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The short answer

Yes — running your air conditioning burns more fuel. The compressor that drives the system is powered by the engine, usually through a belt, so every time you switch the air con on you are asking the engine to do extra work. On a typical petrol or diesel car the penalty works out at roughly 5 to 10 per cent more fuel, and it can be worse in slow town traffic where the engine is already working inefficiently.

How much extra you burn comes down to your speed, the outside temperature, how hard the system is working and whether your car has a conventional belt-driven compressor or a newer electrically driven one.

Why air conditioning costs you fuel

An air conditioning system works by compressing refrigerant gas, then letting it expand and absorb heat from the cabin air. Compressing that gas takes serious effort — a compressor can sap several kilowatts from the engine when it is working flat out on a hot day, which is a meaningful chunk of what a small engine produces at cruising speeds.

That is why the effect is so noticeable at low speed. At 30 mph in traffic, that extra load is a large proportion of the power the engine is making, so the fuel penalty feels steep. At 70 mph on the motorway the engine has plenty of spare capacity, so the same compressor load is a smaller slice of the pie — though you are still burning more than you would with it switched off.

Other things push the figure around too:

• Outside temperature: the hotter it is, the harder the system works to pull heat out of the cabin.
• Cabin size: a seven-seat MPV takes far more cooling than a small hatchback.
• Sun load: a dark car parked in direct sun can hit 50°C inside, and the system has to shift all that heat.
• System health: a system low on refrigerant runs longer and harder for the same result.
• Driving mode: stop-start traffic with the engine idling is the worst case.

Windows down or air con on?

This is the classic pub debate, and the honest answer is that it depends on speed.

Open windows create aerodynamic drag. At low speeds the drag penalty is tiny, so dropping the windows is usually the cheaper option around town. As speeds rise, that drag grows quickly, and by motorway speeds open windows can cost you more in fuel than running the air conditioning would.

A sensible rule of thumb for UK driving:

• Below about 40 mph: open a window or use the vents and save the air con.
• Above about 50 mph: close the windows and use the air conditioning — the drag hurts more than the compressor.
• In between: do whatever keeps you comfortable and alert. A hot, stuffy driver is not a safe one.

Bear in mind that many modern cars run the compressor automatically whenever climate control is set below ambient temperature, so you may be paying the penalty without realising it.

What it actually costs in pounds and pence

Let's put numbers on it. Say your car returns 45 mpg and you pay around £1.40 a litre for petrol. A 10 per cent fuel penalty takes you down to roughly 41 mpg. Over 8,000 miles a year that is the difference between about 808 litres and 887 litres — nearly 80 litres, or something in the region of £110.

That is the worst case of running the air conditioning constantly for a whole year, which almost nobody in Britain does. In practice, using it through a warm summer and on muggy, misty mornings probably costs you £30 to £60 a year. Worth knowing, but not a reason to sweat your way up the M6.

How to keep the cost down

You can have comfortable air and a sensible fuel bill with a few habits.

• Park in the shade or use a windscreen sunshade. A cooler cabin needs far less cooling.
• Open the windows for the first minute when you set off to dump the trapped heat, then close them and let the air con take over.
• Use recirculation mode once the cabin is cool — you are then chilling air that is already cool rather than baking hot outside air.
• Set a realistic temperature. Aim for around 21°C rather than 16°C. The system reaches it faster and then works less.
• Run it regularly, all year round. Ten minutes a week in winter keeps the seals lubricated and the compressor from seizing.
• Change the cabin filter when it is dirty. A clogged filter restricts airflow and makes the fan work harder.
• Get it regassed when it stops cooling properly — typically every two to three years.

Buying advice: checking air con on a used car

If you are looking at a used car, the air conditioning is one of the easiest things to test and one of the most expensive to ignore. A failed compressor or a leaking condenser can easily run into several hundred pounds to put right.

Before you buy:

• Test it properly. Start the car, switch the air con to its coldest setting and let it run for a few minutes. The air from the vents should feel genuinely cold within about 30 seconds — not merely cool.
• Listen for the compressor clicking in, and check that the engine note changes slightly when it engages.
• Look at the condenser through the front grille. It sits ahead of the radiator and takes the brunt of stone damage and corrosion. Bent fins, damp patches or obvious rot are warning signs.
• Check for a musty smell, which usually points to a damp, dirty evaporator and possible mould in the system.
• Ask when it was last serviced. A recent regas and a new cabin filter is a good sign of a cared-for car.

None of this is complicated, but it is easy to skip on a cold, rainy test drive when the seller is keen to get back indoors. Take five minutes, be awkward about it, and you will avoid paying for someone else's neglected air conditioning later. Used sensibly, it is a small price for arriving cool, clear-headed and safe — ignored, it is an expensive problem waiting for the first hot week of July.