Solar, battery and EV payback calculator

Your home Postcode Property type Annual electricity use (kWh) Default: Ofgem medium typical domestic consumption value, 2,500 kWh, in use from 1 July 2026 (Ofgem TDCV decision, May 2026). Use your own bill figure if you can.

Roof and system Roof direction Roof pitch (degrees) Assumption, enter your roof's pitch. System size (kWp) Assumption, change to your quote. Roof type Mounted on the roof uses the PVGIS "free-standing" setting; built into the roof uses the PVGIS "building-integrated" setting. Battery size (kWh, 0 for none) Assumption, change to your quote.

EV miles per year

Efficiency (miles per kWh) Assumption, check your car's figure.

Share of EV charging possible at home in daytime (%) Assumption. Solar only charges the car when export pays less than the off-peak rate.

Tariffs (fetched for your region, editable) Time-of-use tariff (used in every scenario) Day / peak rate (p/kWh) Example value until your postcode is looked up. Night / off-peak rate (p/kWh) Example value until your postcode is looked up. Export rate (p/kWh) Example value until your postcode is looked up. Flexible Octopus single rate, reference only (p/kWh) Example value until your postcode is looked up. Only used for the separate tariff switch line.

Costs (editable defaults, before VAT) Solar install cost per kWp (£) Default: DESNZ Solar PV cost data, TIC basis, 2025/26 (Solar_Costs_2025-26__TIC_.xlsx, published 28 May 2026): mean £2,108.65 per kW for 0 to 4 kW domestic retrofit installs (panels added to existing homes). The median for all 0 to 4 kW installs, including new builds and non-domestic, is lower at £1,595 per kW. Change to your quote. Battery cost per kWh (£) Default: Energy Saving Trust battery storage advice (checked 13 Oct 2026): a 5 kWh battery system costs around £4,600, about £920 per kWh. Change to your quote. EV charger installed (£, inc VAT) Assumption, change to your quote. Install date

Assumptions

Solar yield from PVGIS 5.3 (EU JRC), 14% system losses, on-roof ("free") mounting unless in-roof is chosen.
Modelling parameters marked as assumptions are our own estimates, not sourced figures: the 35/45/20 demand split, the winter weighting, 90% battery efficiency and one battery cycle a day.
Daily profile: each month's solar and demand are split into an average day. Household demand is 35% daytime, 45% evening and 20% overnight, weighted towards winter across the year.
Daytime solar first covers daytime demand. In the EV scenario, surplus then charges the car, up to the daytime share you set, but only when the export rate is lower than the off-peak rate. Otherwise exporting pays more, so the surplus is exported. Anything left charges the battery, and the rest is exported.
The battery cycles once a day at 90% round-trip efficiency. It is limited by its capacity, the day's surplus and evening demand. Any capacity solar leaves empty is topped up overnight at the off-peak rate, so the battery scenarios assume a time-of-use tariff.
The EV baseline assumes the driver is already on an off-peak tariff. Solar charging is valued at the off-peak rate avoided minus the export income given up. The saving on EV charging from moving off Flexible Octopus to the night rate is shown separately and is not counted in payback. Charger cost and grant are shown separately too.
Every scenario assumes the same time-of-use tariff you select. Its day rate is used for household demand that solar or the battery does not cover, and its night rate for EV charging and overnight battery top-ups. Rates come from the public Octopus Energy API for your region, with the date fetched shown. Octopus Go is the default because Octopus lists it as open to new customers. Intelligent Octopus Go is offered for existing customers only. For simplicity, overnight household demand is also priced at the day rate in every scenario; this cancels out because solar does not affect it. Flexible Octopus appears only in the separate tariff switch line. Other suppliers will differ.
Batteries typically last 10 to 12 years (Energy Saving Trust), so one replacement is included in year 12. A warning appears on any battery scenario whose payback, or the battery's own payback, is longer than 10 years. Battery payback = battery cost divided by the extra annual saving over solar only.
Cashflow is worked out year by year for 20 years. Solar output falls by 0.5% a year (assumption: a typical panel degradation rate, check your panel warranty). Payback is the first year in which cumulative savings reach the upfront cost, shown in whole years.
Battery scenarios include one battery replacement in year 12, at today's battery cost with no price change (assumption). It is taken off the cumulative cashflow, so it affects both payback and the 20-year net. The battery's own payback also includes it.
Prices are flat in real terms. Inverter replacement and maintenance are not included.
VAT: 0% on solar and batteries installed until 31 March 2027, 5% from 1 April 2027.
EV chargepoint grant (OZEV): 75% of cost up to £500 for flat owners and renters with off-street parking, and on-street households through the relevant scheme. Not available to homeowners in houses with a driveway.

Illustrative estimate, not advice. Get several MCS-certified quotes before deciding.

Solar, battery and EV payback estimator

Your scenarios

Your running total of savings over 20 years

Estimated monthly solar output (kWh)

Assumptions