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.
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.
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.
Solar heating uses the power of the sun to provide thermal energy for you to make hot water, heat your home, or your pool. The amount of heating you get depends on the solar radiation, surface area, surface reflectance, surface emissivity, ambient temperature, and thermal convection from wind.
Solar heating also can be used to refer to the heating of any objects, including buildings, cars, through solar radiation.
If you divide your requirements for the day by the irradiance, you will get the number of watts your system must generate. For example, if your home in Phoenix requires 600kw per month, or 20kw per day, your system would need to generate about 3600 watts per hour of full sun. Since solar panels quote the watts of power they produce based on the standard of 1,000 watts per square meter you can divide the 3600 watts by the output of your panels to get the number of panels required for your system. In our example, if the panel is rated at 150 watts, you would need 24 panels.
SORCE Solar Irradiance