Irradiance is the rate at which solar energy arrives at a surface. Standard Test Conditions for PV panels use 1,000 W/m², representing a bright clear day at solar noon. In practice, UK horizontal surface irradiance varies from near zero on overcast winter days to approximately 900 to 950 W/m² on clear summer days. The annual sum of irradiance over time gives irradiation, which drives total energy yield.
Irradiance data for specific UK sites is obtained from meteorological databases (PVGIS, Meteonorm, Solargis) and used in solar PV system design and energy yield modelling. Shading objects on or near the roof, including parapet walls, plant, and adjacent buildings, reduce effective irradiance on affected panels, which structural engineers and solar designers account for when determining optimal array layout.