Lasted edited by Andrew Munsey, updated on June 15, 2016 at 2:05 am.
Wp refers to the Watt peak name plate rating for flat plates. There are two Wp test conditions STC and PTC which are quite different.
1) STC = 1,000 W / m2 solar insulation, AIM 1.5 solar spectrum and cell temp controlled / maintained at 25 deg C. This gives the highest panel reading but is very artificial as the operational cell temperature is much higher.
2) PTC = 1,000 W / m2 solar insulation, AIM 1.5 solar spectrum, ambient air temp at 20 deg C, wind at 1 m / sec and elevation at 10 m above the ground
PTC came after STC as an attempt to provide something closer to real world data. The STC test was dreamed up by the flat panel suppliers so they could claim big number.
Another test is NOCT (Normal Operating Cell Temperature) which is tested as follows:
NOCT = Cell temperature at 800 W / m2 solar insulation, AIM 1.5 solar spectrum, air temp 20 deg C and wind at 1 m / sec.
A typical flat panel has a NOCT of 47 deg C. I suspect the flat panel suppliers used 800 W / m2 so as to NOT scare off flat panel purchasers with a real NOCT at 40 deg C and 1,000 W / m2 of about 85 deg C. I would add that at 85 deg C and a cell output temperature derating of 0.55% / deg C a flat panel will loose about 30% of the STC rated output.