Apparatus and methods are provided for thermal energy metering by measuring the average temperature of fluid in a tank, such as a hot water storage tank. Average temperature is measured with an elongated temperature sensor spanning the vertical height of the tank. A controller collects measurements from the temperature sensor and computes changes in thermal energy, from which the system can more accurately attribute gains of thermal energy to sources such as thermal, electric, or gas production, or losses of thermal energy to ambient losses and consumption.