Unless otherwise stated, the values in the following table are lower heating values for perfect combustion, not counting oxidizer mass or volume. When used to produce electricity in a fuel cell or to do work, it is the Gibbs free energy of reaction (Δ G) that sets the theoretical upper limit. If the produced H 2 O is vapor, this is generally greater than the lower heat of combustion, whereas if the produced H 2 O is liquid, it is generally less than the higher heat of combustion. But in the most relevant case of hydrogen, Δ G is 113 MJ/kg if water vapor is produced, and 118 MJ/kg if liquid water is produced, both being less than the lower heat of combustion (120 MJ/kg).