A method of measuring the amount of energy consumed from a water heater having a tank with two or more temperature sensors located a predetermined heights on or in the tank, the method including the step of; for each sensor, determining a corresponding volume of a segment of the tank, measuring the change of temperature at each temperature sensor, calculating the energy for the corresponding volume for each sensor, and summing the energy changes for all the sensors to determine the amount of energy consumed. The energy usage can be recorded with chronological information to construct a usage pattern which can be used for controlling the heater and for providing the user with details of energy usage. The system does not require the use of data derived from a flow meter.