Yes, you will need an Energy Performance Certificate (EPC). An EPC report will detail the energy efficiency of your property, giving it a rating from A (most efficient) to G (least efficient).
All landlords are required to purchase an EPC for a property before they let it and, from 1st April 2018, the property must have a minimum rating of E on its EPC. It will be unlawful to rent a property which breaches this requirement with a penalty of up to £4,000.