How Much Electricity Does an Air Source Heat Pump Use?

Deciding to install a heat pump involves a lot of questions, the main one being how it will impact your energy consumption. Air source heat pumps (ASHPs) run on electricity, which is more expensive than gas in the UK at time of writing. How much electricity an air source heat pump uses will vary depending on the size of the building, the insulation used and the efficiency of the chosen system, so we can’t give a definitive answer that’s true for all.

However, we can talk about the unit for unit efficiency rating, and make some estimates based on average energy consumption that may help you understand how switching to an ASHP will impact your electricity usage. We’ll also discuss costs, and how the energy use of heat pumps compares to gas boilers and electric heaters.

How Air Source Heat Pumps Use Electricity

ASHPs use electricity for three actions within the heating system.

1. The fans create a flow of outside air around the evaporator coil, which is filled with refrigerant fluid. The fluid has a very low evaporation point, which means when it absorbs even low levels of heat from the air, it vaporises.
2. ASHPs use electricity to compress the vapourised refrigerant. The compressor takes in vapour, then rapidly reduces its internal space to pressurise the vapour. This pressure generates heat, and the hot, dense vapour then flows into the heat exchanger to release heat into the water in the next chamber.
3. Finally, the ASHPs keep the refrigerant and the water moving using pumps. The water is pumped from the heat exchanger into a water tank or to radiators. After releasing its heat, the refrigerant condenses into a fluid, which is pumped back to the evaporator coil.

This cycle is highly efficient at generating heat, which makes it easy to compare the electricity use of heat pumps to other heating systems

What Is Coefficient of Performance (COP)?

The coefficient of performance (COP) is a ratio that describes the relationship of useful heat output to energy input, used to measure efficiency in heating and cooling systems.

COP = generated heating energy (kWh) / electricity consumed to generate heat (kWh)

In ASHPs, with each unit of electricity used, heat pumps create between three and four units of heat energy. That’s an energy conversion of 300 – 400%, or a COP of 3 – 4.

Considering electric heaters have a COP of 1, this is a big gain. It means the electricity consumed by your heat pump will be around three times less than electric heaters to heat the same sized building, even without further changes. The COP of a gas boiler is approximately 0.93, creating less heat than the energy used per unit.

What a heat pump’s high COP means for its electricity use is that heating the home will require between a third and a quarter of the electricity that electric heaters use.

Since it takes approximately 100 Watts per square metre to heat a room with electric heaters, we can assume a medium sized house (four rooms of around 15m2 each). That’s 6kW (6,000 Watts) to get up to an acceptable temperature. With a heat pump, the same amount of heat could be generated with just 2kW.

Seasonal Performance Factor (SPF) Explained

Of course, performance is affected by seasonal factors, such as cold weather. Do heat pumps work in cold weather? Of course, they do – but they have to work harder.

To generate enough heat, a certain amount of refrigerant must condense, which means more outside air will be drawn through the system during cold snaps than otherwise. The fans have to run faster for longer, and the system may be slower to heat than usual. This means air source heat pumps use more electricity during cold weather than in hot weather.

While COP is an average performance rating, the seasonal coefficient of performance (SCOP), or seasonal performance factor (SPF), accounts for the difference in outside temperatures between summer and winter.

SCOP or SPF is calculated with the same formula but using the total annual data.

SPF = generated heating energy per year (kWh/a) / electricity consumed to generate heat (kWh/a)

For example, a house with a heating demand of 10,000 kWh/a has a heat pump that uses 2,500 kWh/a of electricity to meet this demand. This gives an SPF or SCOP of 4.

SPF is more useful for real-world applications, as it gives a single figure that encompasses hot and cold weather. It’s the figure required to assess existing heat pump installations for retroactive grant eligibility, because it’s easy to calculate from energy bills and plans of the house.

Modern UK-installed ASHPs have SPFs in the range of 2.5 to 4.

Average Electricity Use of an Air Source Heat Pump

To work out how much electricity air source heat pumps use, we need to understand heat demand. The annual heat demand of an average household in the UK is 145kWh/a per m2. From this we can estimate that heat demands for different sized houses are as follows:

•       average-sized houses (around 82m2) need ~12,000 kWh/a
•       small homes (under 80m2) need ~10,000 kWh/a
•       for larger, detached buildings (over 124m2) need ~18,000 kWh/a

A heat pump with an SPF 3 could meet the 12,000 kWh/a mark using just 4,000 kWh of electricity.

Quality insulation and terracing can make a big difference to the heat demand of a property, as the heat generated will not be lost as quickly. The number of people sharing a house, the size of windows and double glazing will also have an impact.

How Much Does It Cost to Run an Air Source Heat Pump?

Based on current average electricity prices in the UK, we can use the above information about electricity use to make a judgement of how much it costs to run an ASHP on various sized houses over the course of a year.

NB: Electricity Demand based on estimate of UK average heat demand of 145kWh/m2 per year.

