How much will solar panels save me?

Step 1: how much electricity the panels make

MCS installers use a set method from the MCS Solar PV Standard (MIS 3002):

yearly output (kWh) = system size (kWp) × Kk × shade factor

• Kk is the number of kWh each kWp makes in a year. MCS publishes it for each postcode zone, roof pitch and direction, based on European Commission data.
• Shade factor is 1 for a clear horizon, and lower if the panels are shaded.

For a south-facing roof at 35°, Kk ranges from about 736 kWh per kWp in Lerwick to about 1,130 in the Brighton zone. London's zone is 984 and Birmingham's 935. An east- or west-facing roof makes about 80% as much as south at that pitch, and a flat installation about 84%.

Year to year, the weather changes output by a few per cent either way.

Step 2: how much you use yourself

Solar is worth most when you use it as it's made. MCS guidance (MGD 003) gives typical figures. For a home using 2,500–2,999 kWh a year with about 3,600–3,899 kWh of solar:

• No battery: 14% (out all day) to 23% (home all day) of the solar is used in the home
• About 5 kWh of usable battery: 48% to 54%
• About 10 kWh of usable battery: 58% to 60%

MCS installers must give you a written estimate of generation and self-use before you sign, with a warning that the figures aren't guaranteed.

Step 3: what each unit is worth

• Units you use: each saves you your electricity unit rate. Ofgem's price cap average for 1 October to 31 December 2026 is 26.32p per kWh (Direct Debit, single rate). Rates differ by region and tariff, and the cap changes every three months.
• Units you export: each earns your export rate under the Smart Export Guarantee. On 9 October 2026, examples ranged from 3p to 13p per kWh for widely available tariffs.

A worked example

This is an illustration using the figures above, not a prediction:

• 4 kWp, south-facing at 35°, in the Birmingham zone: 4 × 935 = 3,740 kWh a year
• Someone home half the day, 2,500 kWh used a year, no battery: about 19% used = 711 kWh, worth about £187 at 26.32p
• Exported: 3,029 kWh, worth about £363 at 12p
• Total about £550 a year. At a cost of about £6,380, that's a simple payback of about 11.6 years.

That's in line with the Energy Saving Trust's estimate of 9–12 years for a typical system.

Things that change the answer

• Panel output falls slowly over time — a research review found a median of about 0.5% a year.
• Inverters usually need replacing after about 12 years.
• Prices for electricity and export change.
• VAT on household electricity in Great Britain is 0% until 31 March 2027, which slightly lowers the value of each unit you use yourself.

Work out your own

Our savings estimator uses these figures, finds your MCS zone from your postcode and shows every step of the sum.

Sources

1. MCS: MIS 3002 The Solar PV Standard (Installation) issue 5.0
2. MCS: Irradiance Datasets (kWh/kWp by postcode zone)
3. MCS: MGD 003 Solar PV self-consumption guidance
4. Ofgem: Energy price cap unit rates and standing charges
5. Octopus Energy: Outgoing Octopus export tariffs
6. E.ON Next: Smart Export Guarantee tariffs
7. EDF: Smart Export Guarantee tariffs
8. Energy Saving Trust: Solar panels
9. NREL/OSTI: Photovoltaic Degradation Rates — An Analytical Review
10. HMRC Brief 10 (2026): Temporary zero rate of VAT for domestic electricity in GB

This guide is general information, not financial or legal advice. Rules and prices change, and some differ between England, Wales, Scotland and Northern Ireland — check the sources for your situation.