Solar constant - (n.)
The amount of energy received from the Sun at the top of the atmosphere, per time unit and per unit area. The value of the constant is near 2 cal/cm2 per second, which corresponds to about 1350 watts per square meter. The solar "constant" is actually thought to vary by at least 0.1 percent on time scales of decades and perhaps by as much as 0.5 percent on time scales of centuries.