There are two ways to define the energy content of natural gas in common use — Higher Heating Value (HHV) and Lower Heating Value (LHV). The difference can be especially important when reviewing the performance of engine-driven systems. Higher Heating Values for a fuel include the full energy content as defined by bringing all products of combustion to 25° C. Natural gas typically is delivered by the local gas company with values of 1,000 – 1,050 Btu per cubic foot on this HHV basis. Since the actual value may vary from month to month some gas companies convert to therms. A therm is precisely 100,000 Btu. These measures all represent higher heating values. Some engine manufacturers rate their engines using Lower Heating Values (LHV) which can be both confusing and potentially misleading to the casual user of their product literature. Lower heating values neglect the energy in the water vapor formed by the combustion of hydrogen in the fuel. This water vapor typically represents about 10% of the energy content. Therefore the lower heating values for natural gas are typically 900 – 950 Btu per cubic foot. The error can occur when a manufacturer says their engine uses 900,000 Btu/hr but it was expressed on a LHV basis. The engine would actually use about 1,000,000 Btu/hr as purchased from a gas supplier. Therefore, always check the fuel rating method when natural gas is the fuel for the system. This potential confusion almost never exists with liquid fueled systems.