Wondering how much your AC will cost to run and how we estimate what size unit you will need?
- Why air conditioning running costs vary
- The two phases of air conditioning use
- Room size bands we use
- How electricity prices are used
- Why we avoid giving one fixed running cost
- Cooling mode vs heating mode
- What can make your actual running costs higher?
- Why professional sizing matters
- What our estimates should and should not be used for
“How much does air conditioning cost to run?” sounds like it should have a nice, simple answer. Sadly, homes are awkward little boxes full of variables.
The honest answer is - it depends.
It depends on the room, the system, the weather, your electricity tariff, your insulation, your temperature setting, how long you use the unit for and whether someone has decided to leave the patio doors open while trying to cool the room. Bold strategy. Expensive, usually.
At Heatable, we use a consistent method to estimate air conditioning running costs so customers can compare different system sizes and understand likely usage costs before installation.
But these figures are always estimates, not guarantees. They are designed to be useful, logical and transparent, not pretend we can predict your exact future electricity bill down to the penny.
In this guide, we explain how we estimate air conditioning running costs, what assumptions sit behind the numbers and why your actual costs may be higher or lower.
💨 Ready to chill without the guesswork? Find out exactly what size air conditioner you need - and get a fixed price in seconds here.
Why air conditioning running costs vary
Two homes can have the same air conditioning unit and see different running costs.
That is because the unit is only one part of the calculation. The room and the way the system is used matter just as much.
The main factors include:
The size of the room being cooled or heated.
The cooling or heating capacity of the unit.
The actual electrical input of the unit.
The system’s efficiency.
The outdoor temperature.
The desired indoor temperature.
Insulation levels.
Glazing type and window size.
Room orientation, especially south-facing and west-facing rooms.
Whether blinds, curtains or shading are used.
Whether doors and windows are left open.
The number of people in the room.
Internal heat gains from appliances, cooking, TVs, games consoles and gaming PCs.
The local electricity tariff.
How many hours the system is used.
Whether the unit is cooling, heating, dehumidifying or simply maintaining temperature.
A cool bedroom at 10pm and a sun-baked glass kitchen extension at 3pm are not the same job. Your air conditioner knows this. Your bill does too.
That is why sensible running cost estimates need to be based on assumptions, not magic.
The basic running cost formula
The basic calculation is simple:
Running cost = electrical energy used × electricity unit rate
Or, put simply:
Cost = kWh used × pence per kWh
So, if an air conditioning unit uses 1kWh of electricity and your electricity unit rate is 26p/kWh, that hour costs roughly 26p.
Simple enough.
However, there is one very important distinction:
The unit’s cooling or heating output is not the same thing as the electricity it consumes.
For example, a 3.5kW air conditioning unit does not necessarily use 3.5kW of electricity.
In most cases, the 3.5kW figure refers to the unit’s cooling or heating output, not its electrical input.
Fixed split air conditioners and air-to-air heat pumps move heat rather than creating cooling or heat directly, so the amount of electricity they draw is usually lower than the amount of heating or cooling they deliver.
This is where a lot of running cost confusion starts.
If someone assumes a 3.5kW air conditioner uses 3.5kW of electricity every hour, they will usually overestimate the cost.
Cooling capacity vs electrical input
There are two different “kW” figures that often get mixed up:
Term | What it means |
Cooling capacity | How much heat the system can remove from a room |
Heating capacity | How much heat the system can deliver into a room |
Electrical input | How much electricity the system draws from the mains |
Cooling capacity tells you about the unit’s performance.
Electrical input tells you about energy use.
For example, a 3.5kW split air conditioner might deliver around 3.5kW of cooling output, but its electrical input could be closer to 0.8kW to 1.2kW, depending on the model, conditions, efficiency and how hard the unit is working.
That is not a fixed promise. It is a simplified example.
Actual electrical draw changes throughout operation. Modern inverter systems can ramp output up and down depending on the room temperature, set temperature and cooling or heating demand.
This is why Heatable running cost estimates are based on likely electrical input and usage assumptions, not just the headline cooling capacity.
The two phases of air conditioning use
Air conditioning running costs are not always flat.
In normal use, there are usually two broad phases:
The start-up phase.
The maintenance phase.
The unit often works harder at the beginning, then uses less electricity once the room is close to the target temperature.
1. Start-up phase
The start-up phase is when the system first switches on and starts trying to bring the room to the chosen temperature.
If a bedroom is 28°C and you want it at 22°C, the air conditioner has more work to do than if the room is already 23°C.
The amount of energy used during start-up depends on things like:
The starting room temperature.
The target temperature.
The size of the room.
The heat stored in walls, floors, furniture and glazing.
The outdoor temperature.
Solar gain.
Insulation.
Draughts.
Whether doors and windows are closed.
This is usually the most demanding part of the cooling cycle.
