- The rate at which solar energy arrives at the Earth, equal to about 13.5 Watts per square metre in space just above the Earth’s atmosphere. The Solar Constant is not constant, and in recent years there have been many attempts to discover whether its day-to-day and year-to-year variation are accompanied by long-term variations which could have important effects on the evolution of life on Earth, modern-day climate change and the incidence of ice ages.
- The average rate at which radiation energy from the sun is received at the surface of the earth, equal to 430 Btus per hour per square foot (1.94 cal. per min. per sq. cm.).
- The rate at which solar radiation is received just outside of the earth's atmosphere. It is calculated based on a surface perpendicular to the incident radiation at the earth's mean distance from the sun. This value is not constant, and is equal to approximately 1,370 watts per square meter.