How Much Electricity Does a Heat Pump Water Heater Use?
How Much Electricity Does a Heat Pump Water Heater Use? Heat pumps are pretty good at beating conventional electric immersion heaters: they typically use about a third to half the electricity to produce the same hot water. But the amount they use depends on loads of things, like hot water demand, the temperature of the air outside, the water temperature, and the heat pump’s efficiency.
For instance, if a household needs 6 kWh of heat for its hot water and the heat pump works at a Coefficient of Performance (COP) of 3, that would theoretically mean it only needs 2 kWh of electricity to deliver that heat. If you were to use an immersion heater, you’d need around 6 kWh of electricity for the same useful heat output.
Calculating how much electricity a Heat Pump Water Heater uses
One thing to keep in mind is that you need to figure out electricity consumption. It’s actually pretty simple. You just divide the amount of heat needed by the heat pump’s COP.
Electricity consumption = Heat required ÷ COP
For example, let’s say a household needs to reheat its hot water cylinder and requires 6 kWh of thermal energy to do so.
At a COP of 3:
6 kWh ÷ 3 = 2 kWh of electricity
At a COP of 2.5:
6 kWh ÷ 2.5 = 2.4 kWh of electricity
Just keep in mind that real systems also consume electricity for the fan, compressor, controls and other bits and bobs, and performance does change throughout the heating cycle.
How much electricity does it use per day?
Unfortunately, there isn’t a one-size-fits-all answer here. Hot water demand really varies between households.
The amount of hot water you use to have showers, baths, do hand washing or use in the kitchen all influence daily energy consumption. And it’s not just about the size of the cylinder – a 300-litre cylinder doesn’t reheat its entire contents from cold every day.
As a super simple example, if a household needs 7.5 kWh of useful heat for hot water each day and its heat pump does an average COP of 3, the theoretical electrical consumption for the heat pump would be approximately:
7.5 ÷ 3 = 2.5 kWh per day
Over 365 days, that’s approximately 913 kWh of electricity per year.
If the same 7.5 kWh of daily heat were supplied entirely by direct electric resistance heating, the theoretical annual consumption would be approximately 2,738 kWh.
Real household consumption, of course, can be higher or lower than that.
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What affects electricity consumption?
There are several factors that determine just how much electricity an air source heat pump water heater actually uses.
Hot water temperature
Higher storage temperatures generally require more electricity.
The difference between the air temperature and the water temperature you need affects the compressor’s workload. In general, higher hot water temperatures result in a lower COP.
For example, operating at 50°C usually works out more efficient than continuously producing water at 60°C.
Air temperature
Heat pump performance is also affected by the temperature of the air entering the evaporator.
Warm air is great for heat pumps – it provides a more favourable heat source. Cold air, on the other hand, increases the workload on the refrigeration cycle. This is especially important if a unit takes its air from outside or from an unheated space.
Hot water consumption
Households that have long showers and fill baths regularly will naturally consume more energy than households with relatively low hot water demand.
Choosing the right hot water cylinder capacity is therefore vital for both comfort and efficiency.
Does the immersion heater increase electricity consumption?
Yes, it can. An immersion heater provides around 1 kWh of heat for every 1 kWh of electricity consumed at the point of use. If the heat pump is already doing a COP of 3, it can theoretically provide around 3 kWh of useful heat from the same electrical input.
Frequent use of immersion boost, therefore, can significantly increase overall electricity consumption.
The immersion heater is still useful for backup, rapid recovery and high-temperature hygiene cycles, but a correctly designed system should normally allow the heat pump to provide most everyday hot water heating.
Do Legionella cycles use more electricity?
Yes, a periodic Legionella or thermal disinfection cycle can temporarily increase electricity consumption because the cylinder has to be heated to a higher temperature.
Some heat pump water heaters can get to the required temperature using the compressor. Others supplement the heat pump with an immersion heater.
The additional electricity used depends on the starting water temperature, cylinder volume, required final temperature and how the appliance performs the cycle.
How can you reduce heat pump water heater electricity use?
The most effective approach is to let the heat pump do as much of the water heating as practical.
Correct cylinder sizing, sensible hot water temperature settings, low standing heat losses, appropriate operating schedules and avoiding unnecessary immersion heater use can all help. In the end, how much electricity a heat pump water heater sucks up should be compared to the amount of hot water it actually gives you. A decent heat pump water heater designed right, with a good seasonal CoP, can easily save you on your electricity bill compared to using an electric immersion heater to heat just as much hot water – it’s the same volume we’re talking about here.