The pain and the gain

This page provides details of the capital cost of our solar generation, energy storage, electric vehicles and Air Source Heat Pump equipment, and how that cost is being paid back by the savings we are making.

We set out below the full scope of our total capital investment in CleanTech: the ‘pain’ so to speak. That’s because it’s vital to be clear about upfront costs. Basically it essential to know how much pain must be balanced out by the gain which accrues over time in the form of operating cost savings, and which eventually makes the whole thing worthwhile. Ultimately, it’s all about a pretty simple Pain vs Gain Cost:Benefit equation: ‘if we spend £n thousand on X GreenTech, and that saves us £n a month, how long will it take to pay itself back?’

We are, of course, fully aware that not everybody will be able to make substantial financial investments in ‘going green’ on the scale we have made. We’re only able to due to sheer hard work over the years having allowed us to accumulate some savings. We are most definitely not saying “do it on a big scale, or don’t do it at all”. Absolutely the opposite: you can make a real difference by making just one of the CleanTech changes. It’s perfectly fine to do it step-by-step; we did! A possible incremental approach is set out below.

  • Swap your petrol car for an electric model: it’s massively cheaper over time.
  • Swap to an off-peak tariff to ‘fuel’ that car with electricity when the grid has got cheap energy to spare.
  • Then consider a stationary domestic battery to store that cheap energy for later use when electricity is expensive. With the difference between peak and off-peak electricity costs now over at its highest ever level, an investment in a battery will pay back quicker than ever before.
  • Then add some solar panels to charge up your domestic battery with free energy from the sun, further ‘turbocharging’ your savings.
  • And finally, when you’re comfortable with it all, you could make the really big move and replace a planet-killing oil or gas boiler with an electrically-powered heat pump.

Every single one of these steps will produce financial savings and will reduce your carbon impact. There’s no ‘right order’ to do things in either, everyone’s circumstances are different. The key thing is to make a start!

Pain Part 1 – Capital Cost of GreenTech at Home

The table below sets out the purchase costs of the GreenTech equipment we have bought over the period 2016 to 2021. As we mentioned above, we acquired this kit in stages over the six years. We’re setting it out here in a single table for ease of overview. All figures are rounded to the nearest £100 for clarity.

Pain Part 1: GreenTech works to our home (costs include supply and installation)Cost
32x solar panels for 10 kWp array, including 2x inverters & grid export limiter.£13,800
2x 13.6 kWh Tesla Powerwall batteries, for 27.4 kWh total storage capacity.£11,700
Samsung Air Source Heat Pump system from Evergreen Energy. Produces central heating (CH) & domestic hot water (DHW). Cost includes connection to existing CH radiator plumbing. £17,900
Electrical works: include in-house rewiring & grid upgrade from single to multiphase supply.£11,600
Total cost of GreenTech works to our home£55,000
Capital Costs: Domestic GreenTech

The £55,000 capital cost of all the above equipment and associated works is what we’ve spent to turn our home into a ‘Smart Energy Island’, intelligently storing energy we’ve generated or imported off-peak from our 100% renewable supplier, to power an all-electric heating system, Anne’s pottery kiln, and our general home loads.

Let us be clear: this is significantly more than most people would have to invest. We’re really ‘pushing the boat out’ to radically transform the energy economics of a large, old, house. In a typical, more modern, house you’d probably need only half the solar panels, maybe one storage battery, and a much smaller heat pump would be sufficient to drive the heating and hot water. You’ll almost certainly not need the multiphase grid connection either.

In our case, though, the higher investment is well worth it. We’re getting major savings: so it’s right to transparently record the upfront costs we’ve had to outlay to produce them.

Pain Part 2 – Capital Costs of two Electric Cars

A large chunk of our savings occur because we own two Electric Vehicles (EVs), a Tesla Model X and a Nissan Leaf. We trundle about in them in the normal manner, but we simply don’t pay the insane costs of filling cars up with petrol. Between them, these two cars have 130 kWh of battery capacity, which we fill with our ‘home brew’ solar when it’s sunny and Octopus super-cheapo off peak grid energy when it’s not. The savings amount to several thousand pounds a year. Again, those are the benefits, so it’s right and proper to transparently record the capital costs associated with purchasing these two EVs. The table below sets it out.

