The exergy of a process stream is given by the expression:

\[\label{eq1} E = (H - H_o)- T_o (S - S_o) \tag{1}\]

where H is the flow enthalpy (H = Ṁh where Ṁ is the mass rate of flow and h the specific enthalpy) and S is the flow entropy (S = Ṁs, where s is the specific entropy). The subscript o refers to values at the environmental temperature T_o. Strictly speaking, the exergy given by Equation 1 is the thermomechanical exergy and derives purely from the temperature and/or pressure being different from that of the environment or "dead state". No composition changes or chemical reactions are included. These are most easily dealt with explicitly as a separate term as shown later. The temperature is expressed in Kelvin (K) (0 °C = 273.15 K).

Exergy can also be regarded as the product of the mass rate of flow (Ṁ) and a specific exergy e = (h – h_o) – T_o (s – s_o) (i.e. E = Ṁ_e). The molar exergy Ẽ is sometimes used and is the product of the molar flowrate and the molar specific exergy ẽ = (h̃ – h̃_o) – T_o (s̃ – s̃_o) (i.e. Ẽ = Ñẽ).

It is sometimes more convenient to calculate the exergy change (ΔE) for a process stream passing through a piece of equipment. This is because (i) simplified expressions for the exergy change are frequently adequate and (ii) the environment terms H_o and S_o cancel out.