All good advice provided. I have had numerous guests have a similar problem and the only solution that has appeared to be the most efficient is :
Assume the guests will run the AC 24/7
Assume the guests will run the AC even with the doors and windows open, in a hope they will cool Spain and reverse global warming in the process!
Calculate (finger in the air or from the bill) the cost to run the AC. The preferred method is for a trusted key holder to provide before and after meter readings per guest visit
Add it as a sundry item to the booking fee (like a security deposit)
When your next bill comes in, if it is lower than expected, return the relevant, proportional amount to the guest.
All the other suggestions are very valid and I even installed some of the smarter solutions for a customer, whereby windows and doors opened or no movement detection would disable the AC. All these items have install costs attached and require support in the event of a problem.
I think it is very fair and reasonable to be descriptive to any rental prospect what the conditions of rental are and associated costs.
Hope it helps