Sunlight strikes the earth with a power density of about 1 kilowatt per square meter.

The maximum power density of the sun's radiation on the surface of the earth is approximately 1.4 kW/m2.

Stated another way, energy from the sun arrives at the rate of five million horsepower per square mile.

This value varies slightly throughout the year by about 0.1 percent due to the changing earth-sun distance. The power density also varies with the 11-year cycle of sunspots.

On the ground, averaged over 24 hours a day over the entire earth, about 164 watts per m2 are available. That is about 4 kWh a day per square meter.

To make use of this energy, it must be converted into electricity. The efficiency with which this can be done varies greatly.

Increasing the conversion efficiency of solar panels is key to making solar energy practical and economical.

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