If you live in Northern Ireland, there are just five firms supplying households with electricity (this will soon be only four firms, as Electric Ireland is withdrawing from Northern Ireland energy market). But it's still worth checking if you can save by switching supplier or changing the way you pay for energy – some could slash £100s/year off their bill.
However, only some have the option of switching gas suppliers, as there are currently only two in Northern Ireland: Firmus Energy and SSE Airtricity, and which you are supplied by depends on where you live.
For more info, read out Cheap Northern Ireland energy guide.