Estimates indicative annual generation, bill savings and carbon displaced for a rooftop PV array, generation E = kWp × reference yield × performance ratio, split into self-consumed and exported energy.
Indicative only. Verify final design to MCS MIS 3002 and a site-specific yield assessment.
For design guidance only. Always verify with a qualified MCS installer.
About this solar PV generation calculator
This free solar PV generation and savings calculator gives an indicative figure for how much electricity a rooftop photovoltaic array will produce in a year, what it could save on energy bills, and how much carbon it displaces. From the installed capacity in kilowatts-peak (kWp), the local reference yield and a performance ratio, it works out annual generation in kWh, splits that into self-consumed and exported energy, and prices each at the import and export tariff. It is aimed at UK building-services engineers, MCS installers and homeowners scoping a domestic or small commercial PV system at the feasibility stage.
How solar PV generation is calculated
Annual generation follows the standard yield equation E = kWp × Yref × PR, the same form as the MCS MIS 3002 Standard Estimation Method (Annual kWh = kWp × Kk × shading factor) reframed per IEC 61724-1 with an explicit performance ratio. The reference yield (Yref, in kWh/kWp) is numerically the plane-of-array annual irradiation; the performance ratio (PR, typically 0.75–0.85) accounts for temperature, soiling, inverter, wiring and mismatch losses, giving the final specific yield Yf = Yref × PR. Generation is divided into self-consumed energy (used on-site, saving the grid import unit rate) and exported energy (paid at the Smart Export Guarantee rate). Carbon displaced is generation × the DESNZ/Defra grid ‘electricity generated’ conversion factor.
Frequently asked questions
What reference yield should I enter?
Enter the plane-of-array annual irradiation for the location, orientation and tilt, with any shading folded in, around 1,000–1,150 kWh/kWp for a well-oriented UK roof. Do not enter an already-derated ‘specific yield’ or MCS Kk value (~950), because the performance ratio would then double-count losses.
What self-consumption fraction is realistic?
For a UK home with no battery, roughly 0.25–0.35 of generation is used on-site; the rest is exported. Adding a battery or a diverter typically raises self-consumption to 0.5–0.7, which increases savings because the import tariff is usually well above the export rate.
How accurate is the savings figure?
It is an indicative feasibility-stage estimate. The generation method follows MCS MIS 3002 and IEC 61724-1, but the result depends on the tariffs and self-consumption you enter. For a firm figure, use a site-specific PVGIS or MCS yield assessment and your actual supplier tariffs.