"The first, and main reason, why someone would want a thermal storage unit, is to allow the householder to generate pressurised hot water for more than one outlet at once. To make a simple comparison a wall hung gas combination boiler will generate typically 30kW hot water, equal to 12 litres every minute at usable temperatures. A thermal store, typically rated at 100kW, can generate 40 litres per minute at the same temperature - more than 3 times as much." (Quoted from site)