How SolarSums works

SolarSums gives you an estimate of whether solar panels, a battery or smart EV charging would pay for themselves at your home. It's an illustration, not a quote, and this page explains exactly how we get the numbers.

Where you live

Your postcode is turned into a location using postcodes.io, a free service built on Ordnance Survey and ONS data. We use that location to fetch sun data and to find your electricity region.

How much power your panels make

We ask PVGIS, the European Commission's free solar model, how much a system of your size would generate in a year at your location, roof direction and pitch. We use PVGIS's standard 14% allowance for system losses, such as wiring, inverter and dirt. We then assume panel output falls by 0.5% a year as panels age.

What the power is worth

We look up live unit rates from Octopus Energy's public price list for your region: a day rate and a night rate on a time-of-use tariff, and an export rate for power you send to the grid. You can change any of these to match your own tariff. We assume your household uses 35% of its electricity in the day, 45% in the evening and 20% overnight. Each year, the calculator works out how much of your solar you use yourself, how much a battery can store and use later, how much goes to an electric car, and how much is exported.

A battery is modelled at 90% round-trip efficiency, and it can charge cheaply overnight from the grid as well as from your panels.

What it costs

The default prices are £2,109 per kWp for solar panels, £920 per kWh of battery and £1,000 for an installed EV charger. These are starting points, not quotes, so replace them with the figures on your own quotes.

Payback and the 20-year view

Payback is the first year your running total of savings covers what you paid. We look 20 years ahead. Batteries typically last 10 to 12 years (Energy Saving Trust), so a battery replacement in year 12 is included in the battery figures. If a system doesn't pay back within 20 years, we tell you plainly.

What we leave out

We don't model shading from trees or buildings, future changes in energy prices, finance costs or maintenance. Real savings depend on how and when you use electricity, so treat every figure as an illustration.

Sources

PVGIS (European Commission Joint Research Centre); postcodes.io; Octopus Energy public API; Energy Saving Trust; HMRC VAT Notice 708/6; Ofgem, Smart Export Guarantee.

Illustration and general information only, not financial advice.