Before the Industrial Revolution renewable energy was largely the only energy source used by humanity. Solid biofuels, such as wood, are still the main power source for many poor people in developing countries.
Hydroelectricity is the only renewable energy used today that makes a large contribution to world energy production. The long-term technical potential is believed to be 9 to 12 times current hydropower production, but environmental concerns increasingly block new dam construction. There is a growing interest in mini-hydro projects, which avoid many of the problems of the larger dams.
 Solar power
Commercial solar cells can presently convert about 20 percent of the energy of incident sunlight to electrical energy. If built out as solar collectors, 1 percent of the land today used for crops and pasture could supply the world's total energy consumption. A similar area is used today for hydropower, as the electricity yield per unit area of a solar collector is 50 to 100 times that of an average hydro scheme. Solar cells can also be placed on top of existing urban infrastructure and does then not require re-purposing of cropland or parkland. The German government currently has a huge photovoltaic energy initiative, which is being watched with interest by other countries. Researchers have estimated that algae farms could convert 10 percent of the energy of incident light into biodiesel energy. Solar thermal collectors can capture 70 to 80 percent of insolation as usable heat. Passive solar and Solar chimneys can heat and cool residences and other buildings. A solar updraft tower is another concept.
 Wind power
Wind power is one of the most cost-competitive renewables today. Its long-term technical potential is believed five times current global energy consumption, or 40 times current electricity demand. This would require about 13 percent of all land area, or that land area with Class 3 or greater potential at a height of 80 meters. It assumes a placement of six large wind turbines per square kilometer on land. Offshore resources experience mean wind speeds about 90 percent greater than that of land, so offshore resources could contribute substantially more energy. This number could also increase with higher altitude ground based or airborne turbines.
 Geothermal power
Geothermal power and tidal power are the only renewables not dependent on the sun but are today limited to special locations. All available tidal energy is equivalent to one-fourth of total human energy consumption today. Geothermal power has a very large potential if considering all the heat existing inside Earth, although the heat flow from the interior to the surface is only 1/20,000 as great as the energy received from the Sun or about 2-3 times that from tidal power. At the moment Iceland and New Zealand are two of the greatest users of geothermal energy, although many others also have potential. Countries are also researching hot-dry-rock geothermal technologies which have some possibilities.
 Ocean thermal energy conversion and Wave power
Ocean thermal energy conversion and tidal energy are other renewables with large potential. Several other variations of utilizing energy from the sun also exist, see renewable energy. Currently there are plans under way to produce mass scale wave energy plants as those with hydroelectricity.
Biomass (burning biological materials to generate heat), biofuels (processing biological materials to generate fuels such as biodiesel and ethanol), and biogas (using anaerobic digestion to generate methane from biodegradable material & biodegradable waste) are other renewables. Systems such as advanced anaerobic digesters offer the ability to produce medium sized power generation (2MW-10MW) facilities and offer flexibility. They can recover value from biodegradable waste whilst producing power from a renewable energy source.
 Considerations about renewable energy
Some renewable sources are diffuse and require land and construction material for energy production. The large and sometimes remote areas may also increase energy loss and cost from distribution. On the other hand, some forms allow small-scale production and may be placed very close to or directly at consumer households, businesses, and industries which reduces or eliminates distribution problems.
The large areas affected also means that some renewable energy sources may have some negative environmental impact, although populated suburbs have already been impacted by human development. Hydroelectric dams, like the Aswan Dam, have adverse consequences both upstream and downstream. Some flooded areas also contain decaying organic material that release gases contributing to global warming if not captured. The mining and refining of large amounts of construction material will also affect the environment in the short term.
Aside from hydropower and geothermal power, which are site-specific, renewable supplies often have higher costs than fossil fuels if the impacts of pollution, climate change, and resource depletion are ignored, as is common. Renewables like wind and solar are cost effective in remote areas that are off grid because the cost of a grid connection is high, as is the cost of transporting diesel fuel. Many forms of renewables are cost effective in remote, underdeveloped, and/or low population density areas that are off the grid or on unreliable grids. Transmission of electricity through large grids remote from conventional energy sources is also expensive, and embedding small renewable projects in such locations can cut energy losses significantly. The inefficiency, noise, and refueling requirements of small diesel generators are also factors in favor of renewables in this situation.
Renewable sources are economically viable in less developed areas of the world, where the population density cannot support the financial investment of an electrical grid or petroleum supply network. In such situations, fossil fuel energy sources do not realize economies of scale, and distributed, small-scale electrical generation from renewables is usually more economical and operationally reliable.
Solar thermal is already cost effective for water heating. Grid connected solar cells can be cost effective in a spot-priced market because they generate electricity during peak usage periods when electricity is most costly and because they produce electricity at the point of use thereby avoiding transmission costs.
It is widely expected that renewable energy sources will continue to drop in costs as additional investments are made in R&D and as increased mass production improves the economies of scale. Nuclear power has been subsidized by 0.5-1 trillion dollars since the 1950s. No comparable investment has yet been made in renewable energy. Even so, the technology is improving rapidly. For example, solar cells are a hundred times less expensive today than the 1970s and development continues. Solar breeder technologies, where the energy used to make solar cells is itself solar energy, is also being investigated.