If you are on a standard variable tariff (SVT) with your current energy supplier, you can switch to a new supplier at any time without any penalties.
If, however, you are on a fixed term deal with your current supplier this may come with exit fees which may be as much as £60 if you are dual fuel customer i.e... have your gas and electricity with the same supplier.
Don't let these exit fees put you off switching as it may well be that that the savings that you make by switching to a better deal will outweigh the exit fees! Some suppliers are offering deals where they will pay your exit fees if you switch to them!
If you are on a standard variable tariff do the calculations, find the most competitive deal and subtract the exit fees from the savings you would make by switching. If you are still going to make a saving pay the fees and switch!
Another thing to remember is that if you are near to the end of your fixed term deal, i.e. within the last 49 days of your contract, then your supplier is not allowed to charge you fees for switching. If you are nearing the end of your contract and your supplier does try to charge an exit fee then refer them to Ofgem's rules around the switching window.
Of course if you want to stay with your existing supplier and they can offer a better deal than the one you are already on, then most suppliers will not charge exit fees if you want to switch to another of their tariffs.