A PV system needs daylight to work, not direct sunlight. If a panel is exposed to artificial light, it will also produce electricity. However the electrical output of a PV cell is dependent on the intensity of the light to which it is exposed. Therefore if a panel is facing the midday sun on a clear day, then will produce its maximum output electricity (Watt peak = maximum power). You can therefore say that PV cells tend to generate more electricity on bright days than when skies are overcast. Nevertheless, photovoltaic systems do not need to be in direct sunlight to work, so even on overcast days a PV cell will be generating some electricity.