The Big Six Energy Suppliers: Who They Are and What Became of Them

The Big Six were the six incumbent household energy suppliers that dominated Great Britain after privatisation: Centrica plc, the parent company of British Gas, E.ON UK, Scottish and Southern Energy (SSE), RWE npower, EDF Energy and ScottishPower¹. Ofgem's own glossary classes them as large legacy suppliers, defined as firms that have held at least 5% of the market in either fuel since the government transferred the ownership of gas and electricity from public to private ownership in the 1980s¹. EDF joined the group in 2002 and is French-owned².

The grouping is now historical. As a label it survives mostly in search boxes and in older consumer guidance. Two of the six, npower and SSE, no longer trade as household supply brands in Great Britain, while the remaining four still supply more than half of all energy customers, among them British Gas, EDF Energy and E.ON Next³. Concentration itself has barely shifted: Ofgem's retail market reporting of April 2025 records six large companies holding 91% of the domestic market, but notes that Octopus Energy, which was never a Big Six member, is now the largest electricity supplier⁴. Twenty-three domestic energy suppliers and 72 business energy suppliers were active at that point⁴.

For a household, the practical point is that the brand on the bill changed hands more than the market structure did. Six firms still hold the great majority of accounts, the price they may charge on a default tariff is set every three months by Ofgem rather than by competition, and none of the surviving incumbents offers a tariff without a standing charge⁵. Reducing dependence on any one of them is less about the logo and more about how much energy a home buys in the first place.

The Big Six at their peak

At the height of their dominance the six firms were effectively the market. Ofgem's Retail Market Review of March 2011 recorded that the Big Six still held over 99 per cent of domestic customer accounts¹¹, a figure echoed in commentary at the time that the six companies controlled 99 per cent of the market¹². Guidance still in circulation describes 91.2% of the domestic electricity market as dominated by the Big Six¹³, a figure that reflects a later stage of the same concentration.

Ownership was mixed from early on, which matters for any discussion of energy sovereignty. British Gas sat under Centrica, a UK-listed company. E.ON UK and RWE npower were the British arms of German groups. EDF Energy is French-owned and is described as the UK's fifth-largest energy supplier². ScottishPower has been part of a Spanish group, and SSE was a Scottish-headquartered company. A household buying from most of the six was buying from a foreign-owned balance sheet, with the profit and the investment decisions made outside the UK.

Erosion began slowly. Ofgem reported that the six large suppliers' share of GB domestic electricity meter points fell from 90% to 87% between March 2015 and March 2016⁶, and that as of September 2016, 14% of domestic consumers received their electricity from suppliers other than the six large energy suppliers¹⁴. That is a decade of change measured in single percentage points, which is why the term stuck long after it stopped describing a stable group. More detail sits on UK energy supplier market share.

What became of them: consolidation, failure and foreign ownership

Three forces reshaped the group: consolidation among the incumbents, the arrival and collapse of challengers, and continued overseas ownership.

The collapses were the sharpest event. The Public Accounts Committee recorded that between July 2021 and July 2022, 29 energy suppliers failed, attributing this to a lack of financial resilience within many suppliers alongside the rise in wholesale prices⁷. Citizens Advice put the consumer effect at turmoil for over 4 million customers from the beginning of August 2021⁸, and found afterwards that a significant minority of people continue to face severe disruption⁸. Those failures did not bring down a Big Six firm, but they redistributed customers and costs across the whole market, and much of the mutualised cost eventually reached every bill.

Consolidation ran alongside it. Ofgem's scheme reporting notes that ESB merged with So Energy in 2021, with ESB remaining the supplier group name and trading as So Energy¹⁵. Similar absorptions moved former Big Six household books into other brands, which is why four rather than six of the original names still appear on bills³. On the public side, Great British Energy is publicly-owned¹⁶, a different model of ownership from any of the incumbents.

Related detail sits on who owns which energy supplier, energy supplier failures and supplier of last resort.

Which names still trade, and who is largest now

Four of the original six still supply more than half of energy customers, including British Gas, EDF Energy and E.ON Next³. The market beyond them is broader than it was but not crowded: 23 domestic suppliers were active at Ofgem's April 2025 count, against 72 in the business market⁴. The single largest electricity supplier is now Octopus Energy⁴.

