The Isentropic efficiency of a turbine is a comparison of the actual
power output with the Isentropic case. Typical Isentropic efficiencies
range from 70-90%.
Where
To calculate these enthalpy changes, you need to know the initial and
final states, for example, temperature and pressure, of the working
fluid for both the actual and isentropic cases. In the isentropic case,
h2s is found from P2 and (S1
= S2s).
A typical Ts diagram for a steam turbine is shown below. 
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