Customers will be able to lock in prices until 31st March 2018 – three months more than the previous longest fix offered by npower. 

The plan will cost the average household £1,316 a year – just £52 more expensive than the average standard energy bill from the Big 6 suppliers, according to uSwitch.com.

But the comparison site said the new plan from Sainsbury’s Energy is almost £300 a year more expensive than the cheapest tariff on the market – Spark Energy’s Direct Debit Advance 3 at £1,025 a year, which means consumers will be paying significantly more for the lengthy protection.

The new plan also comes with a hefty £50 per fuel early cancellation fee, which customers will have to take into account if they want to switch before the fixed term is over.

Tom Lyon, energy expert at uSwitch.com, said: “While many consumers have been left disappointed by the way suppliers have handled the levy-led price cuts, the launch of a new longest fix term plan on the market shows that competition is still alive and kicking.

“This new offering from Sainsbury’s Energy gives consumers over four years of sanctuary from future price hikes. This peace of mind makes it a strong contender in today’s market albeit that it comes with a hefty price tag.

“I strongly urge consumers, if they haven’t already done so, to see how their current tariff stacks up against the very many new competitive deals that are on the market. There are substantial savings to be made which could significantly improve on the price cuts that suppliers have made.”

For those looking for cheaper, short-term protection, the cheapest fix on the market comes from First Utility, costing £1,037, £279 cheaper than Sainsbury’s Energy’s new offering, uSwitch.com has said.

However prices will only be protected until 31st July 2015, giving 32 months less than the new plan. The First Utility tariff also carries a £30 per fuel early exit penalty.

The next longest fix comes from npower with its Price Protector December 2017 tariff, which is just over £100 a year cheaper than Sainsbury’s Energy’s new plan, but has three months less protection.

However, the npower tariff is free from cancellation fees so customers can move away without penalty.