Yield and sizing
PV annual yield estimate (MCS method)
- where
- estimated annual generation (kWh/yr)
- array peak power (kWp)
- specific yield from the MCS table (kWh/kWp)
- shade factor
Explain this equation
Estimates how much electricity a PV array will generate in a year using the MCS method. This is the figure you give the customer on the quote.
P_pk is the array size: the module STC ratings added up. K_k is how many kWh each kWp produces in a year for your postcode zone, roof pitch and orientation, read from the MCS table. S_F knocks the total down for shading: 1 with no shade, less one hundredth for each shaded segment on the sun-path diagram.
Generation scales directly with array size and with the site's yield per kWp; shading takes a fraction off.
Sense check: 4.3 kWp on a south-facing roof in the south of England with K_k about 850 and light shading (0.94) gives about 3,400 kWh a year.
Watch out: this is an estimate for a typical year. Real output varies year to year, and the MCS method deliberately keeps the shading allowance simple.
The MCS estimate of a PV system's annual AC output, as quoted to customers: array size times the zone yield figure times the shade factor.
Enter your values to see the result.
Good to know
Kk comes from the MCS look-up table for the site's postcode zone, roof pitch (nearest degree) and azimuth from south (nearest 5°). The table already includes system losses, so do not apply a separate loss factor. SF is 1 for an unshaded array; otherwise plot the horizon on the MCS sun-path diagram, count the shaded segments and take SF = 1 − 0.01 × segments. Any object within 10 m of the array gets a shade circle. For several roof faces, run this once per array and add the results. PVGIS is a useful cross-check.
Common mistake
Entering the inverter AC rating instead of the sum of the module STC ratings, or applying an extra 0.8 loss factor on top of a Kk value that already includes it.
Limits
A first-year estimate of average annual output. It is not a guarantee, not a self-consumption figure and not a bill saving. Panel degradation, unusual weather and soiling are not modelled. The full procedure is in MCS MIS 3002 and its supporting guidance.
Sources
- MCS solar PV performance estimate as submitted with a planning application (Camden): estimated annual output = kWp × Kk × SF; Kk from the MCS table; SF = 1 − 0.01 × shaded segments; shade circles for objects within 10 m
- SolarEdge: MCS shade procedure (each affected sun-path segment reduces SF by 0.01; SF = 1 − 0.01 × segments)
- MCS MIS 3002:2025 Solar PV installation standard, Issue 1.0 (3.6.8: 1% voltage drop practicable for domestic AC cabling, up to 3% for larger systems; 3.9.6: no modules within 400 mm of any edge of a domestic roof unless extra measures are taken; BS 7671 takes precedence)
- EU Joint Research Centre: PVGIS grid-connected PV tool (annual yield by tilt and azimuth, optimum-angle option)
Results are calculations from the published methods and figures shown, and depend on the data you enter. Follow MCS standards, manufacturer data, BS 7671, the ENA connection rules and Building Regulations. System design and DNO applications need a competent, certified installer.