This page provides details of the capital cost of our solar generation, energy storage and EV charging equipment and how it is being paid back by the energy cost savings we are making. We start with a summary of the capital costs, and then turn to payback.

You can skip directly to the Payback To Date Summary section if you wish.

Much of our carbon and cost savings are enabled by using ‘Octopus Go’ electricity, which cost us only 5p/kWh (versus the UK average of 15.5 kWh) to fill up our electric cars and Powerwall domestic storage battery during the night. Octopus Energy uses 100% renewable generation, so we can achieve zero emissions at source or point of consumption. Anyone signing up using the link its £50 off their electricity bill. So do we.

Capital Cost

Supply, install, integrate and commission 5.2 kWp solar generation system (Phase 1)
12x solar panels on main house roof +
4x panels on office roof +
Inverter, generation meter and all associated wiring.
Includes all necessary scaffolding and safety equipment for working at height.
Supply, install, integrate and commission 14.3 kWh Tesla Powerwall battery £7,205.10
Supply, install, integrate and commission 5.2 kWp solar generation system (Phase 2)
8 x solar panels on garage roof +
8 x solar panels embedded in ceramics studio roof (when built) +
Inverter, generation meter and all associated wiring.
Price also includes charge to install, integrate and commission 1x zappi EV charger.
Scaffolding not required due to low roof heights.
Purchase zappi EV charger
Note: unless you have already had an EV charger installed at your home, you will qualify for a Government grant of £350 against the cost of buying and installing this item. You can get two grants if you are the registered keeper of two EVs. We did not qualify, due to having previously claimed twice on an earlier scheme.
Summary Of Capital Cost

Thanks to a lot of hard work over many years, we had sufficient savings to invest 100% in cash to cover the up-front purchase cost of the system. Therefore, in our case, every saving we make from Day 1 of operation will go towards paying back the system cost.

We fully appreciate that we’re in a relatively fortunate position and that others may not have the ‘ready cash’ to make this move. In that case, just stick the cost on the mortgage. Assuming a 4% interest rate, a system costing exactly the same as ours would cost £147 per month over a 20 year term. You’d obviously have to adjust the payback calculation to take this finance cost into account.

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This section uses the real world performance data of our system to illustrate how much of the total capital cost is being paid back by the financial savings made during each month of operation.

The table below sets out the ‘benefits stack’ we use to record the savings and benefits which we set against the costs of the system.

As can be seen from Items 4, 5, 6 and 7 in the table, a substantial proportion of the benefits are due to driving electric cars, which we charge using both cheap off-peak electricity and solar generation. As well as daily ‘runabout’ Nissan Leaf, we drive a Tesla Model X. The link below takes you to the Tesla website, where promotional deals are available on new car orders for anybody using this referral. See the Links page for more detail

