Insolation is the incident solar radiation onto some object. Specifically, it is a measure of the solar energy that is incident on a specified area over a set period of time. Generally insolation is expressed in kilowatt-hours per square meter per day.
Not all of the solar energy that reaches the Earth actually reaches the surface of the Earth. Although 1367 W/m2 of sunlight strikes the outer atmosphere, about 30% of it is reflected back into space. After this reflection, a certain spot on the Earth might see almost all or almost none of this sunlight. There are many factors that help determine how much sunlight actually reaches a given area, but some of them include sun angle, air mass, day length, cloud coverage, and pollution levels.
When solar radiation hits an object, some of the energy will be absorbed while the rest is reflected. Generally the absorbed solar radiation is converted to thermal energy, which causes the object to heat up. In some cases, however, the incident energy can be absorbed and converted into another form of energy. This is the case in photovoltaic cells used on solar panels. Thus understanding insolation (the amount of energy that hits an area) is important in maximizing the output of solar panels which absorb and convert this energy. Factors such as the geographic location of solar panels and altitude they should be placed at for maximum output can be determined by understanding insolation values.
It is important to have values for insolation at certain positions on the Earth as these figures are used to help determine the size of solar power systems. Values for insolation can help to determine the expected output for solar panels and determine where on Earth solar panels would be most effective.
As well, insolation is an important consideration in construction. When constructing a building in a particular climate, it is important to understand what the temperature and sun levels will be like to ensure maximum comfort and energy efficiency. Understanding insolation can help to build buildings that are cool in the summer and warm in the winter by installing large windows on the side of the building that faces the equator. Doing this maximizes insolation in the winter while minimizing it in the summer.
Patterns of insolation also help to explain why the polar regions of the Earth are colder than equatorial regions. Figure 1 shows a map indicating where solar insolation is at its highest and lowest. Understandably, the sun shines less directly for less time in colder polar regions, and more in hotter equatorial regions. This helps to partially describe why climates vary around the globe.