Challenger brand Octopus Energy has bought out rival Co-operative Energy, becoming one of the UK's fastest growing energy suppliers. Co-op Energy supplies gas and electricity to c. 300,000 homes. Its acquisition, thought to be worth over £30m, pushes Octopus Energy past one million customers despite only four years in the market. Co-op Energy took on 130,000 customers with the acquisition of Flow Energy last year. Octopus Energy is owned by Octopus Group, an early backer of the property site Zoopla that also backs Big Gym, Secret Escapes and Eve mattresses through its investment arm Octopus Ventures. Octopus Group acquired Seccl earlier this month.