage costs. Like a bunch of us said 10 years ago, because we remembered high school physics.

So TLDR: spend 40K up front to save 3K per year - could be cost effective depending on the functional lifetime of the tesla powerwall.