Renewable energy is energy which comes from natural resources such as sunlight, wind, rain, tides, and geothermal heat, which are renewable (naturally replenished). Renewable energy is derived from natural processes that are replenished constantly. In its various forms, it derives directly from the sun, or from heat generated deep within the earth. Included in the definition is electricity and heat generated from solar, wind, ocean, hydropower, biomass, geothermal resources, and biofuels and hydrogen derived from renewable resources.
Renewable energy replaces conventional fuels in four distinct areas: power generation, hot water/ space heating, transport fuels, and rural (off-grid) energy services. Recently, new policies are adopted in the world community for the use of renewable sources of energy, as the climatic changes render necessary the saving of energy.
In Greece, as member of European Union, policies are promoted for production of energy from photovoltaic systems, installation of photovoltaic stations in homeroofs and in general the energy upgrade of buildings for saving of energy.