When choosing a heat source for your home central heating system, heat pumps are now a mainstream, and well supported, technology. But, to some homeowners they are still not as well known or familiar as boilers.
And, while heat pumps are categorised a type of renewable energy, they do still require electricity which leads to homeowners wondering how much electricity will my heat pump use?
While everyone's costs will vary depend on a number of factors, not least of all the size of your home and how often you require heat, there are ways of understanding potential electricity costs, as energy expert, David Hilton explains.
Article continues belowHow do heat pumps use electricity?
Heat pumps have operational characteristics that are different from other heating technologies such as gas boilers or oil fired central heating. In simple terms heat pumps do not burn a fossil fuel, they move heat from one place to another. The heat pump is usually named after the place it extracts the heat from.
An air-source heat pump gets its heat from the ambient air around the heat pump, while a ground source heat pump extracts the heat from a large network of pipes buried in the ground. The heat is absorbed by a refrigerant circuit and then enhanced before being transferred into the central heating system.
The source of the heat (nature) may be free, but the transfer process uses energy, and that energy is electricity.
Why heat pump efficiency impacts electricity use
Heat pumps operate on an efficiency called a Coefficient of Performance (COP). This is the ratio of electricity used to heat the energy delivered into the home. So, if the heat pump uses 1 unit of electricity to move 4 units of heat to the home, then the COP would be 4 or 400%. The harder the heat pump works then the lower the COP.
So, what causes a heat pump to work harder? Well, there are two main reasons for the heat pump to work hard. Colder temperatures in the air or ground (the source), or the need for higher temperatures in the home.
The ideal scenario for an efficient heat pump, is a small temperature difference between the heat source and the required delivery temperature. If the air is 7°C and the required water temperature is 35°C then the temperature difference is only 28°C. In this scenario you may expect a COP of around 4.
But, if the air temperature is -7°C outside and the required water temperature is 55°C, then the temperature difference is now 62°C – more than twice the temperature rise, and therefore around half the efficiency.
As the season changes and the required heat loads change, so will the COP. So, in order to get a better grasp of the efficiency it is usual to use the Seasonal Coefficient of Performance (SCOP) which is the COP averaged over a year.
From these figures we can see that it is best to let a heat pump run slowly all the time. A common heat pump mistake is to use short start-stop cycles in the same way you might operate a gas boiler. However, this method makes the heat pump work harder as it tries to reheat the water, and needs to achieve a bigger temperature difference in doing so.
There are a number of other influencing factors regarding this matter, but to keep things simple, the COP and SCOP are the main headline contributors to the efficiency of a heat pump – and subsequently how much electricity it will use.
Which heat pump is more efficient – ground source or air source?
When it comes to weighing up the efficiency of an air source heat pump vs ground source heat pump, the efficiency of air source heat pumps tends to be more variable than a ground source heat pump. This is because the air temperature can change very quickly, and very frequently, sometimes many times in a day.
A ground source heat pump however gets its heat from the ground, where the temperature is much more constant. The ground temperature can be around 10°C at a depth of 1.5m, and this temperature may only drop a few degrees across the whole of the heating season in drier ground.
The consistent source temperature can mean that ground source heat pumps are slightly more efficient than air source heat pumps, mainly due to the fact the ground temperature will be higher than the average air temperature during the heating season, when underfloor heating and radiators will be used more often.
How much electricity will my heat pump use?
Given the variable nature of the efficiency of a heat pump, in order to determine how much electricity a heat pump uses will depend on two main factors – how much heat the home needs, and what the actual heat pump efficiency is.
Let’s say your home uses 12000kWh of heat energy per year. If the heat pump ran at a COP of 4 then you would use 3000kWh of electricity. But if the efficiency COP was closer to 2, then you would use 6000kWh of electricity.
You might then think that you simply multiply the amount of electricity by the unit price, but that is too simple an equation.
The exact amount of electricity used will depend a lot on when the heat pump runs the most. The lions’ share of the annual energy use will be in the heating season (mainly winter), and you may also have different electricity costs at different times, especially if you have an electric vehicle tariff or special heat pump tariff.
Although you will want to run your heat pump during the cheaper off-peak times, it is inevitable that it will also need to run during the more expensive peak times. On average your heat pump may have a COP of 3, meaning that you will use around 4000kWh of electricity per year to produce 12000kWh of heat.
If we assume that 50% (and that is an optimistic estimate) of the energy is used during off-peak times, then 2000kWh will be charged at around 8p (£160/year) and 2000kWh at 28p (£560/year).
That’s a total of £720/year. But, if you only use 25% of the energy in the cheap time, then your cost rises to £920.
Please also note that these cost scenarios are based on an average COP of 3 and 12000kWh per year for heating energy, and do not include standing charges or any other electricity use in the home such as car charging, lighting, laundry, cooking or any other appliances. Different rates may also apply to different tariffs from different providers.
In summary, if you are looking at ways to make sure your heat pump uses electricity effectively, you need to specifically manage the use of the energy, and try to target the energy use at cheap tariff times.
Solar PV panels and solar battery systems used with heat pumps can also help reduce electricity costs, by generating electricity that your heat pump can use. And finally, do not turn the heat pump off, make sure that you stay on top of heat pump maintenance with an annual service, and undertake regular checks on the performance and balancing of your central heating system.
