Is there a penalty if I leave my old supplier?

It depends if you are still on a contract. Some plans require you to sign up for 12-month contract, others for a 24-month contract. When your contract is over, you're free to switch to Bord Gáis Energy. However, if you leave before the end of the contract's duration, you'll usually be required to pay an early exit fee. Depending on the provider and plan, that fee could cost between €50 and €100.