The incumbent, RKCR/Y&R, had held the business since 2003. However, it stopped work on the account early last month in the wake of a difference of opinion as to how Powergen's rebrand to E.ON should be handled.
'Our view was to build on the strong heritage of positive energy, especially with the increasing emphasis on environmental issues and sustainable energy,' said a spokesman for RKCR/Y&R.
Powergen's previous campaigns featured giant energy-efficient light bulbs to flag up its green credentials.
Separately, Powergen's sponsorship of ITV's national weather bulletins has been extended until 2008 - taking it into its 18th year. Last year E.ON signed up to sponsor the FA Cup in an attempt to raise its profile. It also sponsors Ipswich Town FC.
E.ON's media business, which is handled by Initiative, will not be affected by TBWA's appointment.
Powergen supplies gas and electricity to about 9m customers in the UK. In the nine months to October 2006, E.ON posted earnings of EUR710m (£467m) in the UK, down from EUR715m (£470m) in the same period in 2005.
Last month E.ON announced that it is to invest EUR25.3bn (£16.6bn) in Europe over the next three years to improve security of supply.