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. SSE and Npower announced a merger of their businesses in 2017, which if approved by the Competition and Markets Authority, would reduce the Big Six to five.
As of Q3 2017, the Big Six hold a combined market share of 81% for electricity supply and 80% for gas supply in Great Britain. The Big Six previously held a combined market share across both markets of 100% in 2004. 
|Supplier||Customers in the UK (Million)||Parent||Other divisions and brands||Previous brands|
|British Gas||15||Centrica||Scottish Gas|
|EDF Energy||5.6||EDF||SEEBOARD, SWEB energy & London Electricity|
|npower||6.5||innogy||Innogy, Northern Electric, Yorkshire Electricity|
|Scottish Power||5.3||Iberdrola||PPM Energy||MANWEB|
|SSE||9.1||SSE Group||SSE||Scottish and Southern, Southern Electric, SWALEC & Scottish Hydro|
In June 2014, energy watchdog Ofgem referred the energy industry to the Competition and Markets Authority (CMA). 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 makes it the seventh-largest energy supplier in the UK and the country’s biggest independent energy provider. Since 2009, other entrants into the market include a number of new energy companies including Economy Energy, Ovo Energy, Good Energy, LoCO2 Energy, Flow Energy, Yü Energy, Spark Energy, the UK's first co-operative supplier - The Co-operative Energy - and, in 2015, the first local authority-owned energy company since 1948 - Robin Hood 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 The Co-op, community renewable energy projects.
- "SSE confirms merger with Npower". BBC News. 2017-11-08. Retrieved 2018-03-27.
- "Electricity supply market shares by company: Domestic (GB)". Ofgem. January 2018. Retrieved 2018-03-27.
- "Gas supply market shares by company: Domestic (GB)". Ofgem. January 2018. Retrieved 2018-03-27.
- Information in this table taken from UK Power "UK Power" Check
|url=value (help). 2017-11-08. Retrieved 2017-11-08.
- E.On profits hit by UK customer woes. BBC News, 9 May 2017.
- Macalister, Terry (26 June 2014). "Ofgem puts big six energy suppliers under CMA spotlight" – via The Guardian.
- "Utility Warehouse buys 770,000 customer accounts from npower in £218m deal".
- "First Utility becomes first independent energy company to reach milestone of 1m customer accounts and pledges 1% of profits to charity". www.first-utility.com.