The review comes just weeks after it hired Vizeum UK on a project basis to handle its buying.
The agency was hired in March alongside the creative agency Isobel on a short-term basis in anticipation of its parent company, RWE, conducting a larger-scale review of its media and creative arrangements.
However, the latest Npower review is said to be unrelated to RWE's group needs and will focus solely on media for the Npower brand.
Npower needed a media agency on a project basis after its previous incumbent, Manning Gottlieb OMD, resigned the account last September following its capture of the larger EDF Energy business.
It has decided to return to above-the-line spot advertising after terminating its £2 million deal to sponsor Channel 4's homes programming and cricket coverage.
Separately, the German-owned RWE has launched its first pan-European ad campaign under the line: "Somebody has to be there." The corporate campaign, which is running in German and pan-European magazines and business press, was created by the German agency Philipp und Keuntje.
RWE also owns Thames Water and recently appointed M&C Saatchi and ZenithOptimedia to handle creative and media for new activity.
Kevin Peake, Npower's head of brand, was unavailable for comment.