Steam Turbine Efficiency Calculator
Estimate the actual shaft power output of a steam turbine given inlet and outlet conditions and isentropic efficiency. Ideal for power plant engineers sizing turbines or auditing existing steam cycles.
Last updated: September 2026
Formula below · 2 sources (NIST, Wikipedia) · Updated Sep 2026
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About this calculator
A steam turbine converts the enthalpy drop of steam into shaft work. The isentropic (ideal) specific work is w_s = h₁ − h₂s, where h₁ is the inlet enthalpy and h₂s is the enthalpy after an ideal expansion to the outlet pressure at constant entropy. Real turbines are less than perfect, so actual specific work is w = η_is × w_s, with η_is typically 0.70–0.90. Shaft power is P = ṁ × w in kW when ṁ is in kg/s and w in kJ/kg. Values come from the IAPWS-IF97 industrial formulation (region 1 for liquid water, region 2 for steam, region 4 for the saturation line), the same equations used in published steam tables. The inlet state is set by inlet pressure and temperature (it must be at or above the saturation temperature), and the ideal exhaust state is found at the outlet pressure: as a wet mixture with quality x₂s = (s₁ − s_f)/(s_g − s_f), or as superheated steam if the entropy is high enough. Higher inlet temperature and pressure, and lower exhaust pressure, increase the enthalpy drop and the power.
How to use
Take inlet steam at 30 bar and 400 °C, an exhaust (condenser) pressure of 0.1 bar, a mass flow of 100 kg/s and an isentropic efficiency of 85%. Step 1 — inlet state from IAPWS-IF97: h₁ = 3231.6 kJ/kg, s₁ = 6.923 kJ/(kg·K). Step 2 — at 0.1 bar (45.8 °C), s_f = 0.649 and s_g = 8.149, so the ideal exhaust quality is x₂s = (6.923 − 0.649)/(8.149 − 0.649) = 0.8366 and h₂s = 191.8 + 0.8366 × (2583.9 − 191.8) = 2193.0 kJ/kg. Step 3 — isentropic work w_s = 3231.6 − 2193.0 = 1038.6 kJ/kg; actual w = 0.85 × 1038.6 = 882.8 kJ/kg. Step 4 — power P = 100 × 882.8 ≈ 88,280 kW, about 88.3 MW.
Frequently asked questions
What is isentropic efficiency in a steam turbine and what values are typical?
Isentropic efficiency compares the actual work output of a turbine to the theoretical maximum work from a perfectly reversible (isentropic) expansion between the same inlet and outlet pressures. It accounts for friction, heat losses, and flow irreversibilities inside the turbine stages. Modern large steam turbines achieve isentropic efficiencies of 85–92%, while smaller industrial units may fall in the 70–80% range. Higher efficiency means more electricity generated per kilogram of steam, directly improving plant heat rate.
How does inlet steam temperature affect turbine power output?
Higher inlet steam temperature increases the specific enthalpy of the steam, giving a larger enthalpy drop across the turbine for the same outlet pressure, which directly raises power output. This is why modern ultra-supercritical plants operate at temperatures above 600 °C — each additional degree adds to the available work. There is a practical limit set by the metallurgical strength of turbine blades and casing materials at elevated temperatures. Superalloys and advanced coatings are used to push inlet temperatures higher while maintaining structural integrity.
Why is a low exhaust pressure important for steam turbine efficiency?
A lower exhaust (condenser) pressure corresponds to a lower saturation temperature and thus a lower enthalpy at the turbine exit, which increases the enthalpy drop and therefore the work extracted. Most large condensing turbines exhaust into a vacuum of 0.03–0.10 bar, well below atmospheric, achieved by water-cooled condensers. Raising condenser pressure by even a small amount — due to cooling water temperature rising in summer, for example — noticeably reduces plant output and efficiency. This is why cooling water temperature and condenser cleanliness are closely monitored in power plants.