peer reviewed ; In the context of the much-needed energy transition, monitoring of residential energy appliances is more and more common in order to evaluate (and improve) their field-test performance in real applications. Many residential heating appliances use natural gas as energy source : residential gas boilers, gas engine heat pumps, gas absorption heat pumps; fuel cell-based cogeneration units, internal combustion engine cogeneration units, etc. Unfortunately, for simplicity and costs reasons, only the volume of the consumed natural is measured at the inlet of the monitored residential heating appliance. This volume, and the equivalent energy level that it contains and that is required for efficiency calculations, is affected by the atmospheric pressure and the temperature at the field-test site, as well as by the always varying natural gas composition. This paper demonstrates the method that has been used in Belgian field-test studies conducted on residential heating appliances to establish the energy content of the consumed natural gas. ; 7. Affordable and clean energy ; 11. Sustainable cities and communities ; 13. Climate action