Solar Panel System Sizing Calculator

A commercial solar decision starts with a single number: the kWp array you can realistically install against your roof or land, set against the kWh you actually consume. The default mistake is to size the array to annual consumption divided by 950 kWh/kWp and call it done. UK irradiance is lower than that headline in the North, the load is rarely coincident with generation in a manufacturing setting, and a 1 MWp on paper might only displace 60% to 70% of daytime demand once cloud cover, weekend patterns and shading are layered in. Better to start with monitored half-hourly demand, then layer the irradiance curve.

The other commercial input is your purchase route. A landlord-funded array with a PPA shifts the question from capex payback to the strike rate negotiated against your wholesale import price, which means the array sizing has a direct line into your supply contract review. A self-funded array with a REGO-backed supply contract is a different conversation again, because the carbon and renewable claims need to be reconciled to avoid double-counting between your on-site MWh and your supplier's REGO certificates. Sizing decisions that ignore the contract structure routinely create downstream SECR and reporting headaches.

Purely Energy does not install solar, but we work with the contract side of every major UK supplier and we run Purely Insights monitoring across thousands of sites. The clients who consistently extract the strongest IRR from an on-site array are the ones who size against monitored consumption data, not nameplate billing data, and who plan the half-hourly profile match before signing the EPC contract. Use this calculator as the first-pass sanity check, then have a competent solar designer model irradiance, shading and inverter losses against verified consumption before committing.