The solar constant is the rate at which solar energy, at all wavelengths, is received per unit area at the top level of Earth's atmosphere. The solar constant actually varies by about 0.3% over the 11-year solar cycle but averages about 1,368 watts per square meter (W/m²). Each planet has its own planetary solar constant, ranging from Mercury (9936.9 W/m²) to Pluto (0.87 W/m²).