1Peak grid cost avoided by solar generation.By definition, the sun does not shine between 00:30 and 04:30, which is the Octopus Go off-peak period for very cheap electricity (5p/kWh as of November 2019).
So, by extension, every kWh generated by our solar kit is produced during peak hours. Therefore, every 1 kWh from solar therefore saves us buying 1 kWh of peak cost electricity (12.93p/kWh, as of November 2019).
2Government payments for solar generation.The Government ‘Feed-In Tariff’ (FIT) only pays us for the 16 panels on Inverter 1. The panels on Inverter 2 were installed too late to qualify.
• On the 16 qualifying panels, FIT pays 3.96p/kWh on 100% of power generated.
• FIT also pays 5.38p/kWh on 50% of the power for ‘deemed exports’. You are ‘deemed’ to be exporting 50% of your generation, even if you are actually using it in the house.
3Savings from Powerwall peak-shifting.This is the benefit from filling up our 14.3 kWh Tesla Powerwall storage battery with a blend of off-peak (5p/kWh) grid electricity and, when available, solar power, which costs 0p. The Powerwall discharges only during peak hours, saving 12.93p for every kWh which we then avoid buying at peak period grid costs.
4Savings from using off-peak grid electricity to charge EVsThis is the benefit from using the TIMED BOOST function to instruct the zappi smart charger to route imported grid energy into the Tesla only during the 00:30 to 04:30 cheap rate (5p/kWh) window. We use an in-car control function on the Leaf to achieve the same result: it is simply plugged in to a 3-pin socket.
5Savings from using solar energy to charge EVsThis is the benefit of using the ECO+ function on the zappi smart charger to route only surplus solar energy to charge the EVs, instead of buying peak-rate electricity. (By definition it is peak rate, because the sun does not shine during the overnight off-peak hours).
FBeMFThis is a subtotal, standing for “Financial Benefit excluding Motor Fuel savings.” It includes only items 1-5 tabulated above.
6Savings from off-peak EV charging vs petrol fuelItem 4 accounts for the electricity cost saving of using off-peak energy to charge the EVs. This item 6 accounts for the petrol or diesel fuel we would have had to purchase to drive an internal combustion (IC) car the same distance as the range we gained from off-peak charging.
7Savings from solar EV charging vs petrol fuelItem 5 accounts for the electricity cost saving of using solar energy to charge the EVs. This item 7 accounts for the petrol or diesel fuel we would have had to purchase to drive an internal combustion (IC) car the same distance as the range we gained from solar charging, for which the total cost was £0.00.
FBiMFThis is the grand total, standing for “Financial Benefit including Motor Fuel savings.” It includes all items 1-7 tabulated above.
Benefits Stack

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Payback in period before Powerwall was installed

As shown in the capital cost table above, our system was installed in two main phases. Phase 1 put the 16 solar panels on the house and office roof. Phase 2 saw the installation of 8 panels on the garage roof, the Tesla Powerwall storage battery and the zappi smart EV charger. We also fully paid upfront for the cost of the 8 panels that will go on the studio roof (it was financially advantageous to do so).

For the period from 21 November 2018 until 19 July 2019, when the Powerwall was installed, the system had no means of storing any surplus solar power. If we could not use it in the house or put it straight into an EV, it was exported to the National Grid.

On 11 September 2019, the 8 panels on the garage roof were switched on. The remaining 8 will follow when construction of the ceramics studio reaches the appropriate stage.

In the light of the above, it is clear that payback of the system started before the full capacity and storage capability of the system was in place. Indeed it won’t be fully complete until the final 8 panels come into service.

System performance in the period from late-November 2018 to mid-September 2019 is summarised in the infographic below. Green represents solar energy generated and the financial benefits associated with it on a month-by-month basis. Blue represents gird electricity bought from our former electricity supplier, OVO Energy. Click the graphic to download a PDF.

Key results for the November 2018 to September 2019 period are as follows.

  • 3.7 MWh (Megawatt hours) – 3,716 kWh – were generated by solar.
  • This avoided the need to purchase that amount of grid energy from OVO for £423.
  • Government payments for our solar generation totalled £246 in the period.
    • ‘Generation Feed-In Tariff’ (FIT) was paid at 3.96p/kWh on 100% of the generation from the 16 panels in Phase 1.
    • ‘Export FIT’ was paid at 5.38p/kWh on 50% of the generation from the 16 panels in Phase 1.
    • These figures are very slightly out, due to the annual uplift in FIT rates every April. A marginally lower rate was paid until that point, when the annual uplift in line with RPI was applied.
    • Please note that these FIT rates were the very ‘last knockings’ of the previously exceptionally generous Government subsidies for solar generation, originally introduced to stimulate the industry. FIT for domestic solar has now been withdrawn. A new scheme is expected to replace it. These ‘end of an era’ FIT payments, although guaranteed for 20 years and index-lined to rise with RPI, are very marginal to the overall payback economics of our system. We’d have gone ahead with a zero FIT rate, so little difference does it make.
  • 1.9 MWh of the solar generated was used to charge the electric vehicles (EVs)
  • This equated to 4,567 miles of driving.
  • Assuming £1.32 per litre and 35 mpg for a conventional car, the solar energy charged to the EVs saved motor fuel costs of £782.
  • Combined financial benefits for the period were therefore £669 + £782 = £1,452
  • 8.6 MWh of grid electricity was purchased from OVO at a cost of £1,081.

