The electrical efficiency is defined as the ratio between the useful power output and the total power input:
[tex]\epsilon = \frac{P_{out}}{P_{in}}[/tex]
In our problem, [tex]P_{out} = 150 W[/tex] (power generated by the solar panel) and [tex]P_{in} = 500 W[/tex] (power hitting the surface of the solar panel), therefore the efficiency of the panel is
[tex]\epsilon = \frac{150 W}{500 W}=0.3 [/tex]
which corresponds to 30%.
