Improvements in the characterization of the efficiency degradation of water-to-water heat pumps under cyclic conditions

•Experimental characterization of a water-to-water heat pump at part load.•High influence of inertia conditions on start-up losses of water-to-water heat pump.•Assessment of the validity of parameterizations on efficiency loss at part load.•New method for improved characterization of water-to-water heat pumps performance.•Advice for improving the measurement of fixed-capacity heat pumps performance.