Qtot

Qtot denotes the total amount of heat transferred during a process. It is a cumulative quantity used in thermodynamics and engineering to describe the energy exchanged with surroundings, as opposed to Q̇, the instantaneous heat transfer rate. Qtot can refer to the entire process or a defined interval.

Mathematically, Qtot is defined as Qtot = ∫_{t0}^{t1} Q̇(t) dt, with units of joules (J). If Q̇ is constant, Qtot = Q̇ × (t1 − t0). In many analyses, Qtot is equivalent to the net enthalpy change when the process involves only heat transfer and pressure-volume work, and when mass remains constant.

Applications include HVAC load calculations, building energy modeling, calorimetry experiments, and industrial chemical processing. In solar thermal systems, Qtot represents the total absorbed solar heat over a period. In open or reacting systems, care must be taken to account for mass flow and phase changes, which can affect the interpretation of Qtot.

Etymology: The symbol Q historically denotes heat, and the suffix “tot” signals a total quantity. See also Q, Q̇, energy balance, and enthalpy.