The confusion in every conversation about air conditioning running costs is the same: people multiply the cooling capacity by their electricity rate. A unit described as 3.5kW does not consume 3.5kW — that is the heat it moves, not the power it draws.
Because a heat pump moves heat rather than generating it, the input power is a fraction of the output. Getting that one distinction right changes the answer by a factor of three or four.
The calculation, in four steps
- 1
Find the input power
The datasheet lists power input in kW for cooling, usually as a rated figure and a range. Use that, not the capacity.
- 2
Multiply by your unit rate
Input kW × pence per kWh gives the cost of one hour at that rated draw.
- 3
Adjust for real operation
Inverter units rarely run at rated power. After the room reaches temperature they throttle back substantially.
- 4
Multiply by realistic hours
Count the hours you will genuinely run it — evenings in July, not twenty-four hours a day for a whole summer.
What COP and SEER are telling you
COP is the ratio of useful heat output to electrical input at a given condition — a COP of 4 means four units of heat moved per unit of electricity. SEER and SCOP are seasonal versions, averaged across a range of conditions, and they are the more honest comparison figures for a whole year.
Those seasonal ratings are also why air conditioning is a serious heating option. A modern unit delivering a SCOP around 4 puts out roughly four kilowatts of heat per kilowatt of electricity consumed — which changes the arithmetic against gas depending on the relative price of the two fuels at the time you look.
The habits that waste the most
- Setting an unnecessarily low target temperature — every degree costs, and 24°C is comfortable in most British summers
- Leaving windows and internal doors open while cooling
- Running with a clogged filter, which forces the fan and cuts airflow
- Never cleaning the outdoor unit's coil, so it rejects heat poorly
- Cooling rooms nobody is in, rather than using zone control
- Ignoring solar gain — closed blinds on a south window save more than any setting
Where the running cost really goes
| Factor | Effect on consumption |
|---|---|
| Target temperature | Large — each degree lower increases the work done considerably |
| Room heat gain | Large — glazing, orientation and insulation set the load |
| Unit sizing | Moderate — oversized units cycle and dehumidify less effectively |
| Filter and coil cleanliness | Moderate — neglected units draw more for less output |
| Fan speed setting | Small — but auto usually beats manual over a whole session |
Questions people ask
Is it cheaper to leave it on all day or switch it on when needed?
For occasional use, run it when the room is occupied. Where a room is used most of the day, holding a steady setpoint on an inverter system avoids repeated hard pull-downs and often works out similar or better.
Does air conditioning cost more to run than a fan?
Yes, considerably — a fan only moves air. What it cannot do is lower the temperature or remove humidity, which is the reason people move on from fans.
Is using it for heating cheaper than my gas boiler?
It depends entirely on the ratio between electricity and gas prices at the time, and on the unit's seasonal performance. It is genuinely competitive in mild weather, which is when heat pump performance is at its best.
Does the outdoor unit cost anything when idle?
Standby consumption is very small. The cost is in the compressor running, which is why controls and setpoint matter far more than standby.
Who does the work
- Gas Safe registered engineers — registration 554328
- REFCOM registered for F-Gas refrigerant work
- Worcester Bosch accredited installer
- John The Plumb Ltd, company registration 10026483
Input power, your unit rate, realistic hours. Three numbers, one minute, and you will have a far better estimate than any figure quoted on a forum.
Ask about efficiency before you buy
We can talk through seasonal performance figures for the equipment we would specify, and how sizing and siting affect what you will actually pay to run it.