What is the relationship between the nominal power of a solar pannel (say 300 Wc) and the actual power it can generate?

The nominal power of a solar panel is the output power at Standard Test Conditions. The actual power is the one that generates under real field conditions that generally differ from Standard ones. You can find more details on this in chapter 4 of our book at The Solar Hub – Solar pumping for water supply: harnessing the power of the Sun downloaded in this webpage.