Does Running Your Air Con Use More Fuel?

It’s a fair question, especially with an eye on fuel costs: does switching the air con on actually make a noticeable dent in your fuel economy? The honest answer is yes, a little — but it’s rarely the full story.

Why it uses fuel at all

The air con compressor is driven by the engine, either directly via a belt or, on hybrids and EVs, electrically. Either way, running it takes some of the engine’s (or battery’s) output, so there’s a real cost. On a typical petrol or diesel car, running the air con can increase fuel consumption by somewhere in the region of 5–10%, more on short journeys or in stop-start traffic where the compressor is working relatively harder against the engine load.

Why a healthy system costs you less

That 5–10% figure assumes a system that’s working properly. A system running low on refrigerant, with a clogged condenser or a struggling compressor, has to work harder to hit the same cabin temperature — which means it costs more in fuel for a worse result. This is one of the quieter reasons to keep on top of servicing: a healthy air con system isn’t just more comfortable, it’s cheaper to run than a neglected one straining to do the same job.

The windows-down alternative isn’t free either

At low speeds and in town, windows down instead of air con is genuinely more efficient — there’s no meaningful aerodynamic penalty and no compressor load. Above roughly 80–100km/h, though, the aerodynamic drag from open windows can cost you more in fuel than the air con would have, so on a motorway run, air con is usually the more efficient choice.

The practical takeaway

For most driving, the fuel cost of air con is small enough that comfort (and, as covered in our piece on running air con in winter, keeping the system healthy) is the bigger factor. If you’re genuinely trying to squeeze out every last bit of economy on a short, slow trip, windows down can save a little. On a motorway journey, just use the air con.

If your air con feels like it’s working harder than it should to get cold, that’s usually a sign the system needs a look — our full air con service checks exactly that.