How solar modules are rated

Panels are rated in Watts of output. This wattage rating is derived by multiplying the panel's peak power voltage times its peak power amperage (Watts = Volts x Amps). These ratings are based on standard test conditions (STC) of 1000 Watts/square metre of light input, a cell temperature (not air temperature) of 25 degrees C (77 degrees F), and an air mass of 1.5 (AM1.5G). These standard test conditions are rarely found simultaneously in "real world" operating conditions.

For example:

1000 Watts/square metre of sunlight can be obtained around solar noon with the panel squarely facing the sun. When you consider that solar cells are dark blue to almost black, they soak up sunshine and get quite hot, so they are operating at temperatures considerably higher than 25° C (77° F). This increased cell temperature translates into a voltage drop and therefore less output. As "real world" operating conditions are so variable, panel manufacturers had to come up with some standard test conditions so that panel ratings are derived after being subjected to the same conditions as every other panel. Therefore, you have a basis for comparison between manufacturers.