Air Conditioning Running Costs Explained
An air-con unit’s cooling capacity is not what it costs to run — the electrical input, its efficiency and how long it runs are what land on the customer’s bill.
Customers routinely confuse the cooling output of an air conditioner with its running cost. A 3.5 kW unit does not use 3.5 kW of electricity — it moves that much heat while drawing considerably less power, because it pumps heat rather than generating it. Understanding that gap is the key to giving an honest running-cost estimate.
This guide sets out what actually drives cost — the electrical input in kW, the efficiency figures (COP and SEER), the run hours and the electricity unit price — with a worked example you can adapt on site, and a note on how modern inverter units change the picture. It closes with how to advise a client sensibly.
Key takeaways
- Running cost depends on electrical input (kW), efficiency (COP/SEER), run hours and the electricity unit price — not the cooling capacity.
- Rough hourly cost = electrical input (kW) × hours × unit price (p per kWh).
- COP and SEER describe how much heat is moved per unit of electricity; higher is cheaper to run.
- In heat-pump (heating) mode a good unit often achieves a COP of around 3–4, moving several units of heat per unit of electricity.
- Inverter units modulate output and are far cheaper to run than older fixed-speed "on/off" systems that cycle at full power.
What actually drives the cost
Four things set the bill. First, the electrical input power — the kW the unit draws from the socket, which is much lower than its cooling output. Second, efficiency: how much cooling or heating it delivers per kW drawn. Third, run hours: a unit cooling a busy office all day costs far more than one used for an hour on a warm evening. Fourth, the electricity unit price in pence per kWh.
Note the distinction between output and input. A unit rated at, say, 5 kW cooling might only draw around 1.4–1.6 kW of electricity at full tilt. It is that input figure — found on the nameplate or in the data sheet — that you multiply by hours and unit price, never the cooling capacity.
Use input kW, not output kW
The headline "kW" a customer quotes is almost always cooling capacity. Find the rated power input on the nameplate or spec sheet before estimating cost, or you’ll overstate the bill by a factor of three or more.
COP and SEER: the efficiency numbers
COP (Coefficient of Performance) is the ratio of heat moved to electricity consumed at a specific test condition — a COP of 4 means the unit shifts 4 kW of heat for every 1 kW of electricity. SEER (Seasonal Energy Efficiency Ratio) is the cooling equivalent averaged across a season, capturing part-load performance rather than a single peak point, so it better reflects real running cost.
Higher figures mean lower bills for the same output. When comparing units for a client, seasonal figures like SEER (cooling) and SCOP (heating) are more honest than a single peak COP, because a unit spends most of its life at part load, not flat out.
Cooling vs heating mode
The same box works both ways. In summer it moves heat out of the room; in winter, run in reverse as a heat pump, it moves heat in. In heating mode a good split system often achieves a COP around 3–4, meaning it delivers three to four units of heat per unit of electricity — which is why air-con-as-heating can undercut direct electric heaters and, at times, other fuels.
Running cost in each mode follows the same arithmetic, but efficiency differs with outdoor temperature: cooling gets harder as it gets hotter outside, and heating gets harder as it gets colder. Quote seasonal figures, and set the client’s expectation that a bitter winter day or a heatwave will pull efficiency below the headline number.
Put the estimate in the quote
An indicative running cost alongside the install price helps a customer decide with eyes open. TradePlanr’s air con running cost calculator gives you the per-hour figure to drop into the quote you build on site.
A worked example and cost by size
Take a unit drawing 1.5 kW electrical input, running for 6 hours, at an electricity price of 28p per kWh. The sum is 1.5 × 6 × 0.28 = £2.52 for the session. Scale the hours to the customer’s usage pattern for a daily or weekly figure. The same method applies in heating mode using that mode’s input power.
The table below gives indicative cost-per-hour figures by unit size at an assumed unit price, based on typical input power. They are a guide only — always use the actual nameplate input and the client’s real tariff for a firm estimate.
| Cooling capacity | Typical input power | Approx. cost per hour |
|---|---|---|
| 2.5 kW | ~0.7 kW | ~20p |
| 3.5 kW | ~1.0 kW | ~28p |
| 5.0 kW | ~1.5 kW | ~42p |
| 7.0 kW | ~2.1 kW | ~59p |
Inverters and how to advise a client
Older fixed-speed units run the compressor at full power then switch off when the set-point is reached, cycling on and off all day. Inverter units instead vary compressor speed, easing back to a low output that just matches the load. That means less start-up surge, steadier temperatures and markedly lower running cost — most units sold today are inverter-driven for this reason.
When advising a client, quote input power not cooling capacity, use seasonal efficiency figures, and base run hours on how they will actually use the unit. Point out that setting a sensible temperature, keeping filters clean and not leaving it running in empty rooms all cut the bill more than any headline efficiency figure.
Frequently asked questions
How do I estimate what an air-con unit costs to run?
Multiply the unit’s rated electrical input in kW by the number of hours it runs and by the electricity unit price in pounds per kWh. For example, 1.5 kW × 6 hours × £0.28 = £2.52. Use the input power from the nameplate, not the cooling capacity.
Is it cheaper to heat a room with air-con than with an electric heater?
Usually, yes. In heating mode a good unit achieves a COP of around 3–4, delivering three to four units of heat per unit of electricity, whereas a direct electric heater delivers one. Efficiency drops as it gets colder outside, so the advantage narrows on very cold days.
Do inverter units really cost less to run?
Yes. Inverter units vary compressor speed to match the load instead of cycling on and off at full power, so they draw less energy holding a steady temperature. This is why they have largely replaced older fixed-speed systems.
From guidance to action
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Air Conditioning Refrigerant Types (R32, R410A, R290)
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