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 wire that can span the vertical height of the tank. The sensor wire can be protected with a waterproofing jacket. The sensor wire can be coupled to a second substantially parallel wire. A processing unit measures temperature from changes in the resistance of the sensor wire and measures rates of change to allow the system to distinguish different sources of heat increase and/or decrease.