By definition, insolation is the incident solar radiation. That is, the amount of solar radiation reading the earth per day. This is usually measured in units of Btu/square ft/d.
This is the sunlight that directly hits a surface and causes energy in the form of heat. This energy that we eventually feel as heat, is due to a nuclear reaction in the sun which is converted into energy. Only about 19% of this is absorbed by the atmosphere, 31% is reflected by the atmosphere, and the remaining 50% gets to earth absorbed almost totally.
