Solar irradiance refers to the amount of solar energy that reaches a given surface on Earth. It is measured in units of watts per square meter (W/m²) and represents the total amount of electromagnetic radiation emitted by the sun, which includes visible light as well as ultraviolet and infrared rays. Solar irradiance can vary depending on factors such as time of day, season, weather conditions, and location on Earth. It is an important measure for understanding how much solar energy is available to power renewable energy systems like photovoltaic panels or to heat water through solar thermal collectors.