Instantaneous solar radiation received on a surface: watts per square metre (W/m2)
1 W/m2 = 0.1 mW/cm2
Integrated (or cumulative) solar radiation over a period of 24 h (= energy): watts x hour or kilowatts x hour per square metre per day (Wh/m2/day) or (kWh/m2/day)
Appendix 1 343
Example: Average cumulative solar radiation per day in Paris in December:
1.12 kWh/m2/day
Other units of energy: langley (Ly), joule per square centimetre (J/cm2) or kilocalories per square metre (kcal/m2)
1 J/cm2 = 2.9 kcal/m2 = 2.78 Wh/m2
1 Ly = 1 cal/cm2 = 11.62 Wh/m2