Pain Part 2: Electric Vehicle Purchase and offsetting capital incentivesCost
Purchase price of Tesla Model X 100D Performance (with Ludicrous Mode)£141,100
Tax break: 100% Writing Down Allowance reduced tax payable in Year 1 of ownership-£63,500
Tax break: benefit of 6 years zero road tax to date vs comparable top-end petrol car-£4,500
Net capital cost of Tesla EV after tax breaks £73,100
Purchase price of Nissan Leaf 30 kWh£38,000
Tax break: 100% Writing Down Allowance reduced tax payable in Year 1 of ownership-£17,100
Tax break: benefit of 5 years zero road tax to date vs comparable mid-market petrol car-£1,000
Net capital cost of Nissan EV after tax breaks£19,900
TOTAL NET CAPITAL COSTS OF EV PURCHASE AFTER TAX BREAKS£93,000
Capital Costs: Electric Vehicles

Note that the gross capital cost of the two cars was substantially reduced by UK Government tax breaks encouraging businesses to own EVs. As we purchased the two cars through our businesses, these very generous incentives were available to us. So the table subtracts these offsetting benefits from the gross cost to arrive at the true net capital cost of the vehicles. We recognise that not everybody owns a business and that these tax breaks may therefore not be applicable. However, employees who receive a company car as part of the remuneration can also benefit from a massive reduction in Benefit in Kind income tax. That’s the main way for EV drivers on PAYE to pocket tens of thousands in hard cash by reducing their tax bills.

Disclaimer: car taxation is a complex field and benefits are highly dependent on the business and/or income taxation bands applicable to any given individual in any given tax year(s). Tax rates can change and incentives can be withdrawn by Government without notice. References to potentially achievable tax benefits in this website are for information and illustrative purposes only and not intended as financial advice. Persons considering the potential tax benefits of EVs are advised to take appropriate professional financial advice. Likewise, incentives supporting the installation of green energy technologies such as renewable microgeneration or heat pumps can (and do) change. Again, thorough research should be undertaken and appropriate professional advice should be sought before entering into any commitment.

The Gain

Totting it all up, we’ve invested capital of £148,000 in our domestic GreenTech and our EVs over the period from 2016 to date. That’s the net spend, taking into account the tax breaks we got on the electric cars. Happily, that’s all the pain out of the way. Now for the gain!

First off, there’s a slightly quirky benefit for which we need to account. As Tesla early adopters, we got Elon Musk’s canny incentive of “free Supercharging for life” (by which he cleverly partly disguised the high prices charged for early cars). From taking delivery of the Model X in February 2017 through to 31 December 2021, the ‘free miles’ we have charged have saved us £9,700 compared to cost of the fuel we would have needed to travel the same distance in a comparable luxury petrol car. For convenience, we’ve simply treated this as an averaged saving of £167.25 per month in our records to date. However, from 01 January 2022, we are now able to log this benefit precisely and present it to the kWh each month in the savings tables.

To keep the payback calculations accurate, we’ve rolled up the Free Supercharging savings-vs-petrol for the February 2017 to December 2021 period and subtracted them from the capital cost. Stripping out the rounding of all the capital cost inputs, the precise figure we use for calculating payback is £138,331

Before we installed the heat pumps, and with only one Powerwall battery, and with only 24 of the full 32 solar panels installed, we were saving around £5,000 a year, compared to paying standard rates for grid electricity and fuelling petrol cars. This blog post has the detailed report for a 12 month period, which also saw us saving around 6 tonnes of CO2 emissions a year.

Now we can go whole hog on the Cost:Benefit calcs. All 32 panels and both Powerwalls are now in service. We’re still driving the two EVs. We’ve now changed from Octopus Go with its 4 hour off-peak window for cheap grid imports to their Go Faster tariff, which is slightly more expensive per kWh, but has a five hour off-peak window which is great for charging the cars and the domestic batteries. And we’ve made the big move: scrapping oil-fired central heating in favour of an Air Source Heat Pump system, entirely running on electricity.

In short, we’re now answering the harshest possible payback question. Essentially, ‘how long will it take to payback the full £138,331 capital investment, taking into account all the savings we can now make with the GreenTech kit and cars we now own?’

