Solar panel calculator
See what panels on your roof could generate and save, how long they take to pay back and what a battery adds. Change the main assumptions and the result updates.
This calculator estimates what solar panels on your roof would generate, how much you would use and export, what that is worth each year and how long the system takes to pay for itself, with or without a battery. The main assumptions appear next to the result, and you can change most of them. It is an estimate to help you plan, not a quote or an MCS performance estimate.
The calculator
My yearly electricity use is
Your yearly electricity use helps us size your solar panel system.
What the calculator asks you
Your address. We use it to find your roof with Google's Solar API and to estimate generation there. The calculator covers England, Scotland and Wales; Northern Ireland, the Isle of Man and the Channel Islands (BT, IM, JE and GY postcodes) have different prices and no Smart Export Guarantee.
Your electricity use. The yearly kWh on your bill or smart meter app is most accurate. A monthly bill works too, if it is for electricity only: we take off the standing charge (54.83p a day) and divide the rest by 26.32p/kWh.
Is someone usually at home during the day?
Solar power is worth most when you use it as it's generated. The choices follow the MCS self-consumption guide that installers use:
- Home all day: someone is usually at home between 9am and 5pm on weekdays.
- In half the day: the home is usually empty for half the day on weekdays, such as all morning or all afternoon.
- Out all day: the home is usually empty on weekdays.
Unanswered, it uses "in half the day", the MCS setting for an unknown routine. For a London home using 2,500 kWh a year with 4 kWp of panels, the share of solar power used at home comes out at 21%, 17% and 12%.
Which electricity tariff are you on?
- Standard (single rate): the same price for every unit, whatever the time of day.
- Overnight / EV tariff: cheap hours at night. The starting rates are Intelligent Octopus Go 12M Fixed's in London on 23 September 2026 (8p/kWh from 23:30 to 05:30, 35.48p/kWh otherwise, without VAT), which need an electric car or compatible charger and Octopus as your supplier. For Economy 7, enter your own day and night rates. A battery is charged from the grid overnight, up to what the home can use from it the next day; a car's charging counts as ordinary night-time use, not smart charging.
- Import-export time-of-use: for example Octopus Flux, which needs a home battery and Octopus for both import and export. The calculator uses Flux's London import and export rates on 23 September 2026 and charges the battery from the grid between 02:00 and 05:00 when that is cheaper.
- Not sure: the calculator uses a standard single-rate tariff.
With a time-of-use tariff, the result shows what solar and a battery save with this tariff, compared with the same tariff without them. It doesn't show whether switching tariff saves money: a higher day rate can make your bill dearer before any solar.
A battery. Add one to see what it changes. Batteries show advertised and usable capacity, for example "10.3 kWh (9.7 kWh usable)": the price follows the first, the estimate the second, with the battery's own power and efficiency. Our battery storage guide explains when a battery pays.
You can change your answers and the rates on the results page, under "Your assumptions".
How the estimate is worked out
- Generation comes from PVGIS, the European Commission's solar model, for your location, roof direction and pitch.
- Use at home comes from an hourly model of your demand, matched to the MCS self-consumption tables for your daytime pattern.
- Value: each kWh you use saves that hour's unit rate and each kWh exported earns your export rate. The standing charge is never counted as a saving.
- Payback adds up each year's value, with output falling 0.4% a year, until it covers the installed price. The projection runs for 25 years, before any repairs or a replacement inverter; check the inverter's warranty.
Three results side by side
The result shows a headline and two other scenarios:
- Headline: exports valued at 12p/kWh, Octopus Outgoing's rate: needs Octopus as your electricity supplier on a compatible tariff. Prices are today's.
- Keep your current supplier: 6p/kWh from E.ON Next Flex Export, which is open to customers of any supplier.
- If electricity prices rise 2% a year: in line with the Bank of England's inflation target, not a forecast. Export rates stay flat.
With a Flux-type tariff the 6p line is left out, because the tariff sets its own export rates; the 2% line still shows. The 12p rate needs you to switch your electricity supply to Octopus: see how switching works and what it costs and the best export tariffs.
For example, a London home using 2,500 kWh a year, in for half the day, with 4 kWp of panels at £7,100 in our calculator's price model (not a quote):
| Octopus, 12p, today's prices | Keep your supplier, 6p, today's prices | Octopus, 12p, prices up 2% a year | |
|---|---|---|---|
| First-year value | £589 | £387 | £589 |
| Payback | 12.3 years | 19.0 years | 11.9 years |
The defaults are:
| Assumption | Default | You can change it |
|---|---|---|
| Electricity price | 26.32p/kWh, Ofgem cap average for Direct Debit, 1 October to 31 December 2026, no VAT | Yes: use the unit rate on your bill |
| Export rate | Headline 12p/kWh (Octopus Outgoing, needs Octopus as your supplier); 6p/kWh line for keeping your supplier (E.ON Next Flex Export, variable); both checked 23 September 2026 | Yes |
| Electricity price rise | 0% a year (today's prices); 2% a year shown beside it | Yes |
| Daytime at home | In half the day | Yes |
| Tariff | Standard single rate | Yes |
| Installed price | Our price model including 0% VAT, checked against DESNZ solar PV cost data for domestic retrofit installations in 2025/26 (mean cost per kW of panels); for 4 kWp it sits between the two size-band averages. Not a quote | Changes with the panels, inverter and battery you choose |
Installed solar panels and batteries have 0% VAT until 31 March 2027; the rate is then due to be 5% unless the relief is extended. The export rate is kept flat because suppliers set and change it themselves.
What the estimate does not include
Your result is an illustrative estimate based on the assumptions shown with it. It is not an MCS performance estimate or a quote. Before you sign a contract, an MCS-certified installer must give you a written estimate of generation and self-consumption. Their figures and price will differ from ours.
- Extras. Bird protection, optimisers, backup power, a new consumer unit and roof repairs are not in the installed price.
- Grants and loans. Nothing is deducted. The estimate assumes you pay upfront; with a loan, the interest adds to the cost.
- Peak-time battery export. The battery is never discharged to the grid for peak export prices, so a battery run for peak export could earn more. Agile and Cosy tariffs are not modelled.
- Your roof. It cannot check the structure, shading from trees or whether you need planning permission.
- Grid connection. If your inverters, including any battery inverter, add up to more than 3.68 kW on a single-phase supply, your installer must consult the network operator before the installation goes ahead.
Using your result
Ask two or three MCS-certified installers for a survey and quote. The solar panel cost guide shows what a complete quote should include.
Frequently asked questions
How accurate is a solar panel calculator?
It is as accurate as its inputs. PVGIS generation is a long-term average that varies from year to year, and the biggest uncertainties are how much solar power you'll use, your export rate and your installed price. Enter your own usage and tariff to narrow the range.
Why does my result differ from other calculators?
Mostly because of the export rate, the daytime-at-home assumption and the date of the prices. Some calculators assume 12p or more for exports without saying that it usually means buying your electricity from the same supplier; we show 12p with that condition and 6p beside it.
How do solar panels work with your electric bill?
Each kWh your panels produce while the house is using power is a kWh you don't buy, so it saves your unit rate (26.32p/kWh at the Ofgem cap, October to December 2026). The standing charge stays. Exported electricity is paid for separately by your export supplier, from export meter readings. Bills fall most in summer.
Is my result the same as an MCS estimate?
No. MCS installers work out generation with MCS's own irradiance tables and must use MCS self-consumption tables. Our use-at-home estimate is matched to those tables, but generation comes from PVGIS, and with a 5 kWh battery our share of solar used at home averages 4 to 9 points above the MCS table. The installer's written estimate is the one to rely on in a contract.