Le lehet-e tartani a kaució egy részét az áramfogyasztásért?

It depends on what rules are in force in your country: a deposit generally applies to damage, not to electricity consumption determined afterwards, and a deduction is usually only permitted if you have clearly documented in advance how you measure consumption and what rate you charge. Without this substantiation, you risk the guest successfully reclaiming the deduction. Ask a legal advisor or a professional organization in your country about this, and as an alternative, consider implementing a system where the guest pays for electricity in advance, independently of the deposit.