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

For most of the period since Britain's gas and electricity industries were privatised in the 1980s, six companies supplied the overwhelming majority of homes. Ofgem names them through their parents: Centrica plc, the parent of British Gas, alongside E.ON UK, Scottish and Southern Energy (SSE), RWE npower, EDF Energy and ScottishPower¹. The term "Big Six" has outlived the group it described: npower's brand has gone, SSE's household customers moved to Ovo, and Octopus Energy is now the largest electricity supplier in Great Britain².

Yet the phrase still shapes how people search and how the market is discussed, because the descendants of those companies still dominate. Six large companies hold 91 per cent of the domestic market as of April 2025, and there are 23 domestic energy suppliers and 72 business energy suppliers active in the market². This page explains who the Big Six were, what happened to each of them, how the price cap and standing charges work, and what the group means for a household that wants more control over its own energy.

The six giants when they ruled the market

The six companies were the inheritors of privatisation. Ofgem's own definition of a "large legacy supplier" is a company that has held at least 5 per cent of the market in either fuel since the government transferred ownership of gas and electricity from public to private hands in the 1980s¹. British Gas, E.ON, SSE, npower, EDF and ScottishPower all met that test, and for two decades the group was effectively the market.

Their dominance was measured precisely. Their share of GB domestic electricity meter points fell from 90 per cent to 87 per cent between March 2015 and March 2016³. As of September 2016, 14 per cent of domestic consumers received their electricity from suppliers other than the six large energy companies⁷, which means roughly six in seven were still with an incumbent.

Ownership was already international at the peak. EDF Energy was the British arm of France's state-owned EDF, RWE npower belonged to Germany's RWE, and E.ON UK to Germany's E.ON. ScottishPower was owned by Spain's Iberdrola. Only Centrica, the parent of British Gas, and SSE were substantial British-listed companies. For a householder, that ownership structure mattered less than the market structure it produced: a small group of incumbents whose tariffs, service standards and profit margins were the subject of repeated regulatory scrutiny, and whose brands most households had inherited rather than chosen.

Their fate: mergers, collapses and overseas buyers

The group was reshaped in two waves. The first was consolidation among the incumbents themselves: SSE sold its household retail business, whose customers moved to Ovo, and npower's customers were transferred to E.ON Next, which retired the npower brand. The second wave was the crisis of 2021 and 2022, when the lack of financial resilience within many energy suppliers and the rise in wholesale prices led to 29 energy suppliers failing between July 2021 and July 2022⁴. Most of the failures were challengers, not the Big Six, but the episode transformed the market the survivors operate in, and it is covered in full on our page on energy supplier failures.

Mergers continued after the crisis. ESB merged with So Energy in 2021, with ESB remaining the supplier group name and trading as So Energy⁸. Bulb, which had grown large enough to be counted alongside the incumbents in Ofgem's reviews, collapsed and its customers were moved to Octopus Energy, a transfer whose cost to the taxpayer is examined separately. The result is a market where the familiar names survive but often under different ownership or different branding, which our page on who owns which energy supplier sets out brand by brand.

Surviving brands today and the current market leader

Four of the six brands still supply households directly: British Gas, EDF, E.ON (trading as E.ON Next) and ScottishPower. SSE's household customers are with Ovo, and npower's are with E.ON Next. The group's replacement at the top of the market is Octopus Energy, which Ofgem's State of the Energy Market report identifies as now the largest electricity supplier in Great Britain².

The headline figure is striking: six large companies still dominate the domestic market with 91 per cent market share, as of April 2025². The composition of that six has changed, but the concentration has not. There are 23 domestic energy suppliers and 72 business energy suppliers currently active in the market², a far smaller domestic field than existed before the 2021 to 2022 failures, when dozens of challengers competed. For a householder choosing a supplier today, the practical effect is that the market looks more like the pre-challenger era than it did five years ago, with a handful of large incumbents and a modest fringe of smaller suppliers, described on our page on challenger and small energy suppliers.

Ofgem's price cap on supplier charges

The energy price cap limits the maximum amount energy suppliers can charge for each unit of gas or electricity used, together with the daily standing charge⁵. Ofgem first introduced the cap in 2019 because of concerns that customers who did not switch were paying more than those who did⁵. The Welsh Government's guidance states the purpose plainly: Ofgem sets the price cap to protect people from a "loyalty penalty"⁹.

