Transcript: The solar constant is the amount of Sun’s radiation that reaches the Earth. Assuming the atmosphere to be perfectly transparent, 1,370 joules reach every square meter of the Earth surface every second. Variations in the solar constant are subtle, only about 0.1 percent per year. However, they can be larger in percentage terms at regions beyond the visible spectrum in ultraviolet rays or x-rays. In particular, in the ultraviolet ozone in the upper atmosphere can affect the amount of radiation that reaches the Earth surface. Small changes in the energy budget from the Sun’s radiation can affect the weather and climate of the Earth in serious and systematic ways. Thirteen hundred and seventy watts is a lot of energy; the average family home intercepts 50 to 100 kilowatts. Even accounting for a relatively low efficiency of a solar cell, and only half to a third of the time of a twenty-four hour day when the Sun is out, solar energy is clearly an abundant energy source.