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Although 77 percent of the sun's energy has the characteristics required for use in photovoltaic systems, the actual efficiency of these systems is much lower.

One reason for this is that more than 40% of the average absorbed photon energy ends up heating the photovoltaic cell. Unfortunately, the efficiency of photovoltaic systems is affected by temperature. As temperatures increase, the output voltage produced by sunlight is reduced which then reduces the amount of energy produced. The brightness of the sun linearly increases the current produced by the cell. Thus, the solar cell is generally most efficient at noon time when the sun is brightest.

In addition, some photons are reflected by the exposed surface of the silicon crystals. Finally, there is some internal resistance in the silicon crystal that inhibits the flow of electrons.

The overall impact of all of these factors is a conversion efficiency that varies widely during the day. The system we have chosen averages 11% when the sun is at its brightest and the module is at its operating temperature of 120 F (50 C).