The cap is not a fixed number. Ofgem reviews and sets a level for how much an energy supplier can charge for each unit of energy and the daily standing charge every 3 months¹⁰, and the House of Commons Library confirms the regulator revises the cap each quarter¹¹. Each reset reflects the supplier's underlying costs, chiefly the wholesale price of energy, so the cap moves up and down rather than ratcheting in one direction. Ofgem states that it will be closely monitoring supplier compliance and will continue to take firm action against suppliers who fall short of their requirements¹².

The cap applies across Great Britain. Northern Ireland is different: energy prices were not capped there, and energy suppliers have the flexibility to set their tariffs independently to reflect their costs of operating¹³, a separate arrangement covered on our page about energy suppliers in Northern Ireland.

Household costs after the turmoil and their causes

Household bills remain well above their pre-crisis levels, and the drivers are itemised in official analysis. Of the rise in bills since pre-crisis levels, 54 per cent is due to higher wholesale prices, while network charges and green levies account for 20 per cent and 6 per cent of the rise respectively¹⁴. In other words, the largest single component of the increase is the cost of the energy itself on international markets, not the supplier's margin, the wires and pipes, or the environmental levies, though all three contribute.

The direction of travel in 2026 has been downward at the margin. The government told Parliament in March 2026 that it took action through the Budget to remove costs from energy bills and reduce energy costs by 7 per cent from April¹⁵. VAT on domestic energy is charged at 5 per cent¹⁶, and a UK Government decision cut VAT on domestic electricity to 0 per cent from 1 October 2026, with the price cap changing on the same date. Every bill also carries the standing charge, which pays for the network whether or not any energy is used, and which is the subject of the next section.

For a household reading these figures, the lesson is about where the leverage lies. Because more than half the rise since the crisis comes from wholesale prices, the bill a household pays is exposed to international gas markets regardless of which supplier it chooses. Switching supplier changes the retail margin and the service; it does not remove exposure to the wholesale cost. Reducing the amount of energy bought, through insulation or generation at home, is the only route that cuts the wholesale component directly.

Why the six majors all set daily charges above zero

A no standing charge tariff, also known as a zero standing charge tariff, is a gas, electricity or dual fuel deal that does not include standing charges⁶. In terms of the big six energy suppliers, none currently offers zero standing charge tariffs⁶. As of April 2026, there are just two suppliers that offer no standing charge electricity and gas tariffs: E and Utilita⁶.

Even those tariffs come with conditions and trade-offs:

• Zero standing charge tariffs currently only exist for prepayment customers and are geared very much towards those who use little electricity and gas⁶.
• These tariffs typically charge higher unit rates for gas and electricity, which could result in higher energy bills⁶.

The arithmetic is straightforward: a household that uses a typical amount of energy will usually pay more overall without a standing charge, because every unit costs more. A household that uses very little, perhaps an empty property or a home that generates most of its own power, may come out ahead. For a self-sufficient household with solar panels and a battery, the standing charge is the part of the bill that self-generation cannot touch, which is precisely why the absence of a zero standing charge option among the Big Six matters to exactly the households most interested in independence.

Energy UK: the industry body representing suppliers

Energy UK is the trade association for the energy industry, and the Big Six suppliers are among its members: Centrica plc, E.ON UK, Scottish and Southern Energy, RWE npower, EDF Energy and ScottishPower¹. Its role is representation, not regulation. It speaks for the industry in policy debates and public commentary, but it holds no licence powers, cannot fine a supplier and cannot resolve a complaint.

That distinction matters because the names are easily confused. The bodies with actual authority over suppliers are Ofgem, which licenses suppliers and enforces the rules, and the Energy Ombudsman, which decides individual disputes. A household that has a problem with a Big Six supplier gains nothing from Energy UK's existence; the routes that matter are the supplier's own complaints process and, if that fails, the Ombudsman, covered on our page about complaining about an energy supplier.

Leaving a legacy provider

The rules on switching are simple and apply to every supplier, large or small. 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 time¹⁷. The new supplier handles the transfer, and the supply itself is not interrupted, a set of promises described on our page about the Energy Switch Guarantee.

