Solar roof calculator

Multiply the system size in kWp by the yearly yield for your area. A south-facing roof produces about 987 kWh per kWp a year in London, 1,014 in Berlin, 1,549 in Madrid and 1,742 in Phoenix, so a 4 kWp system in London makes roughly 3,900 kWh. The calculator works this out for your location, roof direction, pitch and shade.

In the northern hemisphere, south gives the most energy. South-east and south-west roofs produce about 6% less, east or west about 21% less, and a north-facing roof at 35° about 49% less. Add each face of your roof as its own section to see the difference for your house.

Each east or west panel produces about 21% less than a south-facing one, but an east-west roof lets you use both faces and spreads generation across the morning and evening. Enter the east and west faces as two sections to compare the total with a single south face.

They do produce electricity, though much less: about 49% less than south at a 35° pitch in the cities we checked. The flatter the roof, the smaller that gap, so a shallow north-facing roof can still be worth including.

Pitches between about 30° and 45° give the most energy in Europe and the US. The difference near the top is small: in London a 15° roof yields about 6% less than a 30° roof, and a flat panel about 16% less.

Divide the usable roof area by about 2 m² (21.5 sq ft) per panel, leaving out edges, chimneys and roof windows. The calculator assumes 450 W panels, so 30 m² (about 320 sq ft) of usable roof fits about 15 panels, or 6.75 kWp.

Sunlight and temperature come from PVGIS, the European Commission's photovoltaic tool, using satellite records from 2005 to 2023. PVGIS includes shading by distant hills and mountains. Nearby trees and buildings are not included, so set the shade level for each section yourself.