Instantaneous water heaters are electrical appliances used to provide hot water for domestic or commercial use. Their operation is based on a heating element that heats the water in real time, and for as long as the tap remains open, i.e. the hot water does not run out, as can happen with a traditional water heater or solar water heater. Instantaneous water heaters provide water at a constant temperature and the higher the wattage, the faster they heat the water in order to provide a greater flow at the selected temperature. Newer models have a heat exchanger, with heating elements that do not accumulate salts and for this reason offer a multi-year guarantee of good operation. They do not have a storage tank like solar or conventional water heaters, and therefore do not store hot water which may ultimately be unused. Furthermore, they are self-regulating (digital - inverter), that is, they automatically adjust their power to meet our needs with the lowest possible energy costs. They can be placed in series with other hot water supply units (e.g. solar water heater, heat pump, boiler, etc.) and operate only when the temperature of the water received from the other unit is lower than desired, so we have hot water at the temperature we want immediately and with savings in energy and water consumption.