Roof area to system size

Sometimes the question is not “what do I need” but “what will fit”. A dormer, a chimney, a hipped roof or a conservation area restriction can decide the system size before consumption gets a say.

Measure the largest clear rectangle on the plane you mean to use, deduct the clearances listed below, and put the result in. The calculator gives you the capacity that area holds, what it generates at your location and orientation, and what the official statistics say that size costs.

The clearances that decide how much fits

These are the minimum offsets in Approved Document L’s Appendix B, written for new homes in England. They are not binding on an existing house, but they reflect what a mounting system and a competent installer will need:

On flat roofs, further offsets apply to plant, lift overruns and access hatches (1,000 mm), guttering and cable trays (200 mm), and soil vent and rainwater pipes (100 mm). Maintenance access routes need 2,000 mm from an unprotected roof edge, falling to 300 mm where there are railings or a man-safe system.

A mid-terrace loses 750 mm from each party wall, which is why terraces fit less than their plan area suggests — and why the government’s own impact assessment makes the mid-terrace the most expensive dwelling type to bring up to the new build standard.

What we could not establish

Whether your roof structure will take the load. Area is not capacity. See structural surveys.

Shading. Two roofs of identical area can differ enormously in output. Our shading page covers how it is modelled and why a small shadow costs more than its area suggests.

This is information, not advice

Measure carefully, then have someone competent check it. An installer’s survey exists to find the things a tape measure and a web page cannot.