[[Energy balances - variables and forms]] Definition: H = [[Internal Energy (U) of a system is its total energy content|U]] + PV (tabulated in steam tables) Enthalpy is not directly measurable. The change in enthalpy (ΔH) from a process is equivalent to the heat transferred when that process occurs at ==constant pressure==. Enthalpy can be referred to as the total heat content of a system. It's also a product of the relationship between a system and its surroundings.