Solar irradiance is a measure of the irradiance (power per unit area on the Earth's surface) produced by the Sun in the form of electromagnetic radiation, which is perceived by humans as sunlight. Solar irradiance in a specific area may be measured as insolation, the solar radiation energy per unit area during a given time, or as direct insolation, insolation which reaches a location on Earth after absorption and scattering in the atmosphere. Total solar irradiance (TSI), is a measure of the solar radiative power per unit area normal to the rays, incident on the Earth's upper atmosphere. The Solar constant is a conventional measure of mean TSI at a distance of one Astronomical Unit (AU).
Although the energy output of the Sun is fairly constant, solar irradiance "varies significantly from one place to another and changes throughout the year". Sunlight only reaches the parts of the Earth that are facing the Sun at any given time, and the most intense irradiance is experienced by those parts that are not at an angle to the Sun as it passes over. The calculation of solar irradiance for a given area is useful for planning to obtain solar power. Solar activity and irradiance measurement is a concern of space agencies. For example, the American space agency, NASA, has launched its Solar Radiation and Climate Experiment (SORCE) satellite with Solar Irradiance Monitors.
- Michael Boxwell, Solar Electricity Handbook: A Simple, Practical Guide to Solar Energy (2012), p. 41-42.