The biggest energy company in the United Kingdom by customer base is British Gas. Owned by Centrica, British Gas has over 20 million residential and business customers. That total number includes gas, electricity, dual fuel and boiler and maintenance customers. British Gas has four times as many customers as Scottish Power and E.ON UK, and more than twice as many customers as SSE.
British Gas is also the biggest energy company in the United Kingdom by profit growth. British Gas profits grew 31 per cent to £574m for the 12 months to 31 December 2015. However, profits for the 12 months to December 2016 are not expected to see as much growth, due to the rising cost of gas, and the number of customers who are switching to an independent energy supplier; in July 2016, it was reported that British Gas had lost some 400,000 customers (BBC).
We also have to give a special mention to First Utility in this article, because First Utility is the biggest independent energy company in the United Kingdom. In 2014, First Utility announced that they were the first independent energy supplier to reach 1 million customers, and it is estimated that they now serve over 2.5 million residential and business customers in the United Kingdom.
Another energy supplier of note is Utilita. Utilita are the biggest supplier of pay-as-you-go energy in the United Kingdom, and they rank as the 11th biggest energy supplier overall.