## 1. Solar Constant The solar constant is the amount of energy received per area at 1 astronomical unit from the sun. It has been by satellites (above the earth's atmosphere) and corrected to 1 au. to be 1361 W/m^2 Sun behaves like a black body at 5778K. Radiation received on earth is attenuated by atmospheric absorption. ![[solar_radiation.png]] See discussion on [[Solar Absorptance]] ## 2. Air Mass Coefficient It is the length of the atmosphere through which solar radiation needs to travel in a given analysis relative to the length of atmosphere normal to the earth's surface. $AM \approx 1/cos(z)$ z is [[zenith and azimuth angle|Zenith angle]] of sun's posiiton Solar panels do not generally operate under exactly one atmosphere's thickness: if the sun is at an angle to the Earth's surface the effective thickness will be greater. **AM1.5** is typically used **Air mass zero spectrum** - solar spectrum from outer space