The length of a contract is often, but not always, related to its price. You may get a better deal in return for committing yourself for longer. But be sure that the price is really fixed and cannot be increased as the supplier wishes during the contract. Read the contract carefully – some cost elements may not be fixed even though the fuel cost and the period may be.

You need to ask any prospective supplier how long the contract offered is for as, once you sign up for a fixed period, you won’t be able to change your supplier until that period comes to an end unless you pay (sometimes substantial) termination fee.
 
Before signing up remember that, although price is important, and usually most important, there could be other reasons why you might want to change supplier, perhaps if you receive poor service. Unless you have experience of the supplier you are contracting with, be cautious about committing yourself to long term contracts.