These are physically small electrically powered units that heat water 'instantly' as it flows through the product. Instantaneous water heaters are compact, will not run out of hot water and have no stand-by energy losses. Products in this category are three phase 400V units. Three phase operation allows high heating powers to be used so that hot water can be delivered at good flow rates. A three phase electrical supply is essential; these products cannot be used with a standard domestic 230V single phase electrical supply.