They are the smash hit kitchen appliance which have become wildly popular in recent years, especially with energy bills going up and up.

And as energy bills rise another £111 on average in April, the popularity of air fryers is not likely to drop any time soon as the appliances offer a way to cook food cheaper than a traditional big oven.

But advice from Martin Lewis sets out that it’s not always better to use an air fryer or a microwave instead of a traditional oven – the best option really depends on what you’re cooking.

Speaking on his BBC Martin Lewis Podcast on BBC Sounds, Spotify and Apple Music, the finance expert set out exactly how and when you should and shouldn’t use an air fryer when cooking the dinner using his ‘rule of thumb’.

Energy supplier Utilita, in its Power Price List, sets out that an air fryer costs £26.73 to run for a year for an average user, whereas a gas oven costs £33.22, and an electric oven £93.44.

But as Martin says, an oven has a much larger capacity, so if you’re cooking multiple things at once for a long period, you could end up having a lower cost than an air fryer. So an air fryer is only actually cheaper in practice if you don’t need to fill your oven’s capacity and you’re cooking ‘single objects’.

Martin Lewis firstly explained how to work out how much energy the device you want to use actually consumes, then showed what this means for air fryers versus ovens. It’s worth noting that energy units now cost 27p per unit for electricity from April, not 34p, but the comparison is the same because the devices still use the same amount of energy as they did before.

Martin Lewis said: “Let me give you my simple rule of thumb.

“First of all, find the wattage of the appliance you’re using. If you’ve got a heated gilet, it might be 50 watts. When I talk about ‘heating the home’ it is a lot cheaper to have a heated gilet or something that just heats you rather than heating the whole room.

“Then remember 1,000 watts is a kilowatt. And you pay roughly 34p per kilowatt per hour.

“So if you had a heated gilet on for an hour, it’s around a 20th of 34p which is around 1.7p per hour.

“Now an oven is typically 1-2000w so you’re paying around 34p or 68p an hour.

“An oven isn’t always using all the wattage as it’s heating up and then topping up.”

So, Martin said, if you’re cooking a whole turkey, you’re better off using your oven, but a smaller item like a single potato is much better off in an air fryer.

He added: “So if you’ve got a turkey in there that may well be effective if you’re cooking lots of stuff in there and it’s on for a couple of hours.

“But if you’ve got something small and simple in there…. that’s where the microwave and also the air fryers tend to come into their own because they’re better at cooking single objects.”

He also talked about a method to cook a chicken or turkey that was outlined by a caller, called ‘spatchcocking’, otherwise known as ‘butterfly’. This is when you break the bird’s bones and spread it out instead of cooking it whole.

By doing so, you can reduce the amount of time the chicken takes to cook, thereby saving more money on running your oven (or air fryer).