UK Energy Supplier Market Share: Who Has the Most Customers

Six large companies dominate domestic supply in Great Britain with 91% market share, and Octopus is now the largest electricity supplier and second largest gas supplier in Great Britain, according to Ofgem¹. British Gas is described as the largest gas supplier as of March 2026 and the second-largest electricity supplier². Those two positions, held by different companies in different fuels, are why there is no single uncontested answer to the question of who is Britain's biggest energy supplier: gas and electricity accounts are counted separately, and a household with dual fuel appears twice.

The scale behind those percentages is large. There are approximately 53.7 million domestic customer accounts in Great Britain³. Energy UK, the industry trade association, states that its members deliver over 95% of the energy supply for 28 million UK homes and nearly 80% of the UK's power generation⁴. Around 20 energy firms are currently available to choose from, down from more than 50 at the end of 2020⁵; a separate count puts the figure at 17 suppliers operating in the UK as of April 2026².

Market share is not a vanity statistic. It is a working number in regulation. Obligations under the Energy Company Obligation and the Great British Insulation Scheme are apportioned using a calculation based on each supplier's share of the gas and electricity supply market⁶, and Feed-in Tariff costs are spread across all licensed electricity suppliers in Great Britain through the levelisation process, based on their share of the electricity supply market⁷. Share determines who pays for what, and therefore which customers carry which costs.

What market share means, and why the figure moves

Ofgem classifies suppliers by the share of each fuel they hold rather than by customer numbers alone. Its published definitions set out four categories. Small suppliers "hold less than 1% of the market in both fuels (gas and electricity)". Medium suppliers "hold less than 5% of the market in both fuels (gas and electricity) and 1% or more in a single fuel". Large legacy suppliers "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 1980s"¹⁰.

The underlying measurement for electricity is volumetric, not per household. Ofgem's Feed-in Tariff guidance defines a licensed electricity supplier's market share as the amount of electricity supplied to customers in Great Britain less the exempt amount supplied to qualifying Energy Intensive Industries, expressed as a percentage of the total relevant electricity supplied by all licensed electricity suppliers in Great Britain⁷. A share measured in kilowatt hours can differ from a share measured in meter points, because customer mixes differ: a supplier with many low-use flats and a supplier with many large houses do not rank the same on both measures.

Shares move for three reasons: households switching, suppliers failing, and companies buying each other. The first has become the least important of the three. In 2015, approximately 3.3 million domestic consumers switched their electricity supplier and 2.6 million their gas supplier¹¹. By the second quarter of 2021, 20% of bill-payers said they were thinking of switching, down from a peak of 23% in the first quarter of that year, and 44% of those who did switch did so exclusively due to cost or tariff¹². Acquisitions and failures have since done far more to redraw the map than consumer choice has. Further detail sits on who owns which energy supplier.

The big six and beyond: who supplies British homes

The original grouping was defined by Ofgem as Centrica plc (parent company of British Gas), E.ON UK, Scottish and Southern Energy (SSE), RWE npower, EDF Energy and ScottishPower¹⁰. At their height, these companies supplied seven out of 10 British households with gas and electricity². Four of the original six still supply more than half of energy customers today: British Gas, EDF Energy, E.ON Next and ScottishPower¹³.

The concentration figure has moved in an arc rather than a straight line. Between March 2015 and March 2016 the six large suppliers' share of domestic meter points fell from 90% to 87% for electricity and from 90% to 86% for gas¹¹. By September 2016, 14% of domestic consumers received their electricity from suppliers other than the six large energy companies, and the number of active domestic gas or electricity suppliers had more than doubled between December 2012 and September 2016, reaching 48¹⁴. The Scottish Government recorded the big six share of the Great Britain market at around 80% in 2017¹⁵. It has since climbed back: 91% by Ofgem's reckoning¹, and 91.2% of the domestic electricity market for the six biggest companies in the first quarter of 2025 on an independent count¹⁶.

The names inside the 91% are not the names of 2010. Octopus holds the largest electricity position and the second largest in gas¹; British Gas is first in gas and second in electricity²; E.ON Next is described as the UK's third largest energy supplier as of March 2026 and EDF as the fifth-largest². The smaller end has also changed character: in March 2016, 55% of customers of other domestic suppliers with market share above 1% were on fixed-term tariffs, compared to 32% for the six large suppliers¹¹. See also challenger energy suppliers and the big six energy suppliers.

Energy UK: the trade association behind 28 million households

Energy UK is the trade association for the energy industry with over 100 members⁴. Its members deliver nearly 80% of the UK's power generation and over 95% of the energy supply for 28 million UK homes⁴. That reach is roughly the size of the whole domestic electricity market as Ofgem measured it in March 2016, when there were around 28 million domestic electricity and 21 million domestic gas customers in Great Britain¹¹.

