Solar insolation is a measure of solar radiation energy received on a given surface area in a given time. It is commonly expressed as average irradiance in watts per square meter (W/m2) or kilowatt-hours per square meter per day (kW•h/(m2•day)) (or hours/day). In the case of photovoltaics it is commonly measured as kWh/(kWp•y) (kilowatt hours per year per kilowatt peak rating).
The object or surface that solar radiation strikes may be a planet, a terrestrial object inside the atmosphere of a planet, or any object exposed to solar rays outside of an atmosphere, including a spacecraft. Some of the solar radiation will be absorbed, while the remainder will be reflected. Usually the absorbed solar radiation is converted to thermal energy, causing an increasing in the object’s temperature. Some systems, however, may store or convert a portion of the solar energy into another form of energy, as in the case of photovoltaics or plants.
Solar energy is a combination of the hours of sunlight and the strength of the sunlight you can expect at your site. This combination is expressed as insolation and is expressed as an average irradiance measured in kilowatt hours per square meter per day. By definition, solar irradiance of 1,000 watts per square meter is what is expected for noon in the middle of summer when the sun is highest. Normally solar irradiance is measured in the number of kilowatts per square meter per day so if it were possible for the sun to be up for 8 hours and always at the highest strength, solar irradiance would be 8.0
The amount of insolation received at the surface of the Earth is controlled by the angle of the sun, the state of the atmosphere, altitude, and geographic location.
The insolation into a surface is largest when the surface directly faces the Sun. As the angle increases between the direction at a right angle to the surface and the direction of the rays of sunlight, the insolation is reduced in proportion to the cosine of the angle.
This ‘projection effect’ is the main reason why the polar regions are much colder than equatorial regions on Earth. On an annual average the poles receive less insolation than does the equator, because at the poles the Earth’s surface are angled away from the Sun.
Solar insolation will vary significantly during the year particularly in northern latitudes. As an example, the irradiance for New York City is 6.0 in June and only 1.7 in December yielding a average for the year of 4.0. This means that solar energy in December is 70% less than in June. Compare this to Phoenix where the irradiance is 7.8 in June and 3.0 in December or 5.5 for the year.
If you are not tied to the grid, this means your capacity has to be over 2.3 to 3 times the size that would be dictated by June numbers. Values for your location are easily available on the internet since NASA’s weather satellites have been compiling this information around the world for many years.
How much solar energy makes it to a rooftop
After we take away all the factors that affect solar gain, how much solar energy does make it to the rooftop of your home or your yard? More than we imagine. The solar energy making it to a roof in Central Manchester, Connecticut, in a year for example, is about 6-7 times as much energy as a normal house with average insulation in the northeast consumes for space heating or about 25 times as much energy as is consumed to heat water.
If you want to find how much solar energy is available to you at different times of the year, there are good sources available online. This site is one of those places that provides information based on your longitude and latitude on maps or in tables that include data for over 1000 ground sites.
How do we use Solar Insolation to plan the size of your system? If we know how many kilowatts we need to meet the electric requirements of our home, we are now in a position to size our system. If we are building a grid-tie system, you can use the average annual irradiance for your calculation since the objective is to make our payment to your utility company zero for a full year. If we are building a off-grid system, we need to pick the irradiance value for December since we need to produce sufficient power all year.