An EPC (Energy Performance Certificate) has been a legal requirement on a rental property since 2008. An EPC will give a property an energy efficiency rating from A to G, this will allow tenants to compare properties on an energy efficiency basis. You do not need to get a new EPC with every new tenancy as they are valid for 10 years, but any improvements made to a property after an EPC was issued may improve the ratings. A copy must be provided to any new tenant.
From the 1st April 2016 a tenant can request energy efficiency improvements to a property and reasonable requests cannot be refused.
From April 2018 all rental properties must have a minimum rating of E or better, there is also plans for a minimum rating of D or better from 2025 (although this legislation has still not been passed).