The four large pie charts in the infographic summarise the ‘benefits stack’ described above. The lower bar chart and smaller pie charts build up the picture month by month. The marginal difference between total benefits of £1,452 presented in the third large pie chart and £1,446 in the final, fourth, summary pie chart on the right is due to a deduction to account for a small administrative charge.

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Payback To Date Summary

From October 2019, we have been keeping detailed performance records, which are published monthly on the Monthly Performance Updates blog page.

The following table summarises the payback ramp to date, including the period before the detailed monthly records were published. For the 12 months between 01 October 2019 and 30 September 2020 we updated when each monthly system performance update was published. Figures are rounded. Abbreviations and other explanations are as follows:

  • “MF” means ‘Motor Fuel’ that we would otherwise been forced to buy to drive the same mileage in an Infernal Combustion Engine car.
  • The percentages show the proportion of the £24,339 capital cost paid back to date on the two bases of calculation, exclusive and inclusive of Motor Fuel savings benefits.

From the October 2019 entry onwards, the link on the month name takes you to the detailed system performance report for the month in question.

excl MFincl MFexcl MFincl MF
October 2019£154£353£9123.7%£1,9227.9%
November 2019£153£494£1,0654.4%£2,4169.9%
December 2019£151£417£1,2165.0%£2,83311.6%
January 2020£151£460£1,3675.6%£3,29313.5%
February 2020£167£446£1,5346.3%£3,79315.6%
March 2020£110£322£1,6446.8%£4,06116.7%
April 2020£183£267£1,8277.5%£4,32817.8%
May 2020£207£274£2,0348.4%£4,60218.9%
June 2020£124£280£2,1588.9%£4,88220.1%
July 2020£123£385£2,2819.4%£5,26721.6%
August 2020£97£332£2,3789.8%£5,59923.0%
September 2020£180£417£2,55810.5%£6,01624.7%
Cumulative Financial Benefits and Payback up to 30 September 2020

Please note, the payback numbers in the summary table above are highly conservative, for the three reasons set out below.

1: Entire capital cost fully paid, but system not yet fully installed

  • The £24,339 total capital cost, against which payback is calculated, includes an additional 8 solar panels, for which we have paid in full.
  • Due to COVID-19 delays, the building on which these panels were to be installed is not yet built and the panels are not yet connected.
  • Therefore we have paid for approximately 10 kWp solar generation capacity, but currently have only around 7.5 kWp in service.
  • Hence, once completed, the whole system, the capital cost of which is already fully paid, will be generating around 25% more solar energy.
  • This will generate greater savings than those currently reported.
  • Therefore payback will be quicker than the results tabulated above.

2: Powerwall battery not installed for first 8 months of operation

As noted at the top of this page, for the period from 21 November 2018 until 19 July 2019, when the Powerwall was installed, the system had no means of storing any surplus solar power and saving us the cost of peak hours grid imports when the sun was not shining during the day, or during the evening and night hours.

3: Fewer solar panels in service for first 10 months of operation

For the period from 21 November 2018 until 11 September 2019, only 16 solar panels, with an approximate generating capacity of 5 kWp were installed, as opposed to the 24 panels now in service; 75% of the 32 panels which will ultimately be generating when the system is fully completed.

Taking all the above into account, if the full £24,339 system were in operation from the outset, including all 32 solar panels and the Powerwall battery from Day 1, we estimate that payback would take about 3 years 8 months, assuming domestic consumption and EV charging at the levels actually recorded.

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