Simplified correlation methods are powerful tools to compare both the energy performance of buildings and the efficiency of different HVAC control systems. For buildings in which both heating and cooling loads contribute to the energy consumption, widely used methods, like the energy signature models, are not appropriate, since they cannot simultaneously handle both heating and cooling aspects. This paper proposes a new correlation method to overcome this difficulty. The ‘Eta method’ deals with internal gains, as well as solar gains, and can be used to calculate a utilization factor for free gains in the heating and cooling periods. Two examples to illustrate the method are given. The first one is a passive solar office room, on which different blind controllers were tested by simulation. The second example is a non-residential building in which two HVAC controllers were carefully monitored.