The cheapest rates are going to have shorter contract terms. For instance, a 3 month term is cheaper than a 6 month term and a 6 month term is cheaper than a 12-14 month term. So if you are able to remember to call before your contract expires (they will notify via mail you that your contract is about to expire), then the 3 or 6-month term is your best bet. If you don't call to renew/cancel, you will be charged a much higher variable rate on a month-to-month basis. If this is you, sign for a 12-month contract term with the cheapest rate.
On the one hand, long-term, fixed-rate (contract) plans offer stability in pricing. If energy supply costs suddenly go up in your area, you won’t be left paying more than what you bargained for. You’ll have peace-of-mind. If you want to switch out of your contract before it ends with a lower cost plan, you’ll likely face a cancellation fee (early termination fee).