Solar thermal energy (STE) is a technology for harnessing solar energy for thermal energy (heat). Solar thermal collectors are classified as Low-, medium – or High-temperature collectors. Low temperature collectors are flat – plates generally used to heat swimming pools. Medium-temperature collectors are also usually flat-plates but are used for heating water or air for residential and commercial use. High-temperature collectors concentrate sunlight using mirrors or lenses and are generally used for electric power production. STE is different from (and indeed much more efficient than) photovoltaics, which converts solar energy directly into electricity.