Light available to the solar panel—in this case, direct sunlight or indirect light due to cloud cover—was a significant determining factor in its efficiency.

Direct sunlight is the condition most favored to produce maximum output from a solar panel. When the sun's rays hit the cells of a solar panel directly, much more absorbed solar energy is converted to electricity. This is because, when it is bright and sunny, light intensity will be high, and solar cells will have maximum electrical power. It also depends upon the technology used, with ranges like 15% to 22% efficiency in direct sunlight.

Indirect light happens on cloudy days or when the sky is overcast, which decreases the efficiency of the solar panel. Clouds scatter sunlight, reducing its intensity before it reaches the panels. Under such conditions, solar panels can still generate electricity, but certainly the output is lower compared to direct sunlight. The efficiency could go low by as much as 10% to 30%, or even more, depending upon the density of the cloud. However, solar panels are designed to work in varied light conditions, so they can capture and convert diffuse sunlight.

Although it loses some of its efficiency when the sky becomes overcast, better technology in solar material and the use of anti-reflective coatings make it quite efficient even under adverse lighting conditions.