Heating value of a fuel is the amount of energy(Joules) released by burning a unit mass of the fuel initially at 25C, that reacts completely with oxygen and products are returned to 25C Heating Value of a fuel is the change in [[Enthalpy of formation|sensible enthalpy]] between reactants and products or difference in standard heats of formation weighted by the molar mass $\displaylines{Heating Value = \sum_{i=1}^N n_i^{''}\left[{\Delta_{f}}_{STP}H^{\ominus} + \right]_{products}- \\ \sum_{i=1}^N n_i^{'}\left[{\Delta_{f}}_{STP}H^{\ominus} \right]_{reactants}}$ Heating value is reported as Higher Heating Value or Lower Heating Value. In Higher Heating Value, the water in the products is in liquid form and in Lower heating value it is in vapor form.