Hot Water Cost Calculator

How the three systems compare

Whichever system heats your water, the energy needed to warm it is the same, it depends only on the volume of water and how many degrees it needs to rise, based on water's specific heat capacity. What differs is how much input energy each system uses to deliver that heat: a gas boiler loses some heat up the flue (around 90% efficient), an electric immersion heater converts electricity to heat almost 1:1, and a heat pump moves roughly three units of heat for every unit of electricity it consumes (a seasonal coefficient of performance, or SCOP, of about 3). That's why a heat pump is typically the cheapest to run for hot water and an electric immersion heater the most expensive, even though a gas boiler and an immersion heater are heating exactly the same amount of water.