If a supplier stops trading, the household does not need to act. Ofgem's guidance states: "You do not need to do anything if your current supplier goes out of business. We'll automatically move you to a new supplier, and make sure your energy supply is not interrupted."¹⁷ The full process, including what happens to credit balances, is on our page about the supplier of last resort.

Why people switch is well documented. In Ofgem's consumer perceptions survey for Q2 2021, 44 per cent of those who switched their energy supplier did so exclusively due to cost or tariff, and 18 per cent of switchers said their new supplier offered green energy¹⁸. Switching itself, however, is a modest driver of satisfaction: Ofgem's 2025 analysis found that having switched supplier or tariff explained 5 per cent of the variance in satisfaction that its model could explain, and 0.6 per cent of total variance¹⁹. The supplier matters, but less than households often assume.

Earnings, fines and public oversight

Supplier profit is measured as earnings before interest and tax, or EBIT: how much the supplier makes before those costs are met²⁰. The historical figures show how thin domestic retail margins have been at times. The six large suppliers' profits on domestic electricity sales fell from £544.0 million in 2014 to £128.7 million in 2015, and their average non-domestic electricity profit margin fell from 2.0 per cent to 1.0 per cent over the same year³. The price cap now sets the allowance a supplier can build into its charges, and Ofgem monitors compliance with that cap and states it will take firm action against suppliers who fall short¹².

Scrutiny extends well beyond the cap:

• Ofgem monitors energy suppliers and network operators closely 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 them²¹.
• Suppliers must publicly share their performance against smart metering installation targets²².
• Under the Energy Company Obligation, Ofgem calculates suppliers' obligations, tracks performance against them, audits suppliers and reports to the Secretary of State²³; the scheme has delivered 4.4 million measures in 2.6 million homes between 2013 and December 2025²⁴.
• Under the Great British Insulation Scheme, obligations are determined by customer numbers and supply volumes²⁵.
• Ofgem's review of how suppliers support customers in vulnerable situations grouped suppliers by the severity of its findings: British Gas, Bulb, EDF, E.ON, Ovo, Scottish Power and Utility Warehouse were found to have minor weaknesses, while severe concerns were identified at Good Energy, Outfox, SO Energy, TruEnergy and Utilita²⁶.

Complaints that a supplier fails to resolve go to the Energy Ombudsman, which 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 of these, and may make recommendations to prevent issues happening again²⁷. The Ombudsman is funded by the suppliers signed up to its scheme, which pay a fee for each case reviewed regardless of the outcome²⁸, an arrangement designed so that the decision does not depend on who pays.

Customer experience data gives a sense of what to expect. In Ofgem's satisfaction survey of July 2024, which polls roughly 3,500 to 4,000 domestic energy consumers across Great Britain, 7 in 10 (69%) of those who had fallen behind on bills were satisfied with the support they received, and 53% had contacted their supplier for support²⁹. In the January 2025 wave, 29 per cent of those who reported falling behind on energy bills for affordability reasons said they had no contact with their supplier, meaning 55 per cent had made contact themselves³⁰. The support exists, but a household may have to ask for it.

The Big Six and the off-grid home

The Big Six, and the six large companies that have replaced them, remain the default suppliers for the great majority of British homes: 91 per cent of the domestic market as of April 2025². For a household working towards energy independence, that concentration has practical consequences. The supplier controls the tariff, the standing charge, the export payments for any surplus solar generation, and the interface with the grid. Choosing among them changes the retail terms, but it does not change the underlying exposure: more than half of the rise in bills since the crisis comes from wholesale prices¹⁴, which no supplier can shield a household from.

What a household can control is the quantity of energy bought. The standing charge is the clearest example of the limit: none of the Big Six offers a tariff without one⁶, so a home that generates and stores most of its own power still pays a daily charge for the network. The cap, reviewed every three months¹⁰, limits what any supplier can charge per unit, but it does not cap the total bill, which rises with consumption.

The relationships that matter for independence are therefore layered. The supplier remains the counterparty for imports, exports and billing, and the Ombudsman and Ofgem remain the recourse when that relationship goes wrong. The deeper independence comes from the measures the suppliers themselves are obliged to fund, such as the 4.4 million insulation measures delivered in 2.6 million homes since 2013²⁴, and from a household's own generation and efficiency. The Big Six are the backdrop to that project, not the solution to it, and the full picture of how supplier choice interacts with self-sufficiency is on our page about energy suppliers and household energy independence.