I'm totally with you on this Phil. But with guests not appreciating the cost of electricity it would be great if there was a 'friendly' solution.
The sensor gadget would be ideal if it was day v night sensitive and would need to be hardwired since I could see guests removing batteries otherwise!
Our electric meter is on the other side of the site so it would be a fiddle to operate a before & after reading.....& a calculation......by the gardener! Then he has to get the payment from the guest!
Increased rental costs are risky as you run the risk of not having an electricity bill at all !!
We've tried getting the gardener to switch off the mains (access by front door) if he notices aircon working while guests are away for the day but he isn't able to be there at all the appropriate times.
We've tried the key card but it can be circumvented.
So...no easy solution. I await further suggestions with keen anticipation.