An installation includes an in-building hot water supply system, a hot water heat pump, an energy storage arrangement containing a mass of phase change material and a heat exchanger coupled between the hot water system and the heat pump, and a processor to provide a signal to the heat pump based on the opening of an outlet of the hot water supply system. The mass of phase change material has enough latent heat capacity to heat to a predetermined temperature a predetermined quantity of water in the interval from the opening of an outlet of the hot water supply system until at least the heat pump begins to heat water in the hot water supply system. Also provided is a method of controlling a heat pump in such an installation.