There have been long discussions about guests charging their electric cars and who should pay for the electricity.
Here's the latest thread
viewtopic.php?t=28201&highlight=electric+car+automobile
My own feeling is that, unless you already pay for the diesel or petrol that guests put into their cars, why should you pay for the electricity used to run their car? In other words, unless you can direct your potential guest to a charging point nearby, they must expect to pay you some sort of surcharge to cover the electricity used to recharge their car.
More points to consider:
Once you have ascertained whether your electricity connection will stand this extra demand, and maybe checked with your insurance too, where exactly is the power outlet that they will be plugging that extension lead into?
Do you have an outside socket? If not, they will presumably be leaving either a door or window open and trailing the lead through that. If so, the house is not particularly secure, and unless they are ultra responsible, the lead may become pinched if door or window is closed on it, thus leading to risk of electrical fire. Don't think your insurers would be too happy with that scenario.
Plus, is the extension lead an ordinary one for general use, or should it be a special one specifically made for charging cars?
Will they be bringing their own lead or are you expected to supply? If their own, and if it's a general purpose one, how are you to know it has the correct fuse in it to protect your property?
And don't forget about the trip-risk to having trailing cables lying around on your property - how close to your house can they park their car? Would any neighbours have to cross the same area and avoid the extension lead?
Perhaps you had best sit down with a pen and paper and list all the things that MIGHT go wrong if you agreed to this request. (Think Basil Fawlty scenario!) Then list how each might be prevented from occurring, bearing in mind that guests very often leave their "Thinking Heads" at home when on holiday! Then see how you feel about it.
I know it's tempting to accept the booking, but sometimes it's better for your peace of mind to just say no. However, it's your business, and the choice is entirely yours, but preferably made from a position of knowledge
Good luck with whatever choice you make.