Enthalpy is the measure, per unit mass, of the total heat energy contained in a substance. What matters is not its absolute value but the difference: if refrigerant enters the evaporator at 250 kJ/kg and leaves at 400 kJ/kg, every kilogram of refrigerant has removed 150 kJ of heat from the room. Cooling capacity is this difference multiplied by the refrigerant mass flow.
In refrigeration engineering the cycle is drawn on the p–h (pressure–enthalpy) diagram: enthalpy on the horizontal axis, pressure on the vertical. The evaporator and condenser are horizontal lines (heat exchange at constant pressure), the compressor a line sloping up to the right (enthalpy and pressure rise), the expansion valve a vertical line (enthalpy constant, pressure falls). Capacity, compressor work and COP can be read directly from the diagram.
Enthalpy is also defined for moist air and given in kJ/kg of dry air; it contains both the sensible heat of the air and the latent heat of the water vapour. The enthalpy difference between the outside air entering through the door and the room air gives the infiltration load. Air at 35 °C and 50% humidity has an enthalpy of about 81 kJ/kg, air at 0 °C and 90% humidity 8.5 kJ/kg: the 72 kJ/kg difference is the load of every kilogram of air that pours into the room through an open door on a summer day.