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Tesla Powerwall – More of a ‘Luxury’ Than an ROI Winner!

Energy Storage

Last Post by Old_Scientist 3 weeks ago

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Toodles

(@toodles)

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Joined: 3 years ago

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Topic starter 23/02/2025 4:05 pm

We had our first Powerwall 2 installed in November 2022, which provides a storage capacity of 13.5 kWh. The Tesla Gateway, installed alongside, acts as the ‘brains’ of the system, communicating via iOS or Android devices using the Tesla app. The Powerwall is located in our unheated utility area, sheltered from extreme weather conditions. With an average winter ambient temperature of around 12°C, I hope this environment will help extend the battery’s lifespan (I’m only 77 and want to enjoy many more years of use!).

Our 8.1 kWp solar array, consisting of 21 panels, is connected to the Gateway. Each panel has its own Enphase IQ7A micro-inverter. We also have the Enphase Envoy unit, as the solar PV system was installed several months before the Tesla equipment, and we would have had no way to monitor the PV system otherwise.

The Powerwall units feature a sleek white glass front and can be wall- or floor-mounted. They can also be ‘stacked’ one in front of the other, creating a neat installation for the two units that now provide a combined 27 kWh of storage. The Gateway is wall-mounted to the left of the heavy battery and connects to our solar PV system, the grid (after the smart meter), and our consumer unit. We’ve also installed a separate earth rod for grounding when isolated from the grid.

Other articles cover electrical safety and the need for complete isolation from the grid during power outages. Battery owners can continue using stored power or energy from solar PV panels via the Gateway, even charging the Powerwall with solar energy during such times. I won’t delve further into this topic here.

At the time of installation, Powerwalls were as rare as hen’s teeth, and we waited about nine months for delivery of the first unit. We had expressed interest in adding a second unit when installing a heat pump, but our supplier advised waiting to see how the first unit performed. Later, we were told that paying a 25% deposit could secure delivery by February 2024 (15 months later!), but we declined due to the even higher price quoted compared to what we’d paid for the first unit.

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.

The Tesla app offers a wide range of options for managing energy use. We can charge the battery at the lowest rates on our tariff, draw energy from the battery or solar PV as needed, and even export energy from the PV and battery to suit our preferences. We also use the NetZero app to complement the Tesla app, allowing us to make subtle adjustments daily. Currently, we’re on Octopus Energy’s Cosy tariff, which provides three low-rate periods throughout the day. This enables us to charge the battery, power our all-electric home, and avoid higher rates entirely.

ROI? Well… if your primary goal is a return on investment, the Tesla Powerwall system might not be for you! We save significantly by using only the cheapest rates on our tariff, and with energy prices so volatile (read: constantly increasing), it’s hard to predict future savings. However, the ability to choose when to use grid power to charge the battery allows us to maximise our tariff benefits while using the greenest energy available.

In short, ROI isn’t the standout feature of a battery installation. While a Tesla Powerwall isn’t a cheap option (and DIY installations using components from various sources can offer significant savings), the system is sleek, reliable and provides a certain ‘smugness’ during storm warnings!

The latest model, the Powerwall 3, offers a few additional features, including a higher current draw from the battery and an integrated Gateway, which should reduce installation time and cost.

This topic was modified 6 months ago by Mars

Toodles, he heats his home with cold draughts and cooks his food with magnets.

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JohnR

(@johnr)

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Joined: 9 months ago

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23/02/2025 10:45 pm

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.

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Old_Scientist

(@old_scientist)

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28/07/2025 2:12 pm

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.

Samsung 12kW gen6 ASHP with 50L volumiser and all new large radiators. 7.2kWp solar (south facing), Tesla PW3 (13.5kW)
Solar generation completely offsets ASHP usage annually. We no longer burn ~1600L of kerosene annually.

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