The account moves from McCann Manchester, the incumbent since 2013, and FCB Inferno