The same plate of food can cost wildly different amounts to cook depending on which appliance you reach for. None is best for everything; the trick is matching the tool to the task. Here is how the kitchen's three workhorses compare, and when each wins.
The microwave: champion of small and quick
For heating a portion, defrosting, or cooking something small, the microwave is almost always the cheapest, because it puts energy directly into the food and heats nothing else. There is no large cavity to warm and no preheating, so a few minutes of microwaving costs very little. For reheating leftovers or a single serving, nothing beats it on cost.
The air fryer: small meals, crisp results
An air fryer sits between the two. It heats a small chamber fast, so for one or two portions of something that wants a crisp finish, chips, roast vegetables, a couple of fillets, it undercuts the big oven comfortably. Its limit is capacity: cook for a crowd and you are running batch after batch, at which point its edge over the oven disappears.
The oven: built for batches
A full oven is the thirstiest for a small job because of all the space it heats, but it comes into its own when that space is full. A whole roast, several trays at once, or a big batch cooked to portion and freeze spreads the oven's energy across a lot of food, which is the cheapest way to feed many. The oven is not wasteful when it is full; it is wasteful when it heats a vast cavity to warm one small dish.
The rule of thumb
Reheating or a single small portion goes in the microwave. A small crisp meal for one or two suits the air fryer. A big meal, or batch cooking to freeze, justifies the oven. Match the appliance to the quantity and you spend the least without owning anything new. Put your own appliances through the cost calculator with realistic times to see the gaps in your own money.