A method for measuring efficiency of a turbine includes operating the power plant at a first load, wherein a gas turbine is operated with a first fuel flow and a first gas turbine inlet air flow, determining a first shaft output at the first load with steam supplied from a heat recovery steam generator (HRSG) to both the first steam turbine inlet and the second steam turbine inlet, maintaining the first gas turbine fuel flow, the first gas turbine inlet air flow, and the first steam turbine inlet steam flow while isolating the second steam turbine from the steam admission flow, determining a second shaft output while the second steam turbine is isolated from the steam admission flow, and determining an expansion efficiency of the second steam turbine based on the first shaft output and the second shaft output and a measured change in second steam turbine admission steam flow.