UK Energy Supplier Market Share: Who Has the Most Customers

Six large companies dominate the domestic energy market in Great Britain with 91% market share, and Octopus is now the largest electricity supplier in the country, with British Gas the largest gas supplier¹. Below that top tier, the market has thinned dramatically: as of April 2026 there were 17 energy suppliers operating in the UK according to Ofgem, compared with 48 domestic gas or electricity suppliers in September 2016². The whole market serves approximately 53.7 million domestic customer accounts in Great Britain, a figure that counts gas and electricity accounts separately, so most households appear twice⁴.

Market share matters to a household for one plain reason: it shapes what you pay, how quickly your supplier answers the phone, and what happens to you when the market turns. The 29 suppliers that failed between July 2021 and May 2022 affected nearly four million households, and the customers of small failed firms were moved, tariff and all, to suppliers they had not chosen⁵. Concentration also determines who carries the cost of government schemes, because environmental levies are spread across suppliers in proportion to their share of the electricity supply market⁶.

Understanding market share and the reasons it changes

Market share in energy is not a single number. Ofgem sorts suppliers into bands by the share they hold of the gas and electricity markets. Large legacy suppliers are those that have held at least 5% of the market in either fuel since ownership of gas and electricity was transferred from public to private hands in the 1980s. Large suppliers hold 5% or less of the market in at least one fuel, medium suppliers hold less than 5% in both fuels and 1% or more in a single fuel, and small suppliers hold less than 1% in both fuels⁷.

The figure moves for three reasons. The first is switching: in 2015, approximately 3.3 million domestic consumers switched electricity supplier and 2.6 million switched gas supplier, and every one of those moves shifted share between firms⁸. The second is failure and acquisition, which can move millions of accounts at a stroke, as the collapse of 29 suppliers showed⁵. The third is how share itself is measured: for scheme cost purposes, a licensed electricity supplier's market share is calculated from the amount of electricity it supplies to customers in Great Britain, less any exempt amount supplied to qualifying energy intensive industries, expressed as a percentage of the total supplied by all licensed suppliers⁶. A supplier whose licence is revoked before a levelisation period does not form part of the calculation at all⁶.

The market was designed around this structure. Suppliers sit as the primary interface between energy consumers and the energy system, the hub through which bills, levies and policy costs reach the household⁹. That is why share matters beyond bragging rights: the bigger a supplier's share, the bigger its slice of scheme costs, and the more households exposed to its pricing decisions and its financial health.

Which companies supply energy to homes in Britain

The traditional big six are Centrica plc (the parent company of British Gas), E.ON UK, Scottish and Southern Energy (SSE), RWE npower, EDF Energy and ScottishPower⁷. At their peak these companies supplied seven out of 10 British households with gas and electricity². Their dominance has been eroded rather than broken: the six large suppliers' share of GB domestic electricity meter points fell from 90% to 87% between March 2015 and March 2016, and from 90% to 86% for gas over the same period⁸, and by 2017 the big six's share of the Great Britain energy market stood at around 80%¹⁰.

The reshuffling since then has been dramatic. Ofgem's State of the Energy Market report records that six large companies now dominate the domestic market with 91% market share, and that Octopus is the largest electricity supplier and second largest gas supplier in Great Britain¹. British Gas remains the UK's largest gas supplier as of March 2026 and its second-largest electricity supplier². E.ON Next is the UK's third largest energy supplier as of March 2026, and French-owned EDF is the fifth-largest². Octopus's customer numbers make it the UK's overall largest energy supplier as of March 2026².

The challenger wave that once threatened the big six has largely been absorbed. Between December 2012 and September 2016 the number of active domestic gas or electricity suppliers more than doubled, and by September 2016, 14% of domestic consumers received their electricity from suppliers other than the six large energy companies³. Today the count is 17 suppliers operating in the UK as of April 2026². The names at the top have changed, but the shape of the market, a handful of firms serving the great majority of homes, has not. The full history of the big six brands and what became of them is covered in the big six energy suppliers, and the ownership behind today's brands in who owns which energy supplier.

