How do I explain electricity billing to regular guests?

Tell them well in advance, separately from the confirmation email, and explain that the change applies to all guests from a certain date, not specifically to them. Emphasize that it won't automatically become more expensive, because the part that was previously hidden in the rental price now appears separately, and you as the host don't earn anything extra if you set the price at cost. Immediately explain how it works in practice: QR code, prepaid balance and a notification before it runs out, so the refrigerator and wifi always stay on.