 Solar irradiance. Solar irradiance is the power per unit area received from the Sun in the form of electromagnetic radiation in the wavelength range of the measuring instrument. The solar irradiance integrated over time is called solar irradiation, insulation, or solar exposure. However, insulation is often used interchangeably with irradiance in practice. Irradiance may be measured in space or at the Earth's surface after atmospheric absorption and scattering. Irradiance in space is of function of distance from the Sun, the solar cycle, and cross-cycle changes. Irradiance on the Earth's surface additionally depends on the tilt of the measuring surface, the height of the Sun above the horizon, and atmospheric conditions. Solar irradiance affects plant metabolism and animal behavior.