Ofgem's 2022 review of how suppliers support customers in vulnerable situations grouped firms by the severity of the weaknesses found. British Gas, Bulb, EDF, E.ON, Ovo, ScottishPower and Utility Warehouse were in the first group, where Ofgem identified minor weaknesses¹⁷. That review is a reminder that incumbency and service quality are separate questions. Comparative pages including E.ON Next vs British Gas and EDF vs British Gas set out the individual brands.

The tariff cap: how Ofgem limits what suppliers can charge

The energy price cap limits the maximum amount energy suppliers can charge for each unit of gas or electricity used⁹. It was first introduced in 2019, because of concern that customers who stayed put were paying more than those who moved, and it sets a limit to the unit rate of gas and electricity a supplier can charge to protect consumers against this loyalty penalty⁹. The Welsh Government's guidance describes the same purpose: Ofgem sets the price cap to protect people from a loyalty penalty¹⁸.

Ofgem reviews and resets the cap every 3 months, covering both the unit rate and the daily standing charge on a default tariff¹⁰. The Commons Library likewise records that the regulator revises the cap each quarter¹⁹. Recovery of some wholesale cost elements within the cap methodology has been on a six-monthly basis²⁰, which is one reason cap movements lag wholesale market movements.

Two limits matter to households. First, the cap is a cap on rates, not on bills: a home that uses more pays more, and the headline figures quoted around each review describe typical use rather than an individual account. Second, the cap is a Great Britain instrument. Energy prices were not capped in Northern Ireland and energy suppliers there have the flexibility to set their tariffs independently to reflect their costs of operating²¹. See energy suppliers in Northern Ireland.

Bills since the crisis and what is driving them

Bills remain far above pre-crisis levels. The End Fuel Poverty Coalition reported in January 2026 that energy prices were 70% higher than five years ago²². The House of Lords Library analysis of June 2026 breaks the increase down: 54% of the rise since pre-crisis levels is due to higher wholesale prices, with network charges accounting for 20% and green levies for 6%²³.

That split is the heart of most arguments about bills. The largest single driver is the wholesale price of gas and electricity, which no domestic supplier sets and no supplier switch removes. Network charges, the cost of the wires and pipes, are the second largest. Policy costs, the so-called green levies, are the smallest of the three named components.

Tax sits on top. Ofgem's bill guidance identifies VAT at 5% on energy bills²⁴. HMRC guidance records that qualifying supplies of electricity in Great Britain are liable to VAT at 0% from 1 October 2026²⁵. Separately, the Government stated in March 2026 that action taken through the previous Budget would remove costs from energy bills and reduce energy costs by 7% from April²⁶.

Standing charges: why none of the Big Six offers a zero standing charge tariff

A no standing charge tariff, also called a zero standing charge tariff, is a gas, electricity or dual fuel energy deal that does not include standing charges⁵. As of April 2026 there were just two suppliers offering no standing charge electricity and gas tariffs, E and Utilita⁵. In terms of the Big Six, none currently offers a zero standing charge tariff⁵.

The reason is structural rather than reputational. These tariffs typically charge higher unit rates for gas and electricity, which could result in higher energy bills⁵, and independent guidance makes the same point: tariffs with zero standing charges usually charge a higher cost per kWh for each unit of energy used²⁷. Fixed network and metering costs still have to be recovered, so removing the daily charge pushes them into the rate.

They are also narrow in availability. Zero standing charge tariffs currently only exist for prepayment customers and are geared very much towards those who use little electricity and gas⁵. They are described as best suited for households that use little gas or electricity, and properties left empty for long periods such as holiday homes²⁷.

This matters for households generating their own power. A home with solar and storage may cut its imported units sharply while still paying the standing charge every day of the year, so the fixed element becomes a larger share of a smaller bill. Several suppliers, including British Gas, E.ON Next and EDF Energy, offer smart storage battery tariffs, and smart time of use tariffs are offered by British Gas, E.ON Next, EDF Energy, Octopus Energy, Ovo Energy and ScottishPower as well as smaller firms including 100Green, Fuse Energy, Good Energy and So Energy²⁸. See also energy suppliers and household energy independence.

Energy UK: the trade association behind the suppliers

Energy UK is the industry trade body, representing firms including British Gas, ScottishPower and EDF Energy²⁹. It is not a regulator. It does not issue licences, set the price cap, or impose penalties, and it has no role in adjudicating a household's complaint. Its function is representation: putting the industry position to government, to Parliament and to the public.

That position is sometimes contested. On the use of court warrants to enter homes and fit prepayment meters, Energy UK has argued that the warrants are a last resort after exhausting all other options and after vulnerability checks are carried out²⁹. Consumer bodies have taken a different view of how those checks worked in practice. Reading a trade body's statement as industry advocacy rather than as an independent finding is the right frame.

