High above the clouds, satellites have measured the solar intensity to be 1.366 kW per square meter. As the sunlight filters through the atmosphere, some of the energy is reflected back into space while some is absorbed by the air. The remaining amount of solar energy to strike the surface of the earth averages about 1 kW per square meter.
This immense amount of energy powers the wind, weather and water cycle that influence the entire world. Solar panels can tap into this energy but their efficiency is greatly affected by the solar intensity (the strength of the sunlight). Here are some factors that reduce the solar intensity.
At high noon, the angle at which the sun's ray strikes the earth's surface is most direct and the sunlight feels hot. Solar panels produce the peak amount of electricity at this time of day. At sunrise and sunset, the angle is less direct and the sunlight feels weak. Less power is generated when the angle of incidence between the sunlight and the earth is low.
As the earth orbits the sun, the distance between the two celestial bodies change. Solar panels produce more power in summer when the sun is high above the horizon and less power in winter when the sun is low.
In regions near the equator, the sunlight is most direct. The further away from the equator, the more seasonal variations you will see in solar power production.
Atmospheric conditions also reduce the intensity of the sunlight before it reaches the surface of the earth. Air pollution may absorb up to 5% of the solar energy while clouds may absorb up to 30%.
Shadows from trees and other tall obstacles can reduce the efficiency of solar panels up to 70%, even if only a portion of the solar array is blocked. Debris such as leaves and dust can also prevent the solar cells from producing peak amounts of electricity by 10%.
You don't need complex equipment to measure solar intensity. Before installing a solar array and storage system, a small, inexpensive solar panel and data collector can be used to give you a general idea of how much solar energy is available in that location. It will record voltage, current and the time of day and will show how the electricity production changes from day to day and season to season. Place it in an open, unobstructed location, in the same area where you plan to install the larger solar panels.
Solar intensity is not the same as air temperature. Even on a cold winter day, if the solar panels face directly towards the sun, the solar cells will still produce electricity despite the cold air temperature. An automatic mechanical system & sensors, called a solar tracker, can be used to align the panels in the best possible angle, but the extra components may be cost prohibitive for most household applications. A fixed-mount position for solar panels is the easiest and most affordable option for residential home use.