Solar Glossary

Solar Irradiance

Solar irradiance is the power of sunlight hitting a surface, measured in watts per square metre (W/m²), and is the primary factor determining how much electricity your solar panels can generate.

Solar Irradiance explained

Solar irradiance measures the intensity of sunlight falling on a surface at a given moment, expressed in watts per square metre (W/m²). The higher the irradiance, the more electricity your solar panels produce.

Standard Test Conditions (STC) assume 1,000 W/m² — the intensity of strong midday sun in a temperate climate. In the UK, peak irradiance on a clear summer day might reach 800–900 W/m², while a cloudy winter day might drop to 50–150 W/m².

Annual solar irradiance by location (approximate):

  • UK average: 950–1,250 kWh/m²/year (South England higher, Scotland lower)
  • Germany: 1,000–1,200 kWh/m²/year
  • Spain/Southern Europe: 1,400–1,900 kWh/m²/year
  • Arizona, USA: 2,000–2,400 kWh/m²/year
  • Australia (QLD): 1,700–2,200 kWh/m²/year

Your roof's tilt angle and azimuth (compass direction) significantly affect how much of this irradiance your panels capture. A south-facing 35° pitch roof in the UK captures roughly 10–15% more irradiance than a flat roof, and up to 30% more than a north-facing roof.

Installers use irradiance data from tools like PVGIS (Europe) or PVWatts (US) to estimate your system's expected annual output before installation.

How this affects your solar decision

Your location's solar irradiance is the single biggest factor in how much electricity your panels will produce. Use the Solar Energy Calculator with your postcode to get an irradiance-adjusted output estimate.

Get a location-adjusted solar output estimate for your roofSolar Energy Calculator

Related terms

Peak Sun HoursPeak sun hours is the number of hours per day when solar irradiance averages 1,000 W/m² — used to estimate how much energy your solar panels will generate.AzimuthAzimuth is the compass direction your solar panels face, measured in degrees — south-facing (180°) is optimal in the northern hemisphere for maximum energy capture.Roof Pitch / Tilt AngleRoof pitch (or tilt angle) is the angle of your roof from horizontal — for solar panels, the optimal tilt in the UK is around 30–40°, which matches the angle of the sun's path across the sky.Solar Panel OutputSolar panel output is the amount of electricity a panel or system produces, measured in watts (W) at peak conditions or kilowatt-hours (kWh) over a time period.
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