Consumption depends on the heat that has to be removed and the temperature difference the system works across. Setpoint, solar control, filter condition and outdoor unit airflow all have a large effect, and several of them are free.
What you are paying for
Electricity consumed by the compressor, the fans and the controls. The compressor dominates. How much it consumes depends on two things:
- How much heat has to be moved – the room’s cooling load.
- How hard it is to move it – the temperature difference between the room and outside, which determines the coefficient of performance.
Both are influenced by decisions you make, which is why running cost varies so much between apparently similar installations.
Running cost depends on the unit, the load, the setpoint, the outdoor conditions, how many hours it runs and your electricity tariff. Any single number would be wrong for almost everybody. We would rather explain what moves it – see what things cost.
The things that move it most
Setpoint
The single biggest lever you control. Every degree lower means a larger temperature difference for the system to work across, so it runs longer and less efficiently. Setting a room to a comfortable temperature rather than a cold one makes a substantial difference, and in practice most people are comfortable at a higher setpoint than they initially choose.
A related point: setting it very low does not cool the room faster. The system runs at full output either way; the setpoint only determines when it stops.
Solar gain
Frequently the largest single component of the load in a domestic room. Every watt of sunlight entering through glazing is a watt the system has to remove, and removing it costs several times less than not letting it in.
External shading – shutters, awnings, blinds outside the glass, a tree – is dramatically more effective than internal blinds, because internal blinds absorb the heat after it has already entered the room. Solar control film or glazing helps. Closing curtains on the sunny side during the day is free and genuinely effective.
Filters and airflow
Dirty filters restrict airflow across the evaporator, which reduces capacity and makes the system run longer for the same effect. In severe cases the coil ices, which stops it cooling altogether. Filters are user-cleanable on almost every unit and cleaning them monthly in heavy use is the highest-return maintenance task there is.
The outdoor unit
If the condenser cannot reject heat, operating pressures rise and the compressor works harder for the same cooling. Leaves and debris in the fins, a shrub grown across it, a bin in front, or a decorative screen all cost you money continuously. Keeping it clear is free.
Sizing
An oversized unit cycles rather than modulating, losing the efficiency advantage of inverter control. An undersized one runs continuously at full output. Both cost more than a correctly sized unit – see sizing air conditioning.
Why the efficiency rating does not tell you much on its own
Systems carry seasonal efficiency ratings – SEER for cooling and SCOP for heating – which are measured under a defined standard seasonal profile.
They are useful for comparing units and they are not a prediction of your bill, because:
- Your building is not the standard profile. Your load, your hours and your climate differ.
- Efficiency varies with conditions. A rating is a seasonal average; on the hottest day the system is well below it.
- Installation quality matters enormously. A unit with an excellent rating, undersized pipework, a poor vacuum and a fouled condenser will not achieve it.
- Setpoint is not in the rating. Two households with identical units can have very different bills purely from how they set them.
Use the rating to compare units. Use the other factors in this guide to predict your bill.
Cooling versus heating
Worth understanding because it changes when the system is cheap to run.
In heating mode, efficiency falls as the outside temperature drops, because the system is lifting heat across a bigger gap. On a mild autumn day a split system heating one room is frequently cheaper than firing the central heating for the whole house. On a genuinely cold day it is much less impressive.
In cooling mode, efficiency falls as the outside temperature rises – so it is least efficient exactly when you want it most, on the hottest afternoon.
The practical implication: a split system used for heating in the shoulder seasons is frequently good value, and used for heating in deep winter it is competing with your main heating system on unfavourable terms. Using it for spot heating one occupied room rather than the whole house is where it wins.
The free changes, in order of effect
- Raise the setpoint by a degree or two. The largest single lever.
- Close curtains or blinds on the sunny side during the day – and better still, shade outside the glass.
- Clean the filters monthly in heavy use.
- Clear the outdoor unit of leaves, debris and anything growing or stacked against it.
- Close doors and windows in the room being cooled. Cooling a room with the door open cools the corridor.
- Use the timer rather than running it all day in an empty house.
- Use fan or dry mode on humid but not very hot days – dehumidifying alone makes a room feel much cooler for far less energy.
- Do not set it very low to cool faster – it does not, and you will forget to put it back.
- Reduce internal gains where you can – a computer or a television left running in a cooled room is a continuous load.
- Have it serviced, so the charge is correct and the coils are clean.
Bills higher than you expected? Filters, the outdoor unit and the setpoint account for most of it, and all three are free to address.
Questions we get asked about this
How much does air conditioning cost to run?
It depends on the unit, the load, the setpoint, the outdoor conditions, the hours and your tariff. We would rather explain what moves it than publish a figure that would be wrong for almost everybody.
Does setting it lower cool the room faster?
No. The system runs at full output either way; the setpoint only determines when it stops. Setting it very low just means it runs for longer and costs more.
Is it cheaper to leave it on all day or switch it on when I get home?
For an intermittently occupied room, switching on when needed is generally cheaper. For a well-insulated space with a modest load, running continuously at a moderate setpoint lets an inverter modulate efficiently. It depends on the room.
Is air conditioning cheaper than central heating for heating one room?
In mild weather, frequently yes – heating one room with a split system rather than firing the whole central heating system is often good value. In deep winter the advantage narrows considerably.
What is dry mode for?
It prioritises dehumidification over temperature reduction. On a humid but not extremely hot day it can make a room feel considerably more comfortable for much less energy than full cooling.
Do dirty filters really cost that much?
Yes. They restrict airflow, which reduces capacity and makes the system run longer for the same effect. In severe cases the coil ices and it stops cooling altogether. Cleaning them is the highest-return maintenance task there is.
Need this doing?
Call and describe the problem — you will speak to an engineer, not a call centre. Written quotes, no estimates over the phone.