How do you explain separate electricity billing to guests?

Landlords who are already doing this report that it's not as bad as expected: mention it briefly and without apology before arrival, for example in the booking confirmation, and explain the sign with QR code in the accommodation. Don't explain why you're doing it, as that invites discussion. Guests mainly ask what happens if their balance runs out (fridge, freezer and wifi always stay on, and you get a notification in advance) and whether remaining balance is lost (it remains and the guest decides themselves). Unclear deductions afterwards feel like a penalty, a balance purchased in advance and trackable in real time feels like transparency.