Matthew's Dev Blog

Solar panels and Powerwall - one year retrospective

It's a year since we had solar panels and a Tesla Powerwall installed. Was it a good idea or not? Was it worth the money? Does that even matter?

System details

We had an installation from the nice people at Stratford Energy. Here are the details:

The other piece of the puzzle, installed by our friendly local plumber, was a Worcester Bosch SC-210 hot water cylinder. Previously, we had a gas combi boiler which heated radiators and hot water. But now we can choose to heat our domestic hot water (DHW) with the cylinder's built-in 3 kW immersion element, or via heat from a new gas boiler.


Our objectives were simple:

  1. reduce our usage of natural gas
  2. generate green electricity to reduce our dependency on the grid
  3. try to time-shift our energy usage away from peak-times

Rough plans

In my head, I had these rough plans:

  • heat the DHW using electricity as much as possible
    • ideally, it would be fully-heated when we wake up in the morning
  • use solar energy to power the house
  • use excess solar energy to heat the DHW cylinder, via the eddi
  • store excess solar energy in the Powerwall when the DHW is up-to-temperature
  • consider moving to the Tesla Energy Plan, provided in the UK by Octopus Energy
    • when we were planning this, there was zero daily standing charge on the Octopus Tesla Energy Plan

Our energy usage before installation

Approximate annual usage:

  • Gas: 12000 kWh
    • 4000 kWh for DHW and cooking
    • 8000 kWh for space heating (radiators)
  • Electricity: 4000 kWh

Other things to consider:

  • around this time, we swapped our gas hob for an induction hob (AEG IKE64450FB), which will reduce our gas usage, and increase electricity
  • DHW tank, heated with electric immersion, will also reduce gas usage and increase electricity
  • we're also building an extension onto the house, so it's likely that the 8000 kWh of gas for space heating will increase.

I estimated that our gas usage would decrease from 12000 kWh to 9000 kWh, and electricity would rise from 4000 kWh to 6000 kWh.

Planning the installation with Stratford Energy

Jason from Stratford Energy worked with us to plan the installation. He was able to do some calculations about how the system might perform, with an estimated annual energy production of 5270 kWh. There were also estimates for self-consumption, electricity export and import - but they were done before we agreed to also buy the Powerwall, which obviously makes the import/export estimates invalid.

Jason was very patient with my endless stream of questions. This was all done over email/phone, due to Covid. In normal times, there would have been an on-site survey.

The panels themselves are installed on two aspects:

  • 8 panels on a roof facing roughly south-east
  • 6 panels on a roof facing roughly south-west


Installation was done by Stratford Energy over three visits:

  • solar panels were all fitted on 22nd January 2021
  • the inverter and myenergi eddi were installed and provisioned on 16th April 2021
  • Powerwall was provisioned on 19th April 2021

On the first day, in the middle of April, we were delighted to see peak generation of 5 kW.

Use in practice

  • the SolarEdge inverter is noisy at dawn and dusk - we can hear the relays clicking from the other side of the house
  • we've chosen to heat DHW exclusively with electricity
    • this costs more than using gas - but reducing gas usage was our main aim for this project
    • during the summer months, the gas boiler doesn't get used at all. Our plumber recommends we manually fire it up for 10 minutes every couple of weeks
  • myenergi eddi works really well - but there's often not enough excess solar energy to fully heat the cylinder
    • we're a family of four
    • one shower needs approximately 1.5 kWh of hot water
    • one bath needs approximately 3 kWh of hot water
    • so we need around 6 kWh going into the cylinder every day
    • we do put excess energy into the DHW via the eddi, during the day
    • but at night-time we've set a two-hour "boost", using the eddi, to make sure the water is always hot every morning
    • sometimes this boost takes 6 kWh of power overnight; sometimes it takes nothing. This depends on solar generation and hot-water usage the day before
    • in the summer, the power for this overnight boost came from the Powerwall (but not since we changed to the Tesla Energy Plan - more on that later)
  • the Powerwall is a great piece of kit
    • during the summer, I wondered whether we should have bought two Powerwalls
    • during the winter, I wondered whether we should have not bought it, and spent the money on something else
    • when there's a power-outage, there is a good 2-4 second delay before the Powerwall takes over
    • when the engineer provisioned the Powerwall, he switched off the main switch and the Powerwall worked instantly. There wasn't even a flicker with the lights. But when a power cut happens for real, there is a few seconds delay
    • this short delay is "expected" according to Tesla
    • the Powerwall isn't as noisy as I'd feared - but you can hear it whirring when it's doing some high-wattage charging or discharging
  • self-consumption is a cost-saving benefit, not an environmental benefit
    • I think self-consumption is over-rated. Time-shifting energy usage is more important
  • provisioning an export meter is a massive pain in the arse
    • it feels like provisioning a smart meter for export is very much a manual process
    • this requires pieces of documentation from the DNO and the installer - and it's your job, as the homeowner, to understand what's needed and provide all this evidence
    • energy companies (looking at you, Octopus) don't give you much help with this
    • the whole process takes weeks to do - and Octopus didn't even notify us when it was complete

