the most common energy sources for heating water are fossil fuels: natural gas, liquefied petroleum gas, oil or sometimes solid fuels (coal or firewood. these fuels may be consumed directly or by the use of electricity (which may derive from any of the above fuels or from nuclear or renewable sources). alternative energy such as solar energy, heat pumps, hot water heat recycling, and sometimes geothermal power, may also be used as available, usually in combination with gas, oil or electricity.