Determining temperature within a thermal storage device

A method of operating a thermal storage device (1, fig. 1) storing heat in a core (11, fig. 1). The method comprises, by a control system, determining a change in temperature of the core over a time period based on an energy balance of the energy supplied to 202 and released from 203 the core over the time period. Determining a current temperature of the core may be further based upon a temperature drift correction 208 determined using a temperature sensor in the core. The drift correction may also take account of the initial temperature of the core. An electricity meter may measure the energy supplied to an electric heater to determine the amount of heat supplied to the core. A controller and thermal storage device are also claimed.