A method of signalling energy usage to a user of a hot water outlet of a hot water supply system, the hot water supply system including: a thermal energy store that is supplied with energy from a source of renewable energy; a renewable energy source; an auxiliary water heater coupled to a networked energy supply; a flow transducer operable, when a water flow passes through the hot water outlet, to provide flow rate data for the water flow; and a processor coupled to the flow transducer; the hot water supply system being operable, under the control of the processor, to heat water that is to be supplied to the hot water outlet to a target system supply temperature using a selection of one or more of the auxiliary water heater, the renewable energy source, and energy from the thermal energy store; the method comprising: determining that water is flowing through the hot water outlet, and determining a corresponding flow rate; determining the selection required to maintain the target system supply temperature at the flow rate demanded by the user at the hot water outlet; classifying the selection into one of at least two categories, and/or determining the energy usage against a scale of usage; signalling to a remote light source visible in the vicinity of the hot water outlet a colour giving information about the source of heat based on the classification or on the determined energy usage; and heating water using the selection. a thermal energy store that is supplied with energy from a source of renewable energy; a renewable energy source; an auxiliary water heater coupled to a networked energy supply; a flow transducer operable, when a water flow passes through the hot water outlet, to provide flow rate data for the water flow; and a processor coupled to the flow transducer; the hot water supply system being operable, under the control of the processor, to heat water that is to be supplied to the hot water outlet to a target system supply temperature using a selection of one or more of the auxiliary water heater, the renewable energy source, and energy from the thermal energy store; the method comprising: determining that water is flowing through the hot water outlet, and determining a corresponding flow rate; determining the selection required to maintain the target system supply temperature at the flow rate demanded by the user at the hot water outlet; classifying the selection into one of at least two categories, and/or determining the energy usage against a scale of usage; signalling to a remote light source visible in the vicinity of the hot water outlet a colour giving information about the source of heat based on the classification or on the determined energy usage; and heating water using the selection. determining that water is flowing through the hot water outlet, and determining a corresponding flow rate; determining the selection required to maintain the target system supply temperature at the flow rate demanded by the user at the hot water outlet; classifying the selection into one of at least two categories, and/or determining the energy usage against a scale of usage; signalling to a remote light source visible in the vicinity of the hot water outlet a colour giving information about the source of heat based on the classification or on the determined energy usage; and heating water using the selection.