Energy UK: the trade body representing 28 million homes

Energy UK is the trade association for the energy industry, not a regulator and not a consumer body. Its relevance to market share is that it speaks for the firms supplying the bulk of British homes: in March 2016 there were around 28 million domestic electricity and 21 million domestic gas customers in Great Britain⁸, and 84% of UK households, some 21 million, were on dual-fuel bills as far back as 2011¹¹. The suppliers Energy UK represents are, between them, the market.

The trade body also plays a practical role in how the market's numbers are assembled. When Ofgem reviewed whether suppliers had over-recovered costs under the price cap, five suppliers worked with Energy UK to commission a consultancy to collectively quality assure their responses to the regulator's data request¹². That is the association at work: coordinating its members' engagement with regulation rather than regulating them itself.

For a household, the distinction matters when something goes wrong. Energy UK can speak for the industry and publish its members' positions, including on what suppliers actually earn from bills, but it cannot investigate a complaint, fine a supplier or change a tariff. Those powers sit with Ofgem and, for individual disputes, the Energy Ombudsman. The consumer bodies that do advocate for households are covered separately in energy consumer bodies.

How Ofgem caps the amount suppliers can charge

The price cap exists to protect people from a "loyalty penalty", the charge for staying put¹³. The Welsh Government's own explanation of the cap puts it plainly:

The legal basis is the Domestic Gas and Electricity (Tariff Cap) Act 2018, which requires the Gas and Electricity Markets Authority to modify the standard supply licence conditions so that they include tariff cap conditions imposing a cap on all standard variable and default rates that may be charged by holders of supply licences for gas or electricity under domestic supply contracts¹⁴. The tariffs it protects are standard variable tariffs¹⁵.

The cap applies where a customer has not signed up for a fixed-term contract with their supplier¹⁶. Fixed deals sit outside it, priced by the supplier against market conditions. Ofgem revises the cap each quarter¹⁶, a change announced in August 2022, when the regulator moved from updating every six months to updating every quarter⁵. Each level sets maximum prices for a unit of energy and daily standing charges for customers in each energy supply region of Great Britain¹⁶.

Because the cap is regional and varies by payment method, the same household usage produces different annual bills in different places. For the cap period from 1 October to 31 December 2025, a prepayment electricity customer in the Eastern region with 3,100 kWh of single-rate usage faced a capped annual level of £926.96, while a standard credit customer in the Southern region with the same usage faced £1,008.32¹⁷. The table below shows the spread across regions and payment methods for that quarter.

Electricity price cap levels by region and payment method, 1 October to 31 December 2025¹⁷

The cap also protects consumers from the rapid changes observed in the wholesale energy market in the short term¹⁸. During the crisis the government went further, guaranteeing a typical annual bill at the £2,500 level through the Energy Price Guarantee¹², and support schemes delivered help through suppliers themselves: electricity suppliers received and distributed funding to support domestic energy customers in England, Wales and Scotland¹⁹, while local authorities helped get support to over 900,000 households without a direct supplier relationship²⁰.

Current bills and the factors behind the gap

Household bills remain far above where they stood before the 2021 to 2022 crisis, and the drivers are now well understood. The House of Lords Library reports that 54% of the rise in bills since pre-crisis levels is due to higher wholesale prices, with network charges and "green levies" accounting for 20% and 6% of the rise respectively²¹. The Committee on Climate Change made the same point over a decade ago, finding that household energy bill increases were caused primarily by the rising cost of gas, not environmental policies¹¹.

The structure of the bill matters for what a household can influence. Because most of the regulated social and environmental levies are made on electricity rather than gas, electricity's relative price is pushed up, and the Heat Pump Sector Deal Expert Advisory Group judged that rebalancing by the UK Government and Ofgem was urgently needed²². A household heating with electricity carries that levy weight; one on gas does not, but is exposed instead to gas prices set by a market in which the UK is, in the government's own words, "a net energy importer with a high dependence on gas and oil"¹⁸.

There are signs of strain easing at the margins. Ofgem's consumer satisfaction survey found the share of GB energy consumers classified as "doing well" on its Financial Vulnerability Classification rose from 46% in January 2025 to 53% in the latest wave, covering July and August 2025²³. But the underlying position has not returned to pre-crisis levels, and the earlier support measures, such as the £200 up-front discount on bills proposed for domestic electricity customers in Great Britain in 2022, were designed as temporary relief rather than a permanent change to prices²⁴.

