This work investigates the effect of structural parameters (turbine size and type) and operating parameters (inlet steam temperature and pressure, steam flow rate, single or multistage exhaust rate, and pressures) on turbine performance for power generation. The paper extends the steam turbine performance model based on the Willans line. The model achieves high accuracy for a wide range of steam turbines at full-load and part-load operation, which has been verified against data for commercial turbines and from the available literature. The correlation of turbine efficiencies provides realistic insights from the model. The model can be applied in utility system design, operational optimization, and system retrofit with complex multistage turbines. For existing turbines, adjustment of operating conditions and steam header conditions can be explored. This overcomes problems from previous correlations, which did not account for changes in steam main pressures. Regression of coefficients from existing turbine operating data can be used to give better performance prediction for existing turbines, allowing more effective system optimization.