What if a guest charges their car on my electricity?

An electric car sometimes consumes as much power in a couple of nights as the rest of the house does in weeks, and you only notice it at settlement, when the guest is no longer reachable. House rules or loose timers don't work well, because you don't always know in advance that charging will take place. One solution is a separate, chargeable charging point at the driveway: the guest purchases credit there separately at a rate you set yourself, separate from the credit for the accommodation. If you don't have a charging point yet and this rarely happens, then measuring ordinary power consumption is a more logical first step.