A system for estimating energy cost savings during curtailment periods, for example, determines an estimated temperature in the zone during the period of interest as if the thermostat were set at a temperature different than the actual set point temperature. By determining a difference in the estimated temperature in the zone and the actual temperature over time, the amount of energy savings can be estimated. Applying real time energy cost data to the estimated energy savings provides a real time estimation of actual cost savings during a selected period. The inventive estimation technique includes determining an equilibrium condition for the zone as part of the estimation process.