So here goes. The headlines below present a summary of monthly data for our whole setup from 01 January 2022. In due course, we’ll be publishing monthly blog posts to provide more granular detail. But the results are so strong we thought it best to get the news out there without delay.


Over the period 01 January to 15 May 2022, net benefit averaged £53.27 per day. Payback in under eight years is projected.


The Gain: January 2022
£2,082 net financial benefit
2.76 tonnes of CO2 emissions eliminated

  • In this month we imported 4,487 kWh of grid electricity. This electricity cost us £194.97. This was our total energy cost for the entire month, covering everything: heating, hot water, car mileage, ceramics kiln firings and day-to-day domestic loads.
    Note: this very low cost of around 4.34p/kWh was due in very large part to charging EV and Powerwall batteries only in the 5 hour off-peak window and then using energy from the Powerwalls to run the heat pumps during the peak period, as well the cheap rate grid energy itself powering the heat pump during the coldest night hours. In smaller part, the low cost was due to an averaging effect of our energy supplier balancing out an historic overpayment we had previously made to them for a period which included a number of solar-heavy months in 2021. Installation of a polyphase smart meter took place in early May 2022. From this point onward, averaged pricing has been replaced by metered consumption, accurate to the single watt level and measured every 30 minutes in line with SMETS2 smart meter standard practice.
  • We generated 140.4 kWh of solar energy, which cost us nothing.
  • We saved £1,222.71 by not buying ‘standard rate’ electricity from the grid. We use the price capped standard rate of 27.5p/kWh for this calculation.
  • We saved £491.94 by 1,085 kWh of the electricity we produced, imported, or charged for free at Tesla Superchargers fuelling our cars. We travelled 2,658 miles in the month. If we had been filling up comparable cars with petrol, we would have had to pay £491.94 at the prevailing average 147.9p/litre in that month.
    Our 100% renewably fuelled motoring eliminated 0.53 tonnes of CO2 emissions in the month. This is compared to the whole-UK average emissions rate of 124g/km for internal combustion engine (ICE) cars. We use this method of calculation for all future months.
  • We saved £399.98 by 2,267 kWh of the electricity we produced or imported powering our Air Source Heat Pump heating and hot water system. From this electricity input, the ASHP compressor produced a total 7,396 kWh of heat output.
    A conventional boiler would require 755 litres of heating oil to produce this same heat output, using the standard conversion factor of 9.8 kWh of heat output per litre of heating oil burned.
    At the month’s average price of 53p/litre that would have cost £399.98. We bought zero litres of oil for central heating and hot water. We no longer have a boiler.
    By not burning this quantity of heating oil, we eliminated 2.23 tonnes of CO2 emissions in the month. Each litre of heating oil burned emits approximately 2.96 kg. We use this method of calculation for all future months.
  • We received £137.64 from the UK Government’s Renewable Heat Initiative (RHI) scheme, which was designed to enable the decarbonisation of domestic heating by supporting heat pump installations. Note: RHI has now been replaced by the Boiler Upgrade Scheme. Different rates of Government support will apply under BUS. RHI rates were specific to each individual property, its EPC rating, and the size of heat pump installation undertaken.
  • We received £24.25 from the UK Government’s Feed In Tariff FIT scheme, which was designed to enable decarbonisation by supporting solar panel installations. Note: FIT has now been replaced by Smart Export Guarantee tariff arrangement, which pays a set amount for every kWh a household exports to the grid. Different rates per kWh will apply under SEG. FIT rates were also specific to each individual property and the size of solar array. Rates declined over a number of years as the installation costs for solar became cheaper. Our very low FIT rate reflects the fact that we joined the scheme in its very final year, and only 16 of our 32 panels went live before the qualifying date. FIT is calculated on the actual number of kWh produced. It therefore varies month by month. For convenience we simply treat it as an average of 78p per day. It evens itself out between the winter and summer months.
  • The Net Benefit stated in the month’s headline is a simple calculation. We add up all the savings our GreenTech setup and EVs allow us to make, and add the (minor) RHI and (very minor) FIT payments. Then we take away the cost we actually paid for grid energy, which is now our only monthly energy cost.

