Solar energy efficiency is related to the available energy that solar energy converts into electrical energy.

As early as the 1950s, the first solar cells were able to get 6% of the energy from the sun and convert it into electricity.

If you configure them to the same array of 60 cells you see in the image above, you will generate about 20 watts of current, about one-third of the current required to illuminate a 60-watt incandescent bulb.

When we first wrote this page in 2012, solar cells can convert 15% of energy from the sun into energy. By 2018, the efficiency of the most advanced solar cells is close to 23%, while the average solar cell efficiency for residential use is about 18.7%.

If you combine the efficiency of the battery with the size of the panel, you get a number called "rated power." In the solar industry, we say "the panel is rated at X watts."