Solar energy is the energy force that sustains life on the earth for all plants, animals, and people. The earth receives this radiant energy from the sun in the form of electromagnetic waves, which the sun continually emits into space. The earth serves as a huge solar energy collector receiving large quantities of this energy which manifests itself in various forms, such as direct sunlight used through photosynthesis by plants, heated air masses causing wind, and evaporation of the oceans resulting as rain which can form rivers. This solar energy can be tapped directly as solar energy (thermal and photovoltaics), and indirectly as wind, biomass, and hydroelectric energy. (Solar energy is a renewable resource that is inexhaustible and is locally available. It is a clean energy source that allows for local energy independence. The sun's power flow reaching the earth is typically about 1,000 Watts per square meter (W/m2), although availability varies with location and time of year. Capturing solar energy typically requires equipment with a high initial capital cost relative to conventional fossil-fueled systems, however, since there is no need for water, deliveries of fuel or ongoing maintenance, the upfront cost is the only cost during the 20-30 year lifetime of the system.