What happens if the property is vacant?

Even when a property is vacant there will be small amounts of energy used (i.e. the fridge will probably be on) and there may also be a standing charge to pay. This is usually a minimal amount. A property is only classed as "vacant" when there is no active tenancy, if there is a tenancy agreement in place for this period then the tenants are responsible, even if they haven't moved in yet. If the property is truly vacant then a landlord or letting agent would be responsible for the payment of this bill.