2. Maintenance phase
Once the room reaches the target temperature, a modern inverter air conditioner usually reduces its output and uses less electricity to maintain comfort.
Instead of constantly switching fully on and fully off, an inverter system can modulate its output. In normal conditions, this can make it more efficient and more comfortable than older fixed-speed systems.
Think of it like driving.
Accelerating hard uses more energy. Cruising steadily is usually easier.
However, the system only settles into a lower-power maintenance phase if the room conditions allow it.
If the room keeps gaining heat quickly because of poor insulation, large windows, strong sun, cooking, or doors being left open, the system may need to work harder for longer.
The assumptions we use for estimated running costs
Heatable running cost estimates are based on a standardised set of assumptions so customers can compare options consistently.
Without assumptions, every estimate would be so full of “it depends” that it would be technically true but practically useless. Nobody wants a quote that just says, “Maybe 12p, maybe £12, best of luck.”
For modelling purposes, we generally assume:
Typical UK domestic room types.
A sensibly matched air conditioning unit size.
Broadly average UK domestic insulation and glazing.
Double glazing where applicable.
Typical summer cooling use.
A realistic indoor target temperature, usually around 21°C to 24°C, depending on the scenario.
Windows and external doors closed while the system is running.
A maintained, correctly installed system.
Current UK electricity unit rates, updated periodically.
Normal domestic occupancy.
No unusual internal heat loads unless stated.
For modelling purposes, we assume a typical UK home with broadly average insulation and glazing. Older, poorly insulated or highly glazed homes may use more energy. Newer, well-insulated homes may use less.
We do not assume every UK home performs like a new-build property. Many homes vary significantly by age, retrofit history, insulation level, glazing, airtightness and layout.
A Victorian terrace, a 1990s semi, a new-build flat and a glass-heavy kitchen extension are not thermally identical. Anyone pretending otherwise is selling vibes, not estimates.
Room size bands we use
Room size is one of the first things we look at when estimating likely system size and running cost.
As a simple guide, we may use room size bands like these:
Room type | Approximate room size | Typical example |
Small room | 10–15m² | Home office, nursery, small bedroom |
Medium room | 16–25m² | Double bedroom, larger office |
Large room | 26–40m² | Living room, main bedroom suite |
Extra-large room | 41–55m²+ | Open-plan kitchen, diner or lounge |
Room size is only one part of the calculation.
A small south-facing loft room can be harder to cool than a larger shaded room downstairs.
A kitchen with lots of glazing and cooking heat can need more cooling than a bedroom of similar size. A home office with a powerful PC, multiple monitors and direct afternoon sun can behave very differently from a spare room used occasionally.
That is why room size is the starting point, not the whole answer.
How electricity prices are used
Running costs are calculated using a pence-per-kWh electricity rate.
For example, if electricity costs 26.11p/kWh, then every 1kWh of electricity used costs 26.11p.
For simple appliance running cost estimates, we usually use the electricity unit rate, not the standing charge. That is because the standing charge is paid each day for having an electricity supply, whether you run your air conditioner or not.
A few important caveats:
The Ofgem price cap is not a cap on your total bill.
Unit rates vary by region.
Unit rates vary by payment method.
Fixed tariffs, smart tariffs and time-of-use tariffs can differ.
Some customers may pay less than the average unit rate.
Some customers may pay more.
Electricity prices change over time.
So, when we use an average UK electricity unit rate, it is a useful benchmark, not a personal tariff check.
For the most accurate running cost estimate, you should check your own electricity tariff and use your actual pence-per-kWh rate.
Example calculation
Here is a simplified example.
Assume an air conditioning unit draws 0.8kW while cooling.
Electricity unit rate: 26.11p/kWh.
Calculation:
0.8kW × 1 hour = 0.8kWh
0.8kWh × 26.11p = 20.9p per hour
So, in this simplified example, the unit would cost around 21p for one hour of operation at that average electricity rate.
That does not mean the unit will draw exactly 0.8kW every hour.
Real-world power draw changes as the system ramps up, settles down and responds to room conditions.
Here are some simple example costs using the same electricity unit rate:
Example electrical draw | Electricity rate | Approximate cost per hour |
0.5kW | 26.11p/kWh | 13p |
0.8kW | 26.11p/kWh | 21p |
1.2kW | 26.11p/kWh | 31p |
1.8kW | 26.11p/kWh | 47p |
These examples are simplified and based on electrical input, not cooling output. Actual running costs vary as inverter systems ramp up and down during use.
Why we avoid giving one fixed running cost
One fixed running cost would be misleading.
It might be neat for a headline, but it would not be honest.
Short evening cooling in a bedroom may cost relatively little. All-day cooling in a large, sunny open-plan room will cost more. Using the same system for heating in winter has a different cost profile again.
A better approach is to give a realistic scenario or range, based on:
Likely system size.
Likely electrical input.
