To ask guests to pay for air con?

Yes I do, because I wouldn't dream of offering to host my family then a week before arrival tell the no aircon or pat for aircon, on a tropical island.

If money’s that tight they can stay at home for free, no-one has forced them to book flights.

I voted YANBU but only if you pre-warn them. As they aren't paying to stay in your villa, they can afford to spend on the air-con but you can't land them with the charge as soon as they show up. They need to be made aware of it before they depart.
I would be very angry if someone offered their villa to me (even if they didn't charge for the villa) and then as I was leaving they presented me with a bill for the electricity usage during my stay. I'd want to have been pre-warned and prepared for that.

Do you really need to be walked through the fact that aircon is not a given in a private home and if they were that concerned they could have asked?

I do think for the future if people ask to come and stay (who are not close friends) you make this clear at the time of booking their flights. So ‘yes but just to let you know we don’t use the air con as too expensive - you can turn on if you are happy to shoulder the cost’. Then they are aware of total costs.

Yes, I would take the fuse out (or equivalent) to make it stayed "broken"!
If they can't stand the heat they shuldn't go to stay on a tropical island:

I used to live in Bermuda and the cost of electricity was unbelievable. I didn't have air conditioning in my rented place. People I didn't know that well often used to ask if they could come and stay and, in the early days, I agreed so I can see how this happens.

I'd ask them not to switch it on or put signs over the controls saying "do not use" or similar.

When I lived in the tropics, it used to drive me nuts that guests would leave the AC running all day, even when they were out. Don't let them think this is acceptable.

Say the aircon is broken.
Easier.
Disable it if you can.

So when you stay with friends/family instead of at a hotel you don’t consider this a saved hotel cost? How odd.
If they cannot afford the cost of the aircon then they can just tell the host. I agree with the poster who mentions heating costs. If a friend stayed at my place when I was away ( invited or on their own idea) I would say if you increase the heating you’ll have to pay the extra costs. If they didn’t like this they could stay elsewhere and hotels are expensive. If they could not afford the extra then they would have to wear extra clothing which I would provide- shawl, large wrap, slippers etc

To avoid any unawkwardness I would turn off completely and then tell them you only use the fans due to the expenses and tell them £15 to use overnight so the fans are the only things used.
If they then said oh we are happy to pay £15 then I would let them use it.
I wouldn't ask them to though.

Why are they staying with you OP ?

I agree with this tbh. Not a dig, but I just wouldn't have agreed for them to come since they aren't even close friends. I'd suck it up unless finances are super tight. If they are, then ask for some money. Don't let people use you stay in your nice villa in tropical location unless they're good friends though.

Not ridiculous at all, flights to a 'tropical island' are probably very expensive, maybe they usually go to Spain all inclusive for the cost of flights, including a working aircon. I wouldn't invite family to stay with me and then tell them no aircon, this type of thing needs to be mentioned before the time of booking. I've lived in Thailand, electricity was costly, I always had the aircon on at night, my friend never did but I cannot sleep in humid heat as I find it unbearable, so to tell family no aircon unless you pay £15 a night is a joke, just maybe they cannot afford to pay this additional cost they're being told about after booking their expensive flights. Not everyone has spare money for unexpected costs.

I would perhaps email/call them in advance and just give them a heads up so they can budget for it if needed, but personally if I were staying with someone I wouldnt want to be uncomfortable (and hate being overly hot at night) but also wouldnt want to have someone out of pocket so would rather know so I could pay my way.

That does seem so expensive. Is that to run for the whole night? We have 2 air con units and would never use them if they were close to that for use. But we also have timers on ours and usually just run them for an hour at bedtime when its hot, and if we wake up during the night hot, pop it on for a bit but never leave it running the entire night. Is that an option?

If you can't afford to make them comfortable don't invite them

I have heating in my house but I don't have it on all the time as I can't afford to. If someone was using my house as a free holiday home I sure as hell wouldnt want to fork out for them to put it on overnight

And before you ask why I have heating it's because it came with the house 😁

Do we know if the OP invited them? This is crucial I feel.

If you tell them the aircon is broken, they may try to put it on anyway and then tell you it's fixed. Perhaps say your energy has massively increased recently, and because of this if they wish to use the Aircon then you would need to be reimbursed as you do not use it for that reason.

How ridiculous- they are staying rent free so they are not paying accommodation. Therefore they are keeping the money they would have spent on accommodation elsewhere. If you stay at a friends/families place instead of paying for a hotel fo you not think of this as a cost saved. If you have family/friends living in say London and you travel down to see them from Scotland for example you save a lot of cash by staying with family instead of at a hotel. You make it sound like a financial hassle to pay the flights and time off work. I think it must be a good deal to visit somewhere different and be hosted by trusted friends.

Silly answer as using the aircon wasn't explained before they booked the flights.

I think it's reasonable to say you don't use the AC because it's too expensive and the fans are good. Hopefully they will take the hint that the AC is available if they pay for it. We'd probably pay it we like to he cool at night

That. If I was having free use of a villa on a tropical island, I wouldn't be remotely offended by being asked to pay for the Aircon.
But as the owner of the property, I'd feel more secure having it switched off centrally at the beginning, and having the conversation about whether they want it at cost, when they arrive.

Don’t go then.

Absolutely this, they are NOT saving £1000 as if you didn't live there they wouldn't be visiting. If you cannot afford the cost of electricity to host your visitors you shouldn't have invited them, or you should've mentioned before they stumped up the costs of flights and booked time off work.