An féidir leat urrúsúnú a choimeád ar úsáid leictreachais?

This depends on the rules in force in your own country: deposits are typically intended for damage, not for electricity consumption determined retrospectively, and retention is generally only permitted if you have clearly specified in advance how you measure consumption and what fee you will charge. Without that basis, there is a risk that the guest could successfully reclaim the retained amount. Consult a legal advisor or industry organization in your country about this, and alternatively, consider a system where the guest pays in advance for electricity, separately from the deposit.