It is pretty simple to determine the cost you pay the utility per kilowatt hour. The total cost of your electric bill divided by the number of kilowatt hours you used.
EV drivers are not trying to steal from anyone. The biggest convenience of EVs is that they can be fueled up while you sleep. Guests are just trying to avoid an additional stop in the middle of their vacation. I would list it as an amenity FOR A FEE. Simply ask drivers to pay you a certain number of cents per kilowatt hour that is equal to or greater than the price you are paying the utility. EV drivers should be capable of knowing what their car has consumed and will be EXCITED to pay for the charge.
Any loss from the rare person trying to underpay for the juice they used will easily be made up by the delighted guests who paid you more than the expense you incurred.