Co-operative Energy will supply gas and electricity for the first 30,000 households to sign up to 'The Big Switch', the nationwide collective campaign from consumer group Which?.
For those who pay by direct debit and want a fixed rate deal, its main option is one of the cheapest on the market, based on a typical user. But if you prefer a variable rate tariff, it can be significantly beaten.
However, headline energy prices are based on a 'typical' user so true costs can vary wildly depending on where you live and how much energy you use.
Everyone who has registered an interest and provided tariff information will be sent a savings quote showing them how much they could save by switching.
This will include all deals, not just the two Co-op tariffs available. Therefore, households can choose which to plump for, which could make the Co-op option redundant for many.
More than 285,000 registered an interest but there is only space for 30,000 in total on the Co-op deals via Which?.
The consumer group estimates the deals will give typical savings of £123 a year.
The Co-operative Energy dual fuel (gas and electricity) tariff will cost £1,048 a year for a typical household paying by direct debit. The price per unit will be fixed for 12 months.
There is room for 20,000 on this deal. It is also available to existing Co-op Energy customers, although they won't count towards the 20,000.
An alternative fixed deal from First Utility costs £1 less per year.
EDF's Blue +Price Promise fixed tariff costs £1,054 a year. However, it also comes with no exit fees, which is a big bonus.
If customers pay on receipt of the bill, the Co-op deal will cost a typical £1,144. This tariff is variable and is available for 10,000 people.
First Utility's variable iSave v10 tariff costs a lower £1,027 for the average dual fuel user.
How to sign up
Sign-ups will be taken on a first come, first served basis until 28 May, only from those who have registered an interest with Which?. Co-op Energy customers can sign up at any time while the deal lasts.
If more than 30,000 people apply, subsequent switchers will be offered the EDF deal, which is available to anyone.
MoneySavingExpert.com consumer analyst Archna Luthra says: "Most importantly, this has acted as an interrupter to the energy market, which is exactly what was needed.
"Looking at the tariff in detail, it can be beaten, but encouraging people to switch supplier in a relatively flat market is important we'd urge people to be inspired and do a comparison for themselves."