Just a little shade can affect a solar panel’s power output dramatically. Diffuse shade from a “soft” source, like a distant tree branch or cloud, can significantly reduce the amount of light reaching a solar panel’s cells. “Hard” sources stop light from reaching solar cells, such as debris or birds dropping sitting on top of the panel. If even one full cell is hard-shaded, the voltage of a solar panel drops to half to protect itself. If enough cells are hard-shaded, the module will not convert any energy. In fact, solar panel shading can become a significant drain of energy on the entire system over time.
Partial cell shading reduces solar panel power by half.
Partial shading of even one cell on a 36-cell solar panel will reduce its power output. Because all cells are connected in a series string, the weakest cell will bring the others down to their reduced power level. Therefore, whether half of one cell is shaded, or half a row of cells is shaded, the power decrease will be the same and proportional to the percentage of area shaded, in this case, 50 percent.
When a full cell is shaded, it can use energy produced by the remainder of the cells, and trigger the solar panel to protect itself. The solar panel will route the power around that series string. If even one full cell in a series string is shaded, as seen above, it will most likely cause the module to reduce its power level to half of its full available value. If a row of cells at the bottom of a solar panel is fully shaded, the power output may drop to zero. The best way to avoid a drop in output power is to avoid shading whenever possible.
A solar panel affects an array in much the same way a single cell affects a solar panel. In a centralized inverter system, where panels are strung in series if only one of the solar panels is shaded in an array, the rest of the solar panels’ output diminishes.