Irradiation (insolation)
Solar energy received per unit area over a period, usually expressed in kWh/m² per day or per year; the time integral of irradiance.
Irradiation, also called insolation or solar resource, is the quantity that determines how much energy a solar plant can produce. It is the integral of irradiance over time, typically reported as daily averages (kWh/m²/day) or annual totals (kWh/m²/year).
Typical annual global horizontal irradiation (GHI), from the World Bank Global Solar Atlas (Solargis):
- UK, Benelux, northern Germany: about 900–1,100 kWh/m²
- Southern Germany, northern France: 1,100–1,250 kWh/m²
- Southern Spain, Italy, California coast: 1,700–1,950 kWh/m²
- Middle East, Sahara, Australian interior: 2,100–2,500 kWh/m²
- Atacama Desert, Chile: up to about 2,700 kWh/m², the highest on Earth
Irradiation on a tilted plane facing the equator at the optimum angle is usually 10–20% higher than GHI at mid-latitudes. Dividing daily irradiation in kWh/m² by 1 kW/m² gives peak sun hours, and multiplying annual plane-of-array irradiation by the performance ratio gives the specific yield in kWh/kWp.
Year-to-year variability of annual irradiation is typically ±4–6%, so bankable studies use long-term (10–20 year) satellite datasets.
Related terms
All terms →- Irradiance
The instantaneous solar power received per unit area, measured in watts per square metre (W/m²).
- GHI (Global Horizontal Irradiance)
Total shortwave solar radiation received on a horizontal surface, the sum of direct beam and diffuse sky components.
- Peak sun hours
Daily solar irradiation expressed as the equivalent number of hours at 1,000 W/m²; numerically equal to kWh/m² per day.
- Specific yield
Annual energy produced per unit of installed PV capacity, expressed in kWh/kWp per year; combines solar resource and system performance.
- Performance ratio
Ratio of actual AC energy output to the theoretical output implied by plane-of-array irradiation and nameplate capacity; a location-independent quality metric for PV plants.