Using classic thermodynamic principles as a starting point ,it is possible to assess the technical viability of energy-conversion processes. A new concept, the exergetic (useful energy) recovery factor, measures net fraction of the extracted useful energy with respect to the initial exergy of the energy resource.  A negative exergetic recovery factor indicates that more input exergy is required than extracted.  It is possible to give a simplified procedure that allows a non-specialist  to judge both conventional and non-conventional energy resources. Case studies considered comprise the conventional use of fossil fuel (oil gas and coal) the non-conventional use of fossil fuels (shale gas, underground coal gasification) and non-fossil fuel energy resources like geothermal, solar and wind.