What are the typical energy savings of electric instant water heaters compared with stored electric water heaters?
Electric instant water heaters typically save domestic hot-water energy use by around 10-25% compared to your normal electric storage water heater, but the actual savings depend a lot on how much hot water you use and how well the storage cylinder is insulated.
Both systems use electric resistance heating, so when they are heating up, they are practically 100% efficient at turning electricity into heat. The main efficiency advantage of an instant heater isn’t the heating element itself, but rather getting rid of all that stored-water heat loss.
A water storage heater or cylinder has to keep a whole load of water hot, usually at around 55-60°C. Even with good insulation, heat just leaks out of the cylinder casing, pipes and connections all the time. A decent cylinder might lose about 0.8-2.0 kWh over the course of 24 hours, all depending on how big it is, how well it’s insulated, how hot its stored water is and how cold the room is.
An electric instant water heater only kicks in when you turn on a tap or shower, so you don’t have to worry about any tank standing loss between times.
For example, let’s say a household needs to use 6 kWh per day of useful hot water energy, and its electric cylinder is losing 1.5 kWh per day:
Stored system: 6 + 1.5 = around 7.5 kWh per day
Instantaneous system: roughly 6 kWh per day
That’s an energy reduction of about:
(1.5 ÷ 7.5) x 100 = 20%
The bigger the percentage saving, the lower the amount of hot water you use or how frequently you use it, because the standing losses make up a bigger chunk of your total energy use.
However, there’s an important drawback with electric instant water heaters: they pull a lot of power. A whole-house instant unit might need 18, 21, 24 or even 27 kW or more, often requiring 3-phase electricity. A storage heater can get by on a much smaller immersion heater or electric boiler and heat the water over a longer period.
Also worth considering is that an electric storage cylinder can take advantage of cheaper off-peak power, solar power or variable tariffs. So an instant heater may use fewer kWh, but it may not necessarily be cheaper to run.
A reasonable rule of thumb is that you can expect to save around 10-25% of your energy, with even bigger savings if your storage cylinder is particularly poorly insulated or you use only a tiny amount of hot water.
