How much do solar panels for flat roofs cost?

As a rule of thumb, a flat roof needs about 8 to 10 square metres per kilowatt-peak of solar, compared with roughly 5 to 6 square metres on a pitched roof. The difference is the inter-row spacing: on a flat roof the tilted rows have to be spaced apart so they do not shade each other through the low winter sun, which uses more area per panel. So a 250 kWp commercial system needs somewhere in the region of 2,000 to 2,500 square metres of usable flat roof, allowing for plant, rooflights, access walkways and the perimeter set-back. A survey measures the genuinely usable area around obstructions rather than assuming the whole footprint is available.

As an indicative guide, a commercial rooftop solar system is typically in the region of £600 to £900 per installed kilowatt-peak, with smaller systems costing more per kWp and large roofs achieving a lower rate through economy of scale, so a 250 kWp array might fall somewhere around £150,000 to £220,000. The payback is driven by self-consumption — the more of the generation you use on site rather than export, the faster it pays — and for a commercial system with a strong daytime load it typically lands in the region of five to eight years, after which the system generates largely free power for the balance of a 25-year-plus panel life. We build the payback from your actual half-hourly consumption data rather than a rule of thumb.