Even though solar energy is a very small portion of the energy system today, the size of the resource is enormous. The average intensity of light outside the atmosphere (known as the solar constant) is about 1353 W/m2. In order to produce a gigawatt of power, an area of about 5 km2 would be needed, assuming a conversion of 20%. The earth receives more energy from the sun in 1 h than the global population uses in an entire year.