The solar constant is the amount of energy coming in contact with the top of Earth's atmosphere in watts per meter-squared. This includes the energy deposited from the entire spectrum of solar radiation: radio, infrared, visible light, ultraviolet and X-rays. The approximate value of the constant is 1.361 W/m squared.
NASA explains that the term "solar constant" is actually an oxymoron. With the deployment of more advanced satellite-based measuring techniques, it has been observed that the value of the solar constant rises and falls with the sunspot cycle by a significant amount. When the solar irradiance is at a maximum, it is about 0.1 percent brighter than it is at the minimum.