Energy UK is not a regulator. It has no licensing power and cannot fine a supplier. What it does is coordinate voluntary commitments that, because of its members' coverage, apply across almost the entire market. Thirteen energy suppliers covering more than 95% of homes are signed up to Energy UK's Vulnerability Commitment, and fifteen suppliers covering almost the entire market signed the Winter 2024 Commitment¹⁷. The association also serves as a collective vehicle in regulatory processes: in Ofgem's review of price cap wholesale costs, five suppliers worked with Energy UK to commission a consultancy to collectively quality assure their submissions¹⁸.

For a household, the practical consequence of near-universal membership is that a voluntary commitment behaves a little like a rule, without the enforcement that a rule carries. If a supplier falls short of a Vulnerability Commitment promise, the remedy lies with the supplier's own complaints process and then the ombudsman, not with the trade body. The obligations that are genuinely enforceable, on vulnerable customers and on prepayment and debt conduct, come from licence conditions set by the regulator. Around 30% of bill-payers contacted their supplier in the past three months as of the second quarter of 2021¹², so the quality of that relationship is not a marginal matter.

The price cap: how Ofgem limits what suppliers charge

Ofgem first introduced the price cap in 2019 because of concerns that customers who never switched were paying more than those who did¹⁹. Its purpose is described plainly by the Welsh Government: Ofgem "sets the price cap to protect people from a 'loyalty penalty'"²⁰. The cap sets a limit to the unit rate of gas and electricity a supplier can charge, and also caps standing charges on standard variable tariffs²¹.

The legal basis is statutory. Under the Domestic Gas and Electricity (Tariff Cap) Act 2018, the Gas and Electricity Markets Authority "must modify the standard supply licence conditions so that they include conditions ('tariff cap conditions') that impose a cap on all standard variable and default rates that may be charged by the holders of supply licences for the supply of gas or electricity under domestic supply contracts"²².

Coverage and frequency both matter. The default tariff cap protects 22 million default tariff and standard variable tariff customers⁸. It applies where a customer has not signed up for a fixed-term contract with their supplier²³, and does not apply to a household that has signed up to a fixed-term tariff. Ofgem revises the cap each quarter²³. That has been the position since August 2022, when Ofgem announced it would update the cap every quarter rather than every six months⁹; before that it was reviewed twice a year. Suppliers update standard variable tariffs on a regular basis, usually in line with Ofgem's three-monthly changes⁵.

How the level is built

The cap sets maximum prices for a unit of energy and daily standing charges for customers in each energy supply region of Great Britain²³, and varies by payment method as well as region. The headline annual figure is a typical-consumption illustration, not a maximum bill. Regional levels for 1 October to 31 December 2025 show the spread: the table below gives annual electricity figures at nil consumption and at 3,100 kWh single-rate, and at nil consumption and 4,200 kWh multi-register²⁴.

Headline levels have moved sharply. The cap fell to £1,834 in the fourth quarter of 2023 with a planned increase to £1,928²⁵, and for 1 April to 30 June 2026 it was set at £1,641²⁶. During the crisis a separate mechanism, the Energy Price Guarantee, was extended at the £2,500 level¹⁸. Northern Ireland sits outside all of this: energy prices were not capped there, and suppliers "have the flexibility to set their tariffs independently to reflect their costs of operating"²⁷. That difference is covered further on energy suppliers in Northern Ireland.

Bills today, and what is driving the difference

The composition of the increase since before the energy crisis is better evidenced than the total. Wholesale prices account for 54% of the rise since pre-crisis levels, network charges for 20% and green levies for 6%²⁸. The same breakdown is reported independently: green levies and network charges account for just 6% and 20% of the rise in bills since before the energy crisis²⁹.

The reason wholesale dominates is structural rather than commercial. Gas sets the wholesale price of electricity in the UK 98% of the time, according to academic research published in 2023²⁹. A supplier's tariff, whoever the supplier is, is therefore largely an expression of the international gas price, which is why comparisons between suppliers rarely open up the gaps that comparisons between years do. The UK is described in government material as "a net energy importer with a high dependence on gas and oil", with the price cap protecting consumers from the rapid changes observed in the wholesale energy market in the short term³⁰.

Household experience has eased somewhat from the worst of it. In Ofgem's Financial Vulnerability Classification, the share of consumers classified as "doing well" rose from 46% in January 2025 to 53% in the July to August 2025 wave³¹. That still leaves close to half of consumers outside that category. During the crisis, direct support flowed through suppliers rather than around them: the Energy Bills Support Scheme provided a £400 energy discount for electricity customers³², an earlier proposal offered "an up-front discount on their bills worth £200" repayable through bills in later years³³, and over 900,000 households without a direct supplier relationship had to be reached through local authorities in Great Britain instead³⁴.

