How the calculator works

1. Why this calculator exists

Most UK green-home calculators look at one thing. Solar alone. Heat pump alone. EV alone. That misses the point. The big savings show up when you combine them, because they change each other's maths. A heat pump plus a solar array means a chunk of your heating runs directly off the panels. An EV on a smart overnight tariff flattens your battery-charging cost. The order you install things in affects the payback.

This tool models all four together so you can see the combined answer, honestly, with every number sourced.

2. Data sources

• Ofgem price cap: the default electricity and gas unit rates and standing charges come from the April to June 2026 cap.
• Octopus Energy published tariffs: Intelligent Octopus Go (off-peak and peak rates) and the Outgoing export tariff rate (best widely-available Smart Export Guarantee rate).
• MCS: the standard yield estimates for solar PV by UK macro-region (kWh per kWp per year) and median installation costs for MCS-certified heat pumps and solar.
• BEIS Energy Consumption in the UK (ECUK) 2024: annual delivered heat demand by home type.
• BEIS Greenhouse Gas Conversion Factors 2025: the grams-per-kWh numbers for grid electricity, mains gas, heating oil, LPG, petrol, and diesel.
• DUKES 2024: calorific values for heating oil and LPG.
• DfT National Travel Survey 2024: average UK annual mileage and typical EV efficiency.
• RAC Fuel Watch: average pump prices for petrol and diesel, April 2026.
• OZEV: the EV chargepoint grant for flat-dwellers and renters.
• Sutherland Tables: quarterly off-gas heating price averages for heating oil and LPG.

3. How solar is calculated

We estimate a sensible system size for your home, from 3 kWp for a flat to 6 kWp for a 5-bed detached, capped at 8 kWp. We add a nominal 1 kWp of headroom for each of a heat pump and an EV if you are including those, because they shift electrical demand upward. Real installers will do a roof survey and may size differently. The tool assumes you are not roof-constrained.

Annual generation equals the system size multiplied by the regional yield (kWh per kWp per year, from MCS) multiplied by orientation and shading factors. South-facing unshaded is the reference; east or west gets 85 per cent, north gets 65 per cent. Heavy shading is a 25 per cent haircut.

Of that generation, a self-consumption fraction goes directly into your home and the rest is exported. The base self-consumption rate is 35 per cent (solar only, no battery, no electrified loads). A battery lifts it to about 70 per cent. Each electrified load (heat pump, EV) adds 15 percentage points up to a cap of 85 per cent, reflecting the reality that overnight generation is still zero and winter generation is low.

The year-one saving is the self-consumed share priced at the electricity unit rate, plus the exported share at the Smart Export Guarantee rate, minus an annual maintenance cost for insurance and cleaning.

Lifetime saving is nominal, not discounted. Electricity prices are inflated 3 per cent per year (deliberately conservative). Panel output is degraded 0.5 per cent per year. An inverter replacement cost is subtracted in year 12. Anything beyond year 25 is ignored.

4. How battery storage is calculated

Battery value in this tool comes from tariff arbitrage: charge at the off-peak rate, use at the peak rate. We model one full cycle per day at 90 per cent round-trip efficiency, across 365 days, on the Intelligent Octopus Go rates. The default battery size is 10 kWh.

Solar-coupled savings are already captured inside the solar self-consumption uplift above. Double-counting is avoided.

We do not currently model battery replacement within the horizon. A lithium battery that does one cycle a day will typically retain 80 per cent usable capacity after 15 years, which is a gradual de-rate rather than a cliff. An annual 2 per cent capacity decline is applied to future savings.

5. How heat pumps are calculated

The starting point is your home's annual delivered heat demand. We use BEIS averages by home type for an average-insulation home. Poor insulation is a 25 per cent uplift, good is a 20 per cent haircut. If you have provided your actual annual heating bill, we use that instead of estimating.

Current heating cost is converted from delivered heat: gas is divided by a realistic 88 per cent boiler efficiency. Oil uses 10 kWh per litre as the calorific value; LPG uses 7 kWh per litre. Electric-resistance heating is treated as 1-to-1 (no efficiency conversion).

Heat pump running cost is the heat demand divided by the seasonal coefficient of performance (SCOP), times the electricity unit rate, plus an annual service and maintenance cost. We use a SCOP of 3.2, which is the MCS fleet average for air-source heat pumps installed and commissioned in UK homes. Actual performance varies between 2.5 and 4.0 depending on installer, radiators, flow temperatures, and insulation.

Capital cost depends on home size, from a £9,500 flat install to £16,000 for a large detached with radiator upgrades. The £7,500 Boiler Upgrade Scheme grant is subtracted for properties in England and Wales. In Scotland the Home Energy Scotland grant is the same number, with a £1,500 rural uplift not modelled by default.

The honest result: for a well-insulated home heated with cheap mains gas at current 2026 prices, a heat pump may not save money. It may cost slightly more per year to run. The tool surfaces this rather than hiding it. For oil, LPG, and old electric heating, heat pumps typically save hundreds to thousands of pounds per year.

6. How EV charging is calculated

We use your annual mileage and a typical EV efficiency of 0.306 kWh per mile. Charging cost assumes 90 per cent of kWh is drawn at the Intelligent Octopus Go off-peak rate and 10 per cent at the peak rate, which matches published usage data from Octopus on that tariff. The comparison is against your current car's mpg at the pump price for petrol.

Capital cost is a standard £1,050 for a 7 kW home charger and install, with no grant modelled (the OZEV grant for driveway owners ended in 2022; the remaining grants for flats and renters are covered separately).

What the tool deliberately does not include: the purchase price of the EV itself, depreciation, Vehicle Excise Duty, or insurance differences. These are large, driver-specific numbers. The point of the EV card is to show running-cost savings.

7. Grants modelled

• Boiler Upgrade Scheme: £7,500 off an air-source heat pump in England and Wales. The homeowner does not apply directly. The MCS-certified installer applies on their behalf and discounts the quote.
• Home Energy Scotland grant: £7,500 in Scotland, with a £1,500 rural and island uplift. Available as a grant or as a mix of grant and interest-free loan.
• EV chargepoint grant for flats and rentals: £350. Displayed for context.

Not modelled: ECO4 for low-income households, Home Upgrade Grant for off-gas homes, Warm Homes Plan changes expected later in 2026. The tool gets more cautious, not more generous, as new schemes come in; users should always ask an installer what they qualify for.

8. What is not modelled

• EV purchase price, depreciation, VED, and insurance.
• VAT differences on solar (currently zero-rated through March 2027) are assumed priced in.
• Specific supplier tariff eligibility checks.
• Upfront insulation upgrade costs (fabric first is always smart, but this tool models solar, battery, heat pump, EV).
• Detailed monthly or seasonal cashflow. The 25-year chart is annual and nominal.
• Planning permission or party wall considerations.
• Government policy changes after April 2026 (the next review is scheduled for July 2026 after the Ofgem Q3 cap update).

9. How often the numbers are refreshed

Quarterly for Ofgem price cap inputs. Annually for grants and MCS cost data. Any significant change (a BUS uplift, a cap drop) triggers an out-of-cycle refresh. The footer carries the date of last review on every page.

10. Challenging these numbers

If you spot an error or a number that looks wrong for your situation, email methodology@myelectrichome.co.uk with your working. Corrections go in within a week.

Before you spend money on any of these upgrades, get three quotes from MCS-certified installers. The numbers from this tool are a useful sanity check; they are not a substitute for a site survey.