How do you settle with a guest who leaves early?

You don't need to arrange anything here: the guest buys electricity credit in advance at your kWh price, and that credit is not linked to the length of stay. If the guest leaves early, the unused amount simply remains on their phone. They can choose whether to keep it for a next stay or request a refund, without you having to do anything. Note: this only covers electricity costs, not any refund of the rental price, as that is handled through your own cancellation terms.