The Big Six energy suppliers have overcharged their most loyal customers by as much as £7.3 billion.

That’s according to renewable energy company Bulb, which suggests each family that stuck with the same provider for the last five years overpaid by an average of £853 each.

The firms, which include British Gas, SSE, Eon, npower, EDF and Scottish Power, offer cheap fixed tariffs to attract new customers at certain points of the year.

Bulb says when these deals expire after a couple of years, the energy company automatically places the customer on their standard variable tariff (SVT), which is up to 30% more expensive.

In August, British Gas hiked its electricity prices for SVT customers by 12.5%.

Bulb Co-Founder Hayden Wood said: “These latest numbers show that so-called standard tariffs no longer have the customers’ best interests at heart. The Big Six need to show that they’re putting customers first, instead of profits.”

A crowdfunding couple trying to take on the Big Six energy suppliers have launched with 4,500 customers.