The new energy measurement concept is based on the idea that three physical properties will be sufficient for characterising natural gas and providing the information required for energy conversion by way of correlation. The inferential or correlative system uses relative permittivity (epsilon), speed of sound (w) and CO2 mole fraction (xco(2)) as input parameters. For relative permittivity, a measurement unit (a re-entrant resonant cavity cell), has been optimised by National Engineering Laboratory (NEL) for both laboratory and field or on-line applications. With the new system, the calorific value measured at reference conditions agrees with other experimental results, Cutler-Hammer calorimeter and GC analysis, to within 0.1%.