We present a method for quantifying the energy efficiency gap ex-ante. To do this we define the energy efficiency gap as being the difference between the ex-ante market and techno-economic energy savings potentials. The estimation of market potential is based on top-down (econometric) modelling of energy demand using data from the period 1970–2005. The techno-economic estimates are made using a bottom-up building stock model (ECCABS), to assess the effects and cost-efficiency of various energy efficiency measures. Common to these two modelling approaches are two scenarios of energy prices, which differ only with respect to the carbon tax component. We implement the method for the case of useful energy demand for space and water heating in the Swedish residential sector up to 2030. In comparison to the level of energy use in 2005 (74 TWh), the top-down model predicts for 2030 reductions in demand for the two price scenarios of 17 TWh and 21 TWh, respectively. The bottom-up model predicts corresponding reductions in demand of 25 TWh and 31 TWh, respectively. Thus, there is an energy efficiency gap between the two models of at least 8 TWh in 2030. An implicit discount rate of 10% would render the results from the bottom-up modelling identical to those from the top-down modelling. However the presence of the energy efficiency gap indicates that there is a need for enhanced policies in order to make future reductions in energy demand reach the levels predicted by the bottom-up modelling.