Supplier profits: the disputed middle of the bill

Margins on supply are thin relative to the totals that pass through suppliers' hands, but the absolute sums are large. In 2023, suppliers were likely to earn 2.4% profit on every average customer's bill from 1 October, up from 1.9% at the time³⁵. On the same analysis, customers would only pay less profit to energy firms than before if their bill was above £4,000 a year³⁵.

More recent campaign-group tracking puts current figures higher in cash terms. Suppliers are expected to make an additional £140m in profit on the nation's energy bills over the next 12 months following price cap changes, and could take £1.74bn in profits over the next 12 months from customers' energy bills³⁶. Across the wider industry, around £125 billion in profits has been made on UK operations by 30 energy companies since 2020, and energy firms banked £6bn in profits in the first six months of 2026, with UK returns worth over £200 of profit for every household in the country³⁷.

These figures measure different things and should not be added together: a supply margin per bill is not the same as group profit across generation, networks and supply. Ofgem's own review of cap wholesale costs found evidence of recovery beyond allowances in some periods: six suppliers with 65% market share experienced an over-recovery with a range of +£10 to +£77 per standard variable tariff electricity customer across the cap periods examined¹⁸. Concentration matters here, because when six firms hold 91% of the domestic market¹, an allowance set slightly loose or slightly tight is felt by nearly every household at once. Supplier profits and margins sets out the accounting in more detail.

How the market is regulated

The regulator is the Gas and Electricity Markets Authority, referred to for ease as Ofgem³⁸. Its cap duty is statutory²², and market share is the arithmetic on which several of its schemes turn. Environmental and social obligations are set "using a calculation based on their share of the gas and electricity supply market"⁶, and under the Energy Company Obligation a household can contact any of the obligated energy suppliers to find out how they may be able to help, even if they are not that household's energy provider³⁹.

Exits are handled explicitly in that arithmetic. Where a licensed electricity supplier has its licence revoked before levelisation, that supplier "will not form part of the market share calculation"⁷. The consequence is that when a supplier fails, its share of scheme costs does not vanish; it is redistributed across those still trading. The cost of failure on this scale is well documented: between July 2021 and May 2022, 29 energy suppliers failed, affecting nearly four million households in the UK⁹. See energy supplier failures and supplier of last resort.

One further regulatory feature shapes the cost of getting off gas. The Scottish Government's Heat Pump Sector Deal Expert Advisory Group recorded that most of the regulated social and environmental levies are made on electricity rather than gas, increasing electricity's relative price, and that a rebalancing through action of the UK Government and Ofgem is urgently needed⁴⁰. Renewable electricity sources now make up 43% of the UK supply²¹, and emissions in the electricity supply sector are 82% lower than 2008 levels³¹, yet the levy structure still weighs on the fuel that electrified homes depend on.

What concentration means for household energy independence

The market was designed with suppliers as the primary interface between energy consumers and the energy system⁴¹. That design is the root of the dependence. Whoever bills a household, the supplier sits between that home and the wholesale market, administers the cap, delivers government support, and carries scheme obligations set by its market share. A 91% share held by six companies¹ means most households experience the energy system through one of six commercial relationships.

Switching does not change that structure, and the evidence suggests it no longer changes much else either. Ofgem's supplier-level survey for July to August 2025 found that no suppliers saw a statistically significant increase in the proportion of customers who said they were satisfied, or a decrease in dissatisfaction, compared with the January 2025 wave⁴². Satisfaction among those who had switched had fallen to 67% by December 2022⁴³. Earlier work described a market in which "two-thirds of households disengaged and paying more for their energy than customers who have switched suppliers"⁴⁴; the cap now blunts that penalty for the 22 million accounts it covers⁸, but it does so by regulating the supplier, not by removing the household's reliance on one.

Genuine reduction in dependence comes from consuming less bought energy or generating some at home. At the start of 2026, domestic solar comprised 30% of UK solar capacity and 84% of total UK solar installations⁴⁵, so the household sector is already the bulk of installations by count if not by capacity. Even then, the supplier relationship persists: export payments, Feed-in Tariff levelisation, standing charges and metering all run through a licensed supplier. A home with panels and a battery reduces its import volumes and its exposure to the unit rate; it does not leave the market, and it does not escape the standing charge, which the cap sets by region and payment method²⁴. The realistic goal is a smaller, less painful dependence rather than none, and suppliers and energy independence develops that point. The pillar overview sits at /suppliers/.