A turbine extracts shaft work from a flowing fluid as the fluid expands through the machine.

For steady, approximately adiabatic operation with small kinetic and potential-energy changes, the steady-flow energy equation gives

$$w_{out}\approx h_1-h_2.$$

For example, if steam enters with $h_1=3200\ \mathrm{kJ/kg}$ and leaves with $h_2=2600\ \mathrm{kJ/kg}$,

$$w_{out}\approx600\ \mathrm{kJ/kg}.$$

At a mass flow of $4\ \mathrm{kg/s}$, the corresponding shaft-power output is approximately

$$\dot W\approx4(600)=2400\ \mathrm{kW}=2.4\ \mathrm{MW}.$$

The actual enthalpy drop depends on the inlet state, outlet pressure, heat transfer and internal losses. The steady-flow energy equation converts that state change into shaft-work output.