Yield and sizing
Tilt and orientation loss (Kk comparison)
- where
- loss compared with the best orientation (%)
- K_k at the actual pitch and orientation (kWh/kWp)
- K_k at the best pitch and orientation (kWh/kWp)
- annual generation lost (kWh/yr)
- array peak power (kWp)
Explain this equation
Shows how much generation you lose by mounting panels at the roof's actual pitch and direction rather than the best for that location. Useful when a customer has an east-west or shallow roof.
K_o is the best specific yield in the MCS table for the zone, usually around south at 30 to 40°. K_a is the figure for the real roof. Their ratio is the fraction you keep; one minus that, times 100, is the percentage loss. The kWh lost each year is the array size times the difference in yield.
Yields per kWp compare directly, because everything else is the same array.
Sense check: 850 against 1000 is a 15% loss; on a 4 kWp array that is 600 kWh a year.
Watch out: a loss in percentage terms may still be worth it. East-west arrays spread output over the day, which can help self-consumption.
Shows how much annual output a roof's pitch and direction cost compared with the best case, using two Kk values from the MCS table or two PVGIS runs.
Enter your values to see the result.
Good to know
Read both Kk values from the same MCS postcode-zone table: one for the actual pitch and azimuth, one for the best combination (in the UK this is roughly due south at about 30–40°). PVGIS gives the same comparison with its optimum-angle option. East–west roofs typically lose around 15–20% against south, but spread generation across the day, which can help self-consumption.
Common mistake
Comparing a Kk from one postcode zone with a value from another, or mixing MCS and PVGIS figures.
Limits
Annual energy only; it says nothing about the time of day output arrives. Shading is handled separately by the shade factor.
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
- 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.