Tankless water heaters provide hot water at a preset temperature when needed without storage, thereby reducing or eliminating standby losses. Tankless water heaters can be used for supplementary heat, such as a booster to a solar hot water system, or to meet all hot water needs. Tankless water heaters have an electric, gas, or propane heating device that is activated by the flow of water. Once activated, the heater provides a constant supply of hot water. The maximum flow rate at a desired temperature will be determined by the capacity of the heater. Gas tankless water heaters typically have larger capacities than electric tankless water heaters. Large units intended for whole house water heating are located centrally in the house while, in point-of-use applications, the water heater usually sits in a closet or under a sink. Tankless water heaters are rated by the maximum flow rate at which a desired temperature rise is met. Special features may allow the user to set the delivery temperature. Efficiency is higher than an equivalent tank type water heater because standby losses are virtually eliminated. Electric tankless water heaters require a relatively high electric power draw because water must be heated quickly to the desired temperature. Residential gas models are available that can heat more than five gallons per minute by 60°F, generally more than enough for two showers to be run simultaneously. Whole house electric units typically have a capacity closer to three gallons per minute.