Supplier profits: the contested part of your bill

What suppliers earn from the market is contested ground, and the numbers are large. End Fuel Poverty Coalition reporting puts suppliers on track for £1.74bn in profits over 12 months from customers' energy bills, with an additional £140m expected from price cap changes over the same period²⁵.

The regulator's own analysis gives context for how those profits arise. Ofgem's review of wholesale costs under the cap found that six suppliers, holding 65% market share between them, experienced an over-recovery across the cap periods examined, ranging from +£10 to +£77 per standard variable tariff electricity customer¹². An over-recovery means the allowance built into the cap exceeded the supplier's actual costs for those periods.

For a household, the significance is this: the supplier margin is the one part of the bill that market share directly feeds. A supplier with millions of captive standard variable customers earns that margin across a very large base, and the cap's quarterly resets determine how much of it is returned. The detail behind supplier earnings is covered in energy supplier profits and margins.

The way the market is governed

The regulator is Ofgem, acting through the Gas and Electricity Markets Authority. Its duty to cap standard variable and default rates comes directly from the 2018 Act¹⁴. Beyond the cap, the Authority's reach extends across the licence conditions suppliers must meet, and the market's wider security is underwritten by mechanisms such as the Capacity Market, whose purpose is to ensure security of GB's electricity supply at least cost to consumers²⁶.

Regulation also spreads costs by market share. The Feed-in Tariffs scheme's costs are spread across all licensed electricity suppliers in Great Britain through the "levelisation" process, based on each supplier's share of the electricity supply market²⁷. The same principle applies to other environmental and social schemes: the bigger the supplier, the bigger its share of the costs that end up in household bills.

The regulatory framework is built around Ofgem, the Gas and Electricity Markets Authority, which acts as the controller for consultation personal data and oversees the market²⁸. The industry body's positions show how governance works in practice. Energy UK describes the boiler mandate as "working in the way it was designed to work" and the "single biggest driver of emissions reductions" in government climate plans²⁹. On supplier failure, it argues for a simpler approach: a single regulatory backstop, in the form of an appropriate iteration of the Special Administration Regime, managed by Ofgem³⁰. Alongside the rules, the industry reports energy companies investing tens of millions of pounds annually in voluntary programmes to assist their customers³¹. The rules set the framework; much of what households actually receive sits outside it.

Concentration and its effect on household energy independence

A concentrated market means most households depend on a handful of companies for something essential, and the dependence runs deeper than the supplier itself. The supplier is the billing interface, but the energy behind the bill is largely imported, and the UK's "high dependence on gas and oil" ties household costs to international prices no supplier can control¹⁸. The government's British Energy Security Strategy proposals focused on energy supply³², which is the supply side of the system rather than the household side.

Switching supplier changes who bills you, not where your energy comes from. The same wires and pipes deliver the same energy whatever the logo on the bill, and the supplier hub model means the household's relationship with the wider system runs through that one company⁹. When 29 suppliers failed between July 2021 and May 2022, nearly four million households learned how little say they had in who that company was⁵. The mechanics of what happens next are covered in supplier of last resort and energy supplier failures.

The genuine route out of that dependence is generating and storing energy at home. At the start of 2026, domestic solar comprised 30% of UK solar capacity and 84% of total UK solar installations³³, meaning households, not power stations, now own the bulk of the country's solar installations by count. The electricity system those panels feed is also changing: emissions in the electricity supply sector are now 82% lower than 2008 levels³⁴, and the Climate Change Committee's Balanced Pathway sees electricity meet 61% of industrial energy demand by 2040, up from around 26% today³⁵.

Yet the household side lags. In Scotland in 2024, electricity was the primary heating fuel in 10% of social sector dwellings and 8% of owner occupied ones³⁶, so the great majority of homes still heat with gas or other fuels bought through the market. A home with solar and storage still needs a supplier for import and export, but the relationship changes character: the supplier becomes a backup and a market for surplus rather than the sole gatekeeper of the household's energy. That is the practical meaning of market share for independence, the bigger the supplier's share of your energy, the more of your bill and your security sits with a company you do not control.