I had solar panels (nominal 4kW) installed in 2017 and quickly realised that there was a mis-match between generation and domestic demand so a Powerwall was installed in 2018. This meant that much of the domestic use in summer months came from the solar generation. The facility for mains charging came several years later. Initially, Tesla's "intelligent" algorithm was often stupid because it was based on history and would assume that a sunny day would be followed by another sunny day, etc. This was later improved by considering the regional solar generation forecast (OK for very sunny or very cloudy days but less good for mixed sun and cloud).

I had a second Powerwall installed nearly two years ago in anticipation of a heat pump. This additional battery capacity also meant that that there was sufficient ability to carry over power from sunny days to dull ones during the summer months with no usage of mains power. I also had the backup Gateway fitted (this wasn't available when the first battery was installed). The off-line capability came very useful a few months ago when the local supply cable blew up causing a 9 hour outage. There have been about 10 days during the past winter when the electricity consumption exceeded the total battery capacity before the next recharge period arrived (Octopus Go tariff with cheap power 00:30 to 05:30).

My Powerwalls are in the garage which gets down well below 10C in cold winter weather. The batteries have their own thermal management to keep within a comfortable temperature range. The garage, however, never gets hot as it's on the north side of the house. I suspect that the thermal management is one reason why the long term efficiency (power out vs power in) is about 85%. Powerwalls are very neatly packaged.

Any ROI calculation needs to factor in all the unknowns such as the future electricity cost and the potential earnings foregone had the money been invested elsewhere. The convenience of tolerance to electricity suppy interuptions also can't be ignored.

We had our Tesla Powerwall 3 (and Gateway) installed earlier this month, together with a doubling of our installed solar capacity, going from 3.6kWp to 7.2kWp.

Taken alone, I'm not sure we will ever see a financial return on the cost of the battery, but there was more to our decision than ROI alone.

We have signed up for Intelligent Octopus Flux (IOF) tariff which is certainly helping with the financials during summer. We use around 10-12kWh/day in summer and can generate 4 times that on a good day, so a net metering tariff such as IOF that pays 22.3p day rate and 29.8p at peak rate for our net export works out far better than a cheap import tariff paying 15p SEG.

Before I switched to IOF, we were on Cosy ToU for a couple weeks (and will use Cosy for the 6 winter months where we are net importers) and the PW3 worked well, running the house from battery during more expensive periods and charging from excess solar during the day and drawing from the grid in the cheap slots.

Our battery is installed outside on an east facing wall due to lack of other suitable options. One of the reasons we chose a Tesla is their market leading thermal management which is particularly important when installing outside. If we had any suitable indoor locations, I may have considered other brands, but we didn't. We will have to see how well it performs in Winter.

Posted by: @johnr

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My Powerwalls are in the garage which gets down well below 10C in cold winter weather. The batteries have their own thermal management to keep within a comfortable temperature range. The garage, however, never gets hot as it's on the north side of the house. I suspect that the thermal management is one reason why the long term efficiency (power out vs power in) is about 85%. Powerwalls are very neatly packaged.

Interesting what you say about efficiency - I will look at my figures once I have a month's worth of data, correcting for the 36% charge it arrived with. To date I'm seeing ~97% efficiency but I suspect it's a lot more efficient in summer than winter due to operating in it's optimum range, and that it's charged 76% from DC-coupled solar and only 24% from AC grid, so inverter losses are lower than they may be in winter. The PW3 manual quotes 97.5% for Solar to home/grid, and 89% for round trip efficiency.

Posted by: @toodles

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When our heat pump was installed in February 2023, the company offered a Powerwall 2 at a significantly lower price, with delivery in just a few weeks. We placed the order, and the second unit was installed in front of the original one – space we had already planned for. This addition increased our storage capacity to 27 kWh, creating a very tidy setup with all control equipment for solar, storage, Wi-Fi, and network links neatly arranged on the same wall.

 

I know this is pretty old, but wanted to double check something as it is very important for battery upgrades. Did you had the Heat Pump installed by the same exact company that installed the original PW2? Because from what I understand, any expansions MUST be installed by the original installer, or else you void your warranty. Unless the original installer has gone bust, and Tesla can recommend someone else to perform the upgrade, and cover the new and old system.

This seems to be based on the brand, for example Tesla, tightly controlled, very likely your warranty is voided by using any other installer than the original, unless another Tesla certified and approved installer, but worth checking with them first. Other brands, more forgiving but still worth checking with them first.

@drei The second PW2 was installed by the same company who installed our heat pump; the company who installed the first PW were really not interested, they quoted 12-15 months delivery and approx. £2000 above the price of the first one. The company who supplied and fitted the second are accredited installers for Tesla Powerwalls. Regards, Toodles.

Also, though they weren’t prepared to match the price they had indicated for the second Powerwall at the time the first one was installed, they still required a 25% deposit on that order for 12-15 months hence! Toodles.