To buy the right-sized electric water heater, first calculate the hot water requirements for the consumers at various times of the day. Other factors that influence size are the rate of water flow and the heating capacity of the heater.
For tankless water heaters, determine the number of heaters that may be in use during the busiest hours of the day, then calculate the quantity of water required of the various applications in the house that use hot water. This is called the "flow rate" and is calculated in gallons per minute. The size of the heater depends on the water flow rate, which can be reduced by installing low-flow fixtures. Typical applications that require hot water include faucets, washing machines, dishwashers and showers.
Storage tank waters heaters, the most common type, have insulated tanks where the hot water is stored until needed. The tank size is the most important consideration and charts are available to ensure an adequate purchase. Another factor is the number of gallons the unit can heat in a specified time. Look for the stated energy efficiency and annual operating costs. The flow rate and the number of simultaneous usage points are two key factors that decide the size and type of heating.