Identical solar panels perform differently in different environments. Obviously light intensity and all factors that influence it are crucial. Age and temperature decrease the productivity of solar panels. So does dirt, suboptimal "power point tracking", etc.
In order to rate and compare solar panels the concept of nominal power is used. Sometimes the term rated power is used. The nominal power is measured in the lab under very specific conditions, as specified in international norms. For example intensity and temperature have to be:
- light intensity 1000 W/m2
- temperature 25oC (77oF).
In the panel datasheets these conditions are usually referred to as "STC" (standard test conditions).
With the STC fixed the load of the panels is varied and the maximum generated power is recorded. This is the nominal power. It is measured in Watt peak (unit Wp). The 'peak' is meant to emphasize that this is the power generated under optimal load condition, i.e. it is the 'peak' power.
Now, the standard conditions in the lab are hardly ever realistic in in real life, and the load conditions are also not always perfect. But the nominal power rating is useful to compare different panels and allow to introduce the standardized price per kilowatt peak (kWp).