Big Six energy suppliers
The Big Six are the United Kingdom's largest energy suppliers, supplying gas and electricity. The Big Six are British Gas, EDF Energy, E.ON, Npower, Scottish Power, and SSE.
As of Q3 2019, the Big Six held a combined market share of 70% for electricity supply and 69% for gas supply in Great Britain. They previously held a combined market share across both markets of 100% in 2004.
OVO Energy acquired SSE's retail customers, effective January 2020, but will continue to use the SSE brand for the time being.
The companies
Source:| Supplier | Customers in the UK | Parent | Other divisions and brands | Former brands |
| British Gas | 15 | Centrica | Scottish Gas | |
| EDF Energy | 5.6 | Électricité de France | SEEBOARD, SWEB Energy, London Electricity | |
| E.ON UK | 4.6 | E.ON | Powergen | |
| npower | 6.5 | Innogy | Innogy, Northern Electric, Yorkshire Electricity | |
| Scottish Power | 5.3 | Iberdrola | PPM Energy | MANWEB |
| SSE | 9.1 | OVO Energy | Scottish and Southern, Southern Electric, SWALEC, Scottish Hydro |
Possible consolidation to Big Five
In 2017, SSE and Npower announced a merger of their retail businesses, but this was scrapped in 2018.Just after the failed SSE-Npower merger in late 2018, it was reported that a consolidation to the Big Five would still occur, as Npower would be acquired by default by E.ON due to the already-planned asset swap by the respective German parents, innogy and E.ON.
Ofgem investigation
In June 2014, energy watchdog Ofgem referred the energy industry to the Competition and Markets Authority. The 18-month investigation is to conclude whether the big six have been profiteering and should be broken up.Alternatives to the Big Six
In December 2014, Utility Warehouse, part of FTSE 250-listed company Telecom Plus became the UK's biggest independent energy supplier through a £218m deal to buy 770,000 existing customers from npower. In September 2014, First Utility announced it was the first independent utility supplier to reach the milestone of 1m customer accounts for gas and electricity – the equivalent of 550,000 customers, which made it the seventh-largest energy supplier in the UK and the country's biggest independent energy provider. In June 2015, a Cornwall Report stated Opus Energy had broken the dominance of the 'Big Six' energy suppliers in the business market.The Competition and Markets Authority published an investigation into the energy supply market in June 2016, following a referral by Ofgem in June 2014. The report identified 34 suppliers of both electricity and gas to households, and described the three largest suppliers outside the Big Six as "mid-tier suppliers"; these were First Utility, Ovo Energy and Utility Warehouse. Other significant suppliers were Co-operative Energy, Extra Energy and Utilita Energy. The combined market share of suppliers outside the Big Six had increased from less than 1% in 2011 to around 13% in the first quarter of 2016.
Since 2009, other entrants into the market include a number of new energy companies including Bulb Energy, Good Energy, Ecotricity, and Octopus Energy. Many of these newer entrants are seeing significant growth in customer numbers, in part due to their greater commitment to renewable energy and, in the case of Co-op Energy, community renewable energy projects.
Local authority-owned companies
There are currently three energy companies owned by local authorities. The first such company since 1948 was Robin Hood Energy which entered the market in 2015 and is owned by Nottingham City Council. Bristol Energy, also launched in 2015, is owned by Bristol City Council. Both Robin Hood Energy and Bristol Energy are available to consumers throughout the country.In January 2020, London Power was launched by the Mayor of London. Contrary to the other two companies, it only provides gas and electricity to London homes. London Power is not itself an energy supplier, instead it is a partnership between the Greater London Authority and a providing partner, currently Octopus Energy.
Defunct competitor companies
By January 2019, ten small energy suppliers had ceased trading or been taken over by others. This number increased to sixteen by the end of that year.| Company | Ceased | Customers | Customers transferred to |
| Affect Energy | September 2018 | 22,000 | Octopus Energy |
| Breeze Energy | December 2019 | 18,000 | British Gas |
| Brilliant Energy | March 2019 | 17,000 | SSE |
| Economy Energy | January 2019 | 235,000 | OVO Energy |
| ENGIE UK | January 2020 | 70,000 | Octopus Energy |
| Extra Energy | November 2018 | 108,000 domestic, 21,000 business | Scottish Power |
| Flow Energy | May 2018 | 130,000 | Co-op Energy Brand continued until acquired by Octopus in 2019 |
| Future Energy | January 2018 | 10,000 | Green Star Energy |
| GB Energy Supply | November 2016 | 160,000 | Co-op Energy Brand continued until acquired by Octopus in 2019 |
| Green Star Energy | October 2019 | 200,000 | Shell Energy Green Star was a subsidiary of Canada's Just Energy |
| Iresa | July 2018 | 100,000 | Octopus Energy |
| OneSelect | December 2018 | 36,000 | Together Energy |
| Our Power | January 2019 | 31,000 | Utilita Energy |
| Solarplicity | August 2019 | 60,000 | EDF Energy |
| Spark Energy | November 2018 | 290,000 | OVO Energy |
| Toto Energy | October 2019 | 134,000 | EDF Energy |
| Usio Energy | October 2018 | 7,000 | First Utility |