In short, an air source heat pump with SPF 3 could meet the annual heat demand of an average-sized house for a running cost of £1,107.60.

Unfortunately, because electricity is up to 4 times more expensive than gas in the UK, this may not be the savings you’d hoped for if you’re switching from a gas boiler. The added efficiency makes up for the price difference per unit, so gas heating and electric heat pumps ultimately cost a similar amount to run.

However, if you currently have electric storage heaters or radiators instead of central heating, your bills could be reduced by up to 75% thanks to the efficiency of heat pumps.

Factors That Affect How Much Electricity a Heat Pump Uses

Heat pump usage varies significantly from household to household, as different lifestyles and situations influence how people use their heating. Also, each property in the UK has vastly different heat demands because of their insulation, layouts, ventilation and radiator placements.

Home Insulation and Heat Loss

Houses built before 1960 in the UK generally do not have cavity wall insulation, in which walls are built in two layers with insulation between. Insulating cladding can help amend this, adding a layer outside the house, but many homeowners do not wish to cover their brick frontages.

Floors are easily insulated using thicker tog rated underlay, and loft insulation is vital to retain heat properly, as heat rises. Double glazing and properly sealed doors can also prevent heat loss. When all these areas are well insulated and double glazing is installed in all windows, the amount of electricity your heat pump will use.

This is because the heat your ASHP generates will be less able to escape into the outside world when you have good insulation, which means the heat pump won’t have to “top up” the heat as frequently.

Heat Pump Size and System Design

It’s extremely important that your heat pump system is the appropriate size for your property. An oversized system will use more energy than it needs to, while an undersized system will struggle to heat your home effectively.

A professional heat pump installer will calculate your heat loss and advise you on the best ASHP for you, as well as other amendments you can make to the property to improve efficiency, such as insulation.

Flow Temperature and Heating Controls

The temperature of heated water flowing through your radiators is the ‘flow temperature’. Lowering this can improve efficiency, as hotter water cools more quickly. So, heating your home with water that is too hot can actually lead to greater heat loss, even though you’re using more energy. This is true whether you have a gas boiler or a heat pump, so a quick win is to adjust your flow temperature on your boiler.

ASHPs typically heat your water to between 35oC and 55oC and have weather compensation controls built in to reduce energy use when the temperature is warmer. This avoids overheating by matching the flow temperature to the real heating needs and providing a steady flow of warmth, rather than a quick blast of heat.

Can Solar Panels Reduce Heat Pump Electricity Usage?

Solar PV can offset electricity usage, optimising your heat pump system and saving you thousands of pounds per year. With a photovoltaic cell or solar panel installation on your roof, you can generate your own electricity, meaning the ASHP does not take anything from the national grid. You won’t have to pay to heat your home again unless your solar panel stops working (which is unlikely).

While solar panels generate electricity during the day, heat pumps tend to see more use in the early morning and evening hours, so solar energy storage is an important part of linking solar and heat pump systems.

Is an Air Source Heat Pump Expensive to Run?

Compared to electric heaters and gas boilers, air source heat pumps are not expensive to run at all.

Heat pumps are so efficient that they use up to a quarter of the energy that other heating systems consume, whether gas or electricity.

This means that if you’ve been using electric radiators to heat your home or business, an air source heat pump will save thousands of pounds every year. There will be an initial investment to install wet central heating, such as radiators and a water tank, but the long-term running costs of heat pumps are worthwhile.

If you currently have a gas boiler, your household heating costs with an ASHP will be marginally higher, but not as much as you might expect. Even though electricity is more expensive than gas, the heat pump generates almost three times more heat per unit of energy, or almost 5 times with the most efficient heat pumps when correctly installed.

With improved insulation and a high efficiency system, switching to electricity is more likely to save gas users money in the long term when they install a heat pump. It’s important that these are properly designed and installed to get the best efficiencies.

The main motivator for many people to switch to an air source heat pump is that it uses electricity, not natural gas. Removing fossil fuel use from your heating system can dramatically reduce your carbon footprint, and while it’s important that governments regulate corporate emissions more tightly, homeowners can focus on what we can control to make a real impact.

In the long term, gas prices are likely to rise as supplies diminish and governments seek to discourage use. Over the next ten years, we hope to see electricity prices reduce to balance this. It will also ultimately increase your property value as buyers seek more eco-friendly options. Either way, in the long term, switching to a heat pump offers long term savings.

Get an Accurate Estimate for Your Home

Personalised assessments can help you understand whether a heat pump is right for your property. Usage depends on whether your building has appropriate insulation and radiator types, and there may be work you need to carry out before a heat pump will be effective at heating your home.

To get a professional assessment in the West Midlands, you can get in touch with the experts at Rollings Renewables. Our team can help calculate your heating demand and assess how much electricity an air source heat pump will use for your particular property, depending on insulation, property size and proximity to other houses.

Contact us to book a free consultation.