An electric steam boiler is a type of boiler where the steam is generated using electricity, rather than through the combustion of a fuel source. Such boilers are used to generate steam for process purposes in many locations, for example laundries, food processing factories and hospitals. Although they are more expensive to run than gas-fired or oil-fired boilers they are popular because of their simplicity and ease of use. Because of the large currents required, they are normally run from a three-phase electricity supply. They convert electrical energy into thermal energy with almost 100% efficiency but the overall thermal efficiency is variable, depending on the efficiency with which the electricity is generated.