The Sun contains an inexhaustible supply of energy that can be used instead of traditional fuels. Photovoltaic solar cells are used in the space shuttle, deep space probes, commercial satellites, solar-powered cars, and even roadside construction signs on highways and handheld calculators. Then, the heated water flows into the building and is used directly as hot water, or for heating. Large glass windows let warming sunlight into a building. Greenhouses use passive heating to stay warm, even in cooler seasons.
Solar ovens concentrate solar energy in a small space and are used for cooking and drying food.
Like the larger greenhouses, solar ovens are designed to let sunlight shine in but keep the heat from flowing out. All of these solar collectors can save money that would otherwise be spent on traditional fuels. A typical American family of four uses about 80 gallons of heated water daily. An electric hot water heater would require 14 kilowatt-hours of electricity to heat that water. A single solar roof panel can generate the same amount of heat as 10 kilowatt-hours of electricity each day.