A simple calculator for early stage energy performance assessment in residential buildings was developed and partially validated in this study. The calculator is based on the correlation of heat loss and net energy need for heating and has fixed internal heat gains as intended for compliance assessment. With the calculator the effect of any parameter implemented can be immediately seen on delivered and primary energy. Because of simple equations, it is suitable for implementation into CAD design tools including basic BIM data. Results showed that the accuracy of the correlation for the cases with practical relevance was not worse than 14% of the net energy need of space heating. This applies for passive house insulation level; for less insulated building envelopes the deviations were smaller. This 14% equals to only 3 kWh/(m² a), i.e. the percentage difference in the total delivered and primary energy was much lower because of other components in the energy balance. The deviations determined were implemented in the calculator as safety margin. Results prove that simple energy performance assessment based on specific heat loss coefficient is well justified for early stage energy analyses as well as for compliance assessment implemented in Estonian regulation.