Solar thermal energy is primarily used domestically for space heating, hot water, and in some cases air conditioning. Solar thermal energy is renewable and no fuels are required during the process to generate electricity or mechanical energy. It is also carbon free except for production and transportation, otherwise it is non-polluting. Solar thermal can also be combined with photovoltaics (PVs), in highly efficient cogeneration systems. Some downsides are that solar thermal generates low grade energy, installation and construction costs are very high, it is expensive compared to conventional water heaters, and is hard to compete against cheap natural gas. An example of a Solar Thermal Electric Generation Plant is the Ivanpah Solar Electric Generating System that uses 300,000 computer-controlled mirrors, that focuses sunlight to the top of 459-foot towers, where water is turned into steam to power turbines.