By the time we realised that our export was being metered correctly, Octopus had added a 22p daily standing charge to the Tesla Energy Plan. This was massively annoying, because we could have got the better deal had Octopus worked a little quicker, and properly notified us that the job was done.

While we waited for the export meter to be provisioned, our generation was easily powering the house for the whole day; our only cost to Octopus was the daily standing charge.

When setting up the export tariff, the quality of help we received from Octopus was way below my expectations.

Tesla Energy Plan

I considered waiting for a whole year before trying the Tesla Energy Plan (TEP), to give us a baseline set of usage figures... but in February we moved to the Tesla plan, two months early.

There were three reasons for this:

  1. the cost of electricity is heading towards 30p/kWh - and getting the TEP now fixes my import (and export) at 11.7p for a whole year
  2. "self-consumption" isn't as important as time-shifting. I could set some peak/off-peak times for the Powerwall to do time-shifting for my house, but the TEP actively puts energy into the grid at peak time
  3. on the TEP, the import rate and export rate are exactly the same, which removes the cost argument for self-consumption

The TEP isn't as "smart" as you might think: it appears to be a set of fixed peak and off-peak times, which change every 6 months. As I write, we're in "Season 1 Nov - Apr". Peak is 16:00-19:30, off-peak is 01:30-07:00 (local time). The Powerwall charges from the grid during off-peak, and discharges at 5 kW during peak time.

Actual energy usage, 12 months on

All these numbers are from 17th April 2021 - 17th April 2022.

  • Solar generation: 5020 kWh
    • 1010 kWh of that directly powered the house
    • 1249 kWh was diverted to DHW, via the eddi
    • 1958 kWh went into the Powerwall
    • 753 kWh was exported to the grid, when the DHW and Powerwall were full
  • Total house usage (including DHW): 6820 kWh
  • Net grid import: 2140 kWh
  • Battery usage: 2200 kWh

(Yes, I know those figures don't add up exactly)

Gas usage 12049 kWh.


Gas usage is approximately the same. This is disappointing, but not surprising - because the house is bigger now.

DHW was heated entirely by electricity - so there's at least 2 MWh of gas saved there.

Solar provided 74% of our annual electricity usage.

Would I do it all again?

What would I do differently?

  • consider more panels. 5.6 kWp is a lot, but outside the summer months I've found myself wishing for more
    • even with the interver capped at 5 kW output, I think we could add another 2 kW without noticing severe clipping
  • find somewhere else for the SolarEdge inverter to live, where I can't hear its relays chattering
  • properly fix my roof before having panels fitted. It seems obvious now, but I didn't know that 50-year-old felt would tear easily
  • use a big energy company while provisioning equipment and hooking up the export meter, and then switch to Octopus after everything is working
  • request an on-site survey from the solar installers. There were a couple of surprises (like the Powerwall needing its own earth-rod) which could have been avoided if we'd had an in-person survey
  • seriously investigate heat-pumps instead of buying a new gas boiler for space-heating

... but why no talk about money?

There are countless blogposts and Youtube videos about Powerwalls and solar panels - and they invariably focus on money. "How much does it cost?" "What's my price per kWh of electricity?" "What's the payback time?" "When will I start making a profit?"

But this post has hardly mentioned money, because I don't really care about that. Here's why:

  • yes, it's expensive... but if you can afford to fit a system like this, then I think you have a moral obligation to do so
  • don't do it to save money. Do it to reduce your reliance on fossil fuels. Do it to contribute some green energy into the national electricity grid. Do it to time-shift your import from the grid away from peak times
  • don't think about payback time, think about energy. How much you contribute to the national energy pool, and how much you take
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First published 17 April 2022