Can a guest get an electricity invoice for his employer?

Yes, a business guest purchases electricity like any other guest and receives a proof of purchase showing the amount, date, and the accommodation it was for, which is usually sufficient for a reimbursement claim. You as the landlord appear as the supplier on that proof and do not need to make any adjustments yourself. If an employer wants everything on a single invoice or with a cost centre number, this is not a standard feature and must be resolved manually or in consultation with the guest.