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Solar irradiance measures the amount of solar energy reaching a surface, with the Earth’s distance from the equator, meteorological conditions, and time of day/year affecting it. It is essential for life and solar energy production. Insolation decreases as the angle gets lower, causing seasonal changes and temperature fluctuations. Solar energy production and building design can exploit insolation.
Solar irradiance 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 place on Earth to small space objects such as artificial satellites to the surface of an entire planet. The solar irradiance of a particular area of the Earth depends on its distance from the equator, its meteorological conditions and the time of day and year. It is essential for the continued existence of life on Earth, as plants rely on the sun’s energy to survive, as well as being a major factor in the construction and location of equipment to generate electricity from solar energy.
Usually measured in watts per square meter, an area’s average solar irradiance over longer periods of time is often expressed in kilowatt-hours per square meter per day. The watt is the standard metric unit of power, or energy over time; one watt of power equals one joule of energy per second. A kilowatt-hour, a term more commonly used to refer to electricity generation, is energy sufficient to produce 1,000 watts of power for one hour, or 3,600,000 joules (3.6 megajoules).
The more directly a surface faces the sun, the greater its solar radiation. Maximum solar insolation is produced when sunlight strikes at a 90 degree angle. Insolation decreases as the angle gets lower, because a lower angle spreads the same amount of radiant energy over a larger area. This is why the area around the Earth’s equator, which receives the most direct sunlight, is the hottest part of the Earth, and the polar regions are the coldest. It also causes the change of seasons, because the Earth’s tilted axis means that the angle of sunlight reaching a given part of the planet changes throughout the year. This is also why the temperature on any 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 approaches the horizon later in the day. .
The total solar insolation of Earth’s outer atmosphere from direct sunlight averages about 1.366 watts per square meter at a 90-degree angle over the course of a year, most of which is in the form of visible light. The dimming of sunlight passing through the atmosphere reduces it to about 1,000 watts per square meter at a 90-degree angle by the time it reaches the earth’s surface. This figure decreases steadily as a person moves to higher latitudes and decreases during the hours of the day furthest from solar noon, dropping to nearly zero at night. The average insolation of the Earth as a whole over the course of a year is about 250 watts per square meter.
Areas at similar latitudes may still have significant differences in average insolation due to local factors. Insolation of an area can be further reduced by atmospheric conditions that interfere with sunlight, such as clouds or atmospheric haze. Insolation increases at higher altitudes, because there is less atmosphere for solar radiation to pass through and be attenuated. Measurements of the amount of solar radiation at different locations can be compiled to create a specialized map called an insolation map.
Solar energy production is highly dependent on insolation. Arid or semi-arid regions are commonly home to solar power plants to minimize interference with solar radiation from cloud cover, and are built at higher elevations if possible. Photovoltaic solar panels are mounted at angles intended for incoming sunlight to strike them as close to a 90 degree angle as possible to maximize the power received. The optimum angle for this varies depending on geographic location and time of year.
The insolation of an area can also be exploited in building design. 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 relatively less when it is high in the sky during the summer. This moderates seasonal temperature extremes within the building, making it more comfortable and reducing the amount of energy needed for heating or air conditioning.
