Air to air heat pump running costs: how to work them out

Why there is no single answer

You will see confident running cost claims online. Treat them carefully. The Energy Saving Trust says plainly that there is not much data available on how air to air systems perform in the UK, so it is difficult to know how much money you would save. It depends on what you are replacing, how the system is designed and how efficiently you use it, and the Trust recommends asking your installer to estimate running costs based on their design for your home.

What you can do is understand the calculation, so you can check any estimate you are given.

The basic formula

Running cost for heating = heat needed ÷ SCOP × electricity price

Term	What it means	Where it comes from
Heat needed (kWh per year)	the heat your rooms need over a year	a heat loss calculation, or your current heating use
SCOP	heat delivered per unit of electricity, averaged over a season	the product label and data sheet for the exact system
Electricity price (p per kWh)	what you pay per unit	your bill or tariff

If you are replacing electric heaters, a good starting point for heat needed is how much electricity you currently use for heating, because an electric heater turns one unit of electricity into one unit of heat. If you are replacing gas, take your annual gas use for heating and allow for the boiler’s efficiency.

An illustrative example

These numbers are made up, round and only for showing the method. They are not a quote or a prediction for your home.

Say a flat needs 4,000 kWh of heat a year. The system’s SCOP is 4. Electricity costs 25p per kWh.

• Electricity used: 4,000 ÷ 4 = 1,000 kWh
• Cost: 1,000 × 25p = £250 a year

The same 4,000 kWh from direct electric panel heaters would use 4,000 kWh of electricity, so four times as much. That is why air to air systems can make a big difference in flats heated by electric heaters. Replace the made up figures with your own and the method works the same way.

Replacing gas in a Barnet semi

Many homes in Finchley, Hendon and Mill Hill are 1920s and 1930s semis with gas boilers and radiators. For these, the comparison is with gas, and it is closer. Gas is cheaper per kWh than electricity, so the heat pump’s efficiency has to make up the difference. Whether it does depends on the SCOP you actually achieve, your gas and electricity tariffs and how well the house holds heat. Solid walled houses and large bay windows lose more heat than cavity walled ones with modern glazing. Our Barnet area page looks at the local housing in more detail.

Two practical points for Barnet homes:

• most air to air systems do not heat water, so you would still need a hot water source, which has its own running cost
• the Boiler Upgrade Scheme grant of £2,500 for air to air requires the system to replace your existing heating and be sized for the full space heating need, so a top up system running alongside a gas boiler is unlikely to qualify

What raises or lowers your costs

Pushes costs down	Pushes costs up
good loft and wall insulation	draughty sash windows and open fireplaces
system sized for the real heat loss	oversized units cycling on and off
steady, lower set temperatures	big swings and boosting from cold
clean filters and coils	clogged filters, blocked outdoor coil
a tariff with cheaper off peak periods	heating mostly at peak rates
outdoor unit with free airflow	unit boxed in or blocked by leaves

The Energy Saving Trust also points out that air to air units recirculate room air rather than bringing in fresh air, so you still need normal ventilation.

Cooling costs

In summer the same system cools, and its cooling efficiency is shown as SEER. The method is the same: cooling needed ÷ SEER × electricity price. In London, the number of hot days varies a lot from year to year. The Met Office’s London City Pack lists increased energy demand for summer cooling as one impact of warmer summers. A well sized system cooling a bedroom at night uses far less than one cooling a whole open plan floor all afternoon.

Questions to ask an installer

1. What heat loss did you calculate for each room?
2. What SCOP and SEER does this exact combination of units achieve?
3. What electricity price did you assume?
4. How will hot water be provided, and what does that cost to run?
5. What happens to efficiency on the coldest days?

A good installer will show the working. Our air to air heat pump page explains how we design for winter heating, and our guide to using air conditioning for heating explains how reversible systems behave in cold weather.