Introduction

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

Item
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.
£9,290.12
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.
£7,282.79
Purchase zappi EV charger
Note: unless your have already had an EV charger installed at your home, you will qualify for a Government grant of £500 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.
£642.03
Total£24,339.04

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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Payback

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

ItemExplanation
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).
Example with October 2019 data:
Solar production: 308.6 kWh x 12.93p grid cost avoided: £39.90
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.
Example with October 2019 data:
FIT Generation payment £8.88 + FIT Deemed Export: £6.03: Total benefit: £14.92
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.
Example with October 2019 data:
£21.04 total cost for charging: 420.8 kWh off-peak + 93.3 kWh solar.
£66.46 peak grid cost avoided by Powerwall discharging 514 kWh during peak hours.
For info: Net benefit of peak shifting alone (£66.46 – £21.04): £45.42
Because all electricity going in to the Powerwall is 100% booked as a financial cost, we account the entire kWh coming out of it as a benefit: £66.46 in this example month.
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.
Example with October 2019 data:
• 449.2 kWh off-peak energy charged to EVs at £22.46 off-peak grid cost.
• Saving versus the cost of charging the same amount of range using peak electricity: 12.93p – 5.00p = 7.93p/kWh x 421: Total benefit: £35.62
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).
Example with October 2019 data:
19.57 kWh solar energy charged to EVs at £0 cost. Saving versus peak-rate grid electricity cost: Total benefit: £2.53
FBeMFThis is a subtotal, standing for “Financial Benefit excluding Motor Fuel savings.” It includes only items 1-5 tabulated above.
Example for FBeMF with October 2019 data: £159 (rounded)
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.
Example with October 2019 data:
• 449.2 kWh off-peak electricity charged to EVs at cost of £22.46
• 449.2 kWh equates to 1,049 miles @ average of 0.428 kWh/mile
• 1,049 miles would require 30 gallons = 136.1 litres @ 35mpg in an IC car
• 136.1 litres would cost £179.70 @ £1.32 per litre cost of motor fuel.
Total benefit: £179.70 fuel cost minus £22.46 electricity cost paid: £157.24
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.
Example with October 2019 data:
• 19.6 kWh solar charged to EVs at cost of £0.00
• 19.6 kWh equates to 45.7 miles @ average of 0.428 kWh/mile
• 45.7 miles would require 1.3 gallons = 5.9 litres @ 35mpg in an IC car
• 5.9 litres would cost £7.76 @ £1.32 per litre cost of motor fuel.
Total benefit: £7.76 fuel cost minus £0.00 electricity cost paid: £7.76
NOTE: obviously, we expect this benefit to be substantially greater in the brighter summer months when more surplus solar energy will be available.
SubtotalMotor fuel savings versus conventionally-fuelled car: £165
FBiMFThis is the grand total, standing for “Financial Benefit including Motor Fuel savings.” It includes all items 1-7 tabulated above.
Example for FBiMF with October 2019 data: £324 (rounded)
Total Financial BenefitThis figure is presented in the detailed monthly report.
• It takes the total cost we actually pay to Octopus Energy for the total energy we have consumed and the 25p/day standing charge.
• From this we work out the average total cost per kWh for the grid electricity we have actually paid, including the standing charge.
• We then contrast this with the UK average cost of electricity for those who simply purchase it from the grid, and work out the saving we have made by using our own blend of solar, off-peak power and peak-shifting to generate, store and use electricity for home and motoring use.
Example with October 2019 data: £370 (rounded)
NoteWe expect significantly higher financial benefits in the summer months.

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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. Henceforth it will be updated when each monthly system performance update is published. Figures are rounded. Abbreviations and other explanations are as follows:

  • FBeMF: Financial Benefit excluding Motor Fuel savings.
  • FBiMF: Financial Benefit including Motor Fuel savings.
  • CeMF: Cumulative excluding Motor Fuel savings.
  • CiMF: Cumulative including Motor Fuel savings.
  • 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.

PeriodFBeMFFBiMFCeMFCiMF
22 Nov 18 – 11 Sep 19£669£1,446£6692.7%£1,4465.9%
12 Sep 19 – 30 Sep 19£90£123£7583.1%£1,5696.5%
October 2019£159£324£9183.9%£1,7957.6%
November 2019£136£402£1,0544.3%£2,1979.0%
December 2019£131£417£1,1854.9%£2,61410.7%
January 2020£130£386£1,3155.4%£3,00012.3%
February 2020£158£390£1,4736.1%£3,39013.9%
March 2020£206£382£1,6786.9%£3,77215.5%
Cumulative Financial Benefits and Payback

Please note, the payback numbers in the summary table above are stated on the most conservative possible basis, with FBeMF calculated as (Cost We Would Have Paid If Every kWh Was At Peak Rate) minus (Cost We Actually Did Pay For All kWh Consumed). This makes sense for us, in our specific situation, paying 5p for off-peak electricity and 12.93p for peak. In order to make our results more generally relevant to other people, the (slightly higher) numbers we present in the Monthly Performance Summary posts are calculated using savings versus the UK average cost per kWh.

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