Can you withhold a deposit for electricity consumption?

This depends on the rules in your country: a deposit is usually intended for damage, not for retroactively determined electricity consumption, and withholding is generally only permitted if you have clearly established in advance how you measure consumption and what rate you charge. Without that substantiation, you run the risk that a guest will successfully claim back the withholding. Check this with a legal advisor or industry organization in your country, and consider as an alternative a system where the guest pays for electricity in advance, separately from the deposit.