Typical usage hours.
Room size.
Electricity tariff.
Whether the system is cooling, heating or maintaining temperature.
Even then, the estimate is only as good as the assumptions.
A poorly matched system can affect both comfort and efficiency. An undersized unit may run hard for longer. An oversized unit may be inefficient in other ways, depending on the system type, controls and room conditions.
This is why we prefer transparent estimates over fake precision.
Cooling mode vs heating mode
Many fixed air conditioning systems are also air-to-air heat pumps.
That means they can provide cooling in summer and heating in colder months.
However, heating costs should be estimated separately from cooling costs.
That is because:
Outdoor winter temperatures differ from summer cooling conditions.
Heat demand can last for longer periods.
The comparison may be against gas central heating, direct electric heating or another heating system.
System efficiency varies with outdoor temperature.
Room-by-room heating is different from whole-home heating.
Air-to-air heat pump heating can be efficient, especially when used to heat specific rooms. But it should not be lazily described as “always cheaper than gas central heating”.
Sometimes it may be cheaper. Sometimes it may not.
It depends on electricity prices, gas prices, system efficiency, room usage, the existing heating setup and how the home is used.
What can make your actual running costs higher?
Your actual air conditioning running costs may be higher than an estimate if the system has to work harder or run for longer.
Common causes include:
Factor | Why it matters |
Room size | Larger rooms usually need more cooling or heating output |
Insulation | Poor insulation means the system has to work harder |
Glazing | Large windows can increase heat gain |
Orientation | South and west-facing rooms can get hotter |
Temperature setting | Lower cooling targets usually increase energy use |
Electricity tariff | Higher p/kWh means higher running costs |
Usage hours | More hours means more kWh used |
System efficiency | More efficient systems deliver more cooling per unit of electricity |
Maintenance | Dirty filters and poor airflow can increase energy use |
Why professional sizing matters
Accurate sizing is not just about comfort.
It can affect:
Efficiency.
Noise.
Running costs.
Dehumidification.
System lifespan.
How quickly the room reaches temperature.
How comfortable the room feels once the system is running.
An undersized unit may run hard for too long and still struggle to cool the room properly.
An oversized unit may reach the temperature quickly but perform less smoothly, depending on the system design, controls and room conditions. In some cases, oversizing can also affect dehumidification and comfort.
A correctly specified system is more likely to maintain temperature efficiently and comfortably.
That is why Heatable does not treat room size as the only sizing factor. It is important, but so are layout, glazing, orientation, insulation, usage and heat gains.
What our estimates should and should not be used for
Our running cost estimates are designed to help customers make better decisions.
They can help you:
Compare likely running costs between system sizes.
Understand broad cost ranges.
See how energy tariffs affect costs.
Make a more informed buying decision.
Avoid confusing cooling capacity with electricity consumption.
Understand why your home may use more or less energy than another property.
They should not be treated as:
A guaranteed bill forecast.
A promise of exact savings.
A substitute for a technical survey.
A substitute for checking your own electricity tariff.
A prediction for every property type or usage pattern.
A guarantee of performance in unusual room conditions.
In other words, the estimate is there to guide the decision. It is not a crystal ball wearing a hard hat.
Important disclaimers
Running cost estimates are for illustration only. Your actual running costs may be higher or lower depending on your property, room size, insulation, glazing, system specification, installation quality, local climate, thermostat settings, electricity tariff and usage habits.
Electricity prices change over time. Any example costs based on Ofgem average unit rates should be treated as a snapshot, not a permanent price.
The Ofgem price cap is not a cap on your total bill. Your bill depends on how much energy you use, where you live, how you pay and which tariff you are on.
Standing charges are generally excluded from simple per-hour running cost examples because they are paid regardless of whether the air conditioning system is used.
Manufacturer efficiency data is usually based on standard test conditions. Real-world performance can vary.
Heating mode and cooling mode should be considered separately because the operating conditions and comparison fuels may differ.
Any running cost estimate should be read alongside the system specification, room assumptions and your own electricity tariff.
Next Steps For Your AC Journey:
When planning to install air conditioning for your home, there are several important factors to consider.
Make sure to refer to the following guides to help you make informed decisions:
To dive deeper into these topics, head over to our advice section, check out our YouTube channel for informative videos.
Get a Quote for Air Conditioning Today ❄️
Here’s why homeowners choose us:
Thousands of Happy Customers: We have an average score of 4.9 on Trustpilot, outperforming the market leader.
Which? Trusted Trader: Heatable is proudly recognised as a Which? Trusted Trader.
FGAS Certified: Our FGAS certification ensures all air conditioning work is carried out to the required industry standards.
Flexible Payment Options: Choose from multiple payment methods, including finance options.
Fixed Price Guarantee: Enjoy transparency with no hidden costs.
Save Your Quote: Save your quote and come back to it whenever you’re ready.