The Gain: February 2022
£1,519 net financial benefit
2.17 tonnes of CO2 emissions eliminated

On the same basis as January’s results, February was another superb month

  • We imported 3,490 kWh of grid electricity. This electricity cost us £176.10. Again, this was our total energy cost for the entire month.
    Note: again the averaging effect of our energy supplier balancing a previous overpayment played a minor role in generating a low cost of around 5.04p/kWh. Once again, though, the major reason for low monthly cost was charging EV and Powerwall batteries only in the 5 hour off-peak window and then using energy from the Powerwalls to run the heat pumps during the peak period, as well the cheap rate grid energy itself powering the heat pump during the coldest night hours.
  • We generated 325.4 kWh of solar energy, which cost us nothing.
  • We saved £951.03 by not buying ‘standard rate’ electricity from the grid.
  • We saved £270.81 by 554 kWh of the electricity we produced, imported, or charged for free at Tesla Superchargers fuelling our cars. We travelled 1,513 miles in the month. The saving reflects an average 151.9p/litre fuel cost in that month.
    100% renewables-fuelled electric motoring eliminated 0.30 tonnes of CO2 emissions in the month.
  • We saved £327.23 by 1,999 kWh of the electricity we produced or imported powering our Air Source Heat Pump heating and hot water system. From this electricity input, the ASHP compressor produced a total 6,050 kWh of heat output. A conventional boiler would require 617 litres of heating oil to produce this same heat output. The saving reflects this month’s average price of 53p/litre.
    Not burning this quantity of heating oil eliminated 1.87 tonnes of CO2 emissions in the month.
  • We received £124.32 in RHI payments.
  • We received £21.90 in FIT payment.

The Gain: March 2022
£1,620 net financial benefit
1.91 tonnes of CO2 emissions eliminated

  • We imported 2,737 kWh of grid electricity. This electricity cost us £194.97. As usual, this was our total energy cost for the entire month.
    Note: the averaging effect of balancing historic ledgers with our energy supplier generated a higher cost of around 7.12p/kWh in this month.
  • We generated 711.8 kWh of solar energy, which cost us nothing. The spring sunshine was beginning to have its benign effect.
  • We saved £745.83 by not buying ‘standard rate’ electricity from the grid.
  • We saved £387.87 by 697 kWh of the electricity we produced, imported, or charged for free at Tesla Superchargers fuelling our cars. We travelled 1,807 miles in the month. The saving reflects an average 166.9p/litre fuel cost in that month.
    100% renewables-fuelled electric motoring eliminated 0.36 tonnes of CO2 emissions in the month.
  • We saved £327.23 by 2,349 kWh of the electricity we produced or imported powering our Air Source Heat Pump heating and hot water system. From this electricity input, the ASHP compressor produced a total 6,442 kWh of heat output. A conventional boiler would require 657 litres of heating oil to produce this same heat output. The saving reflects this month’s average price of 79p/litre.
  • We received £137.64 in RHI payments.
  • We received £24.25 in FIT payment.

The Gain: April 2022
£1,372 net financial benefit
1.81 tonnes of CO2 emissions eliminated

  • We imported 1,950 kWh of grid electricity. This cost us £78.90: our total energy cost for the entire month.
    Note: this is the final month affected by averaging past balances with our energy supplier: from May 2022 metered pricing will be used.
  • We generated 1,039 kWh of solar energy, which cost us nothing. April was nice sunny month!
  • We saved £531.46 by not buying ‘standard rate’ electricity from the grid.
  • We saved £397.49 by 728 kWh of the electricity we produced, imported, or charged for free at Tesla Superchargers fuelling our cars. We travelled 1,964 miles in the month. The saving reflects an average 166.9p/litre fuel cost in that month.
    100% renewables-fuelled electric motoring eliminated 0.39 tonnes of CO2 emissions in the month.
  • We saved £365.42 by 1,446 kWh of the electricity we produced or imported powering our Air Source Heat Pump heating and hot water system. From this electricity input, the ASHP compressor produced a total 3,770 kWh of heat output. A conventional boiler would require 385 litres of heating oil to produce this same heat output. The saving reflects this month’s average price of 95p/litre. As the warmer weather arrives, clearly the ASHP is not having to work so hard to heat our home, so its absolute energy consumption declines markedly.
  • We received £133.20 in RHI payments.
  • We received £23.46 in FIT payment.

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