Back-billing is when a gas or electricity supplier charges you for energy used more than 12 months ago. Ofgem, the energy regulator, introduced new rules in 2013 that stopped suppliers from back-billing customers like this. nnThe rules say that if a customer has been with their current supplier for less than 12 months, the supplier can only bill them for energy used in the past 6 months. And if the customer has been with their current supplier for more than 12 months, the supplier can only bill them for energy used in the past 12 months. This means that customers are only ever billed for energy used in the past year. If you think you've been back-billed, you should contact your energy supplier. nnThey should be able to give you a refund if you've been overcharged. And if they can't help, you can contact Ofgem for advice.