Electric generator losses usually include the same kind losses as of the power transformer, copper losses in the winding and magnetizing losses in the core, plus the rotational losses.
According to RETScreen Software model for small hydro project, generator efficiencies can range from 93% to 97%. Since the turbine used for small hydro power plants has a slow output rotational speed, a gearbox or a speed increaser, between the turbine and the generator is required to match the ideal rotational speeds of each. This is to permit the use of a high-speed, lower cost generators but reduces the overall efficiency of the plant by approximately 2%.
According to RETScreen, a value of 95% represents the efficiency of most generators used for small hydro plants. This value should be reduced to 93% for plants that require a gearbox.