Calculator methodology

Overview

The calculator estimates how much electricity a small plug-in solar system could generate at your postcode, and then separates that generation from the part that actually reduces your bill. Nothing you enter is stored.

1. Geocoding the postcode

Your postcode is converted to a location using Postcodes.io, which returns the latitude, longitude and region. We display the matched area so you can confirm it found the right place.

2. Modelling generation

We request modelled output from the European Commission's PVGIS service for your system size, orientation and tilt. PVGIS uses long-run satellite irradiance data, which makes it a reliable basis for an annual and monthly estimate. We then apply a shading factor based on your selection.

If PVGIS is unavailable, we fall back to a regional model based on typical UK specific yields (kWh per kWp per year) and a representative monthly profile, adjusted for orientation, tilt and shading. The result clearly states whether it used PVGIS or the fallback model.

3. From generation to savings

Generation is not the same as savings. A small plug-in system mostly offsets daytime background use. We estimate the share you use at home (self-consumption) from your daytime-use profile, and only that share reduces your bill — valued at the electricity unit rate you enter. Electricity you do not use is treated as export.

4. Export income

Export income defaults to zero, because eligibility and supplier treatment for self-installed plug-in systems is not settled. You can enter a confirmed export rate manually if you have one. See our Smart Export Guarantee guide.

5. Payback

Indicative payback is the system cost you enter divided by the estimated annual benefit (bill saving plus any export income). If the annual benefit is zero, no payback is shown.

Assumptions & limitations

• Figures are directional estimates for planning, not guarantees.
• Real output varies with weather, equipment quality, soiling and exact placement.
• A standard system loss factor is applied to generation.
• Self-consumption is modelled from a simple daytime-use profile, not from your actual half-hourly usage.
• Energy prices change; the saving reflects the rate you enter.

The underlying calculation functions are unit-tested. See also our sources.

The calculator and its methodology are overseen by Christopher Panteli, founder and editor of MyPlugInSolar.