What if more guests arrive than booked?

Extra guests are rarely malicious, but they do result in higher electricity consumption than expected, while asking questions afterwards or strict checks beforehand rarely yield results in practice and create hassle. If a guest settles their own electricity per kWh, the purchased credit will simply run out faster with increased usage and the guest will buy more themselves, without you needing to discuss it. This does correct consumption, but not the occupancy itself; rules about maximum number of guests remain a matter of your house rules.