How often do you use your old deep fryer, roughly?
Yeah, it's true that air fryers consume less than a regular one, but not as much as people often say. A regular deep fryer eats up like 1500-2000W when it's heating, while an air fryer is more like 800-1500W depending on the model. So yeah there's a difference, but don't expect to cut your bill in half or anything like that. If you only use them a couple times a month, honestly the difference in euros per year is gonna be pretty minimal, like a few dozen bucks at most.
What you really need to consider isn't just the power consumption - it's also the purchase price versus how much you're actually gonna use it. A decent air fryer costs at least 80-150 euros, and if you're planning to keep using your old one for a few more years, do you really want to invest in that? Though there are other advantages to it: it's less messy, faster, and there's less steam everywhere. But if it's just to save on electricity, honestly it's gonna take a long time before you break even on your investment.