A separate voluntary commitment, the Energy Switch Guarantee, covers the switching process itself. All of the big six energy suppliers have signed up, along with several smaller suppliers³⁰, and So Energy is among the signatories as of May 2026³⁰. It is an industry promise, not a licence condition, and is set out on the Energy Switch Guarantee.

Switching away from an incumbent

Ofgem's position is direct: if you pay a supplier directly for the electricity or gas you use at home, you can choose to switch to a different supplier or tariff at any time³¹. Residents of some alternative and communal arrangements are in a different position; where a household holds its own contract with a supplier, it may be able to switch.

Exit fees are the usual friction. Tariff Watch reported in September 2026 that just one in twenty, 6%, of British Gas tariffs come with no exit fees³². Where a supplier has entered an Energy Supply Administration Order, Ofgem states that customers can switch to another supplier without any exit fees³³.

If a supplier stops trading, no action is needed:

Related pages cover exit fees when switching, what happens if my energy supplier goes bust and final bills and credit refunds.

Profits, penalties and scrutiny

Supplier profit is measured on a bill as earnings before interest and tax, or EBIT, described by Ofgem as how much the supplier makes before interest or tax³⁴. The historic numbers show how volatile supply margins are: across the six large suppliers, profits on domestic electricity sales fell from £544.0m in 2014 to £128.7m in 2015, while the average non-domestic electricity profit margin fell from 2.0% to 1.0% over the same period⁶.

Contemporary estimates are larger and cover the whole industry rather than supply alone. The End Fuel Poverty Coalition stated in January 2026 that around 24% of every household energy bill is taken as profit by the energy industry²², and reported in September 2026 that suppliers are expected to make an additional £140m in profit on the nation's energy bills over the next 12 months following price cap changes³². Those two figures come from a campaigning body and should be read as its estimates. More sits on energy supplier profits and margins.

Scrutiny runs through several bodies:

• Ofgem, formally the Gas and Electricity Markets Authority³⁵, monitors suppliers and network operators to make sure they meet the rules set out in licences, regulations and law, provide good customer service and reply quickly to customers who contact them³⁶. On the cap, Ofgem states it will be closely monitoring supplier compliance and will continue to take firm action against suppliers who fall short of their requirements³⁷.
• Guaranteed Standards. Ofgem requires energy suppliers to meet Guaranteed Standards and sets the level of compensation payable if they breach a standard³⁸. See switching compensation and guaranteed standards.
• Enforcement. Ofgem has previously used its enforcement powers for smart meters to require OVO Energy to contribute more than £1 million to a redress fund, and has been considering more severe financial penalties³⁹.
• Obligations. Ofgem calculates suppliers' obligations under the Energy Company Obligation, processes notifications, audits suppliers, runs counter fraud work and reports to the Secretary of State⁴⁰. Which suppliers carry obligations under the Great British Insulation Scheme is determined by customer numbers and supply volumes⁴¹, and suppliers have had to publicly share their performance against smart metering installation targets⁴².
• The Energy Ombudsman can tell suppliers to take practical action, such as crediting or cancelling an account or changing a tariff, make an apology, or offer a financial award, or a combination, and may make recommendations to prevent issues happening again⁴³. It is funded by the suppliers signed up to its scheme, who pay a fee for each case it reviews regardless of the outcome⁴⁴.

That last point is worth stating plainly: the redress scheme a household uses against a supplier is paid for by suppliers. The fee is charged per case whatever the decision⁴⁴. See complaining about an energy supplier.

What the Big Six mean for a self-sufficient household

The dependence that remains is easy to state. Six large companies hold 91% of the domestic market⁴, most of them under foreign ownership, and the rate they may charge on a default tariff is reset by a regulator every three months¹⁰ rather than negotiated. The largest driver of the increase in bills since the crisis, at 54%, is wholesale price²³, which sits entirely outside a household's control and outside any individual supplier's control. Switching brand changes the customer service and the standing charge; it does not change the wholesale exposure.

What a household can influence is volume and timing: the units imported, and when they are imported. Time of use and storage tariffs from several of the incumbents and from smaller firms make timing worth money²⁸. But the daily standing charge persists whatever the meter records, and no Big Six supplier offers a way around it⁵. Energy independence, in this market, is built on the fabric of the building and on-site generation, with the supplier relationship reduced rather than replaced.