On Mon, 17 Dec 2012 20:33:22 -0800 (PST),
A particular solar panel product will specify the output power , Pout,
measured at standard test conditions (STC) which are defined as 1000 W/m^2,
25oC cell temperature, and 1.5 Atm air pressure.
1 atmosphere is the amounts of air directly above you i.e 1 kg/cm² or
10 000 kg/m². If the air density would not drop with altitude, but all
air would have the same density as the air at sea level, the
atmosphere would thus be 8 km thick.
If the light would come directly from above (zenith), it would
propagate through this 8 km amount of air i.e. one air mass, usually
AM 1 notation would be used. about a quarter of the solar radiation is
lost in the atmosphere.
Since the sun very rarely shines directly from above, a bit more
realistic standard condition is used, i.e. AM 1.5, in which the light
travels through 12 km sea level density air (instead of 8 km for AM1).
