When considering engine efficiency one should consider whether this is based on the lower heating value (LHV) or higher heating value (HHV) of the gas. Engine manufacturers will typically quote efficiencies based on the lower heating value of the gas, i.e. the efficiency after energy has been taken to evaporate the intrinsic moisture within the gas itself. Gas distribution networks will typically charge based upon the higher heating value of the gas. "i.e.", total energy content. A quoted engine efficiency based on LHV might be say 44% whereas the same engine might have an efficiency of 39.6% based on HHV on natural gas.