Maxus takes the UK offline account from the incumbent Vizeum, after winning the digital media planning and buying account earlier this year after a pitch.
Npower is the trading name of RWE npower, which is based in Germany.
The company announced the review of the media business in September last year, and chose to handle the search in-house and not use an intermediary.
The RWE offering spans the Czech Republic, Germany, Netherlands, Poland, Slovakia and the UK.
In the UK, the review included npower’s offline media business only, but other countries are understood to have pitched a full offline and online brief.
Sabine Schmittwilken, head of group brand communications at RWE AG, sais: "For the first time, we looked for a media agency on an international level in order to maximise synergies for the group. Group M provides us with cutting-edge expertise on excellent terms."
The decision was taken jointly by marketing heads for the RWE subsidiaries as well as RWE Group procurement.
Jürgen Blomenkamp, global investment chairman at Group M, said: "The GroupM agencies Maxus and Mediacom impressed RWE at a pan-European level and will advise this blue chip customer on innovative communication solutions in the future. We are very much looking forward to working together."
RWE is the latest energy supplier to review its media arrangements, following EDF Energy and Scottish supplier SSE.
The company uses McCann Manchester as its lead creative agency, which recently worked on a marketing drive to convey its "customer-centric" approach. Npower previously worked with VCCP Blue.