Solar insolation is a measure of the amount of solar energy reaching a surface, or irradiance, on a given surface over time. The surface in question can be anything exposed to sunlight, from a particular object or location on Earth to small space-going objects such as artificial satellites to an entire planet's surface. The solar insolation of a particular area of the Earth depends on its distance from the equator, its weather conditions, and the time of day and year. It is essential to the continued existence of life on Earth, as plants rely on energy from the sun to survive, as well as being an important factor in the construction and location of equipment to generate electricity from solar power.
Usually measured in watts per square meter, the average solar insolation of an area over longer periods of time is often given as kilowatt hours per square meter per day. The watt is the standard metric unit of power, or energy over time; one watt of power is equal to one joule of energy per second. A kilowatt hour, a term most commonly used in reference to electric power generation, is enough energy to produce an output of 1,000 watts for one hour, or 3,600,000 joules (3.6 megajoules).
The more directly a surface faces the sun, the higher its solar insolation will be. Maximum solar insolation is produced when the sun's light strikes at a 90-degree angle. Insolation decreases as the angle becomes lower, because a lower angle spreads the same amount of radiant energy over a wider area. This is why the area around the Earth's equator, which receives the most direct sunlight, is the warmest part of the Earth and the polar regions are the coldest. It also causes the changing seasons, because the Earth's tilted axis means that the angle of the sunlight reaching a given part of the planet changes in the course of the year. This is also why the temperature on a given day will tend to peak around solar noon, when the sun is at its highest point in the sky, and then decrease as the sun drops closer to the horizon later in the day.
The total solar insolation of the Earth's outer atmosphere from direct sunlight averages about 1,366 watts per square meter at an angle of 90 degrees over the course of a year, the majority of which is in the form of visible light. Attenuation of the sunlight as it passes through the atmosphere reduces this to about 1,000 watts per square meter at an angle of 90 degrees by the time it reaches the Earth's surface. This figure steadily drops as a person moves to higher latitudes and decreases at times of day further from solar noon, dropping to almost nothing at night. The average insolation of the Earth as a whole over the course of a year is around 250 watts per square meter.
Areas at similar latitudes can still have significant differences in average insolation due to local factors. An area's insolation can be further decreased by atmospheric conditions that interfere with sunlight, such as clouds or atmospheric haze. Insolation rises at higher altitudes, because there is less atmosphere for the solar radiation to pass through and be attenuated by. Measurements of the amount of solar irradiance at different locations can be compiled to create a specialized map called an insolation map.
Solar power generation relies heavily on insolation. Arid or semiarid regions are commonly home to solar power stations to minimize interference with solar radiation caused by cloud cover and are built at higher altitudes if possible. Photovoltaic solar panels are mounted at angles intended to make incoming sunlight strike them at as close to a 90-degree angle as possible to maximize the power received. The optimal angle for this varies according to the geographic location and the time of year.
The insolation of an area can also be exploited in the design of buildings. For example, large windows on the side of a building facing the equator will let in more light and heat during the winter, when the sun is low in the sky, and comparatively less when it is high in the sky during the summer. This moderates seasonal temperature extremes inside the building, making it more comfortable and reducing the amount of energy needed for heating or air conditioning.