NRG Energy, which has been fighting off the takeover advances of rival electricity supplier Exelon since last October, rejected a higher offer worth $7bn made by Exelon last week and said Exelon should be willing to pay more.

NRG, which claims Exelon’s initial offer “grossly undervalued” the company, said on Wednesday that the higher, all-stock bid still did not recognise NRG’s growth potential and the additional value it has created in recent months.