"The grand necessity, then, for our bodies is to keep warm, to keep the vital heat in us."
                                   -- Henry David Thoreau, Walden


The same solar heat that can burn your skin at the beach produces a daily average of 4 to 5 kilowatts of energy on each square meter of the earth's surface. Taken as a whole, our planet's surface receives quadrillions of kilowatts, far more than we need for our energy needs. All this energy can be directly captured or indirectly utilized in the form of wind and water power or biomass fuels. Direct radiation from the sun can be used for solar heating, cooking or crop drying, to power industrial processes, to produce solar electricity, to pump or distill water, and for many other purposes.

For a better idea of the availability of solar, wind or water power see the Weather Data tables later in this web page. You'll find information on sun angle, average daily solar radiation, average temperature and annual degree days, average wind speed, and annual precipitation, in addition to longitude, latitude and elevation, for 168 cities and towns throughout the U.S.


Solar Heating

The sun's thermal energy is commonly used for space or water heating and increasingly for industrial processes. For these applications the radiant energy of the sun is absorbed as thermal energy and then transmitted to the liquid or gas to be heated. Because dark colors absorb more of the sun's energy than light colors, the surface that receives the solar heat is generally blackened or at least darkened.

Solar heating is generally divided into passive and active applications.

  • Passive space heating systems collect and utilize solar energy by design changes and other natural means; it generally excludes the use of mechanical power or electronic controls.
  • Active space heating systems utilize a collector, a circulator, and thermal storage.

The most common application of solar energy is for solar water heating - for use in showers, dish or clothes washing, swimming pools, etc.

  • Active solar water heating systems - using pumps, controllers and valves - are usually more expensive than passive systems, but they are more efficient and often easier to retrofit. However, they will not operate during an electric power outage. If the circulating fluid in either an active or a passive system is water, then provision must be made to prevent freezing during the colder months. In most active solar water heating systems the sun's heat is trapped in flat plate collectors or concentrating collectors that heat a fluid that is then stored in a tank.
  • Passive solar water heating - relying on the natural convection of heated and cooled fluid - is usually less expensive but often less efficient. In this case the storage needs to be installed above or quite close to the tank. The two primary types of passive water heaters are batch heaters and thermosyphon systems.

Solar water heating is generally cost competitive when you account for the total energy expenses over the life of the system. You can expect a simple payback of 4 to 8 years on a well-designed and properly installed water heater. Performance is dependent on how cold the water to be heated is as well as how much solar radiation is available at the site. Information on solar radiation, sun angles and temperature in 168 towns and cities across the USA is given in the Weather Data tables.

For an index of manufacturers of solar collectors, glazing, heating and cooling equipment, sun spaces and greenhouses, as well as solar water tanks and water pumps, see the Renewable Energy Manufacturers Index.


Solar Electricity

The sun's energy can also be used to heat fluids to high temperatures in order to produce solar thermal electric power. A simpler method produces electricity by using photovoltaic cells (PV cells), which convert the radiant energy of the sun directly into electrical energy.

Because they were initially quite expensive, PV cells were first used in remote locations (where other sources of electricity were not available) for applications such as water pumping, highway lighting or signs, weather stations, maritime signals, and forest lookouts. On a small scale, PV cells have proven to be very practical in powering millions of watches, calculators, radios and other electronic devices. Medium-sized modules can produce domestic electricity or charge electric automobiles. At the other end of the spectrum, because of their modularity, PV arrays can be joined to form small power plants linked to a public utility grid. If these small power plants are distributed throughout the grid, each one close to the source of demand, they can reduce transmission losses and costs.

The efficiency of PV cells in converting solar energy to electricity has risen from about 4% when the first silicon cell was developed at Bell Laboratories in 1954 to over 30% for some concentrating cells today. During the same time period the cost of PV cells has fallen sharply - so that we are now approaching the time when solar electricity will be economically feasible for many homes and businesses. In most sections of the U.S., about 30 square feet of roof space can currently supply electricity for an average house. One cost-effective approach is to integrate solar electricity into the building structure by using PV roofing tiles, PV curtain walls, etc.

For an index of manufacturers of photovoltaic cells, modules and equipment - including equipment for water pumping, outdoor lighting, battery charging and other applications - see the Renewable Energy Manufacturers Index.

Browse our glossary of renewable energy terms or our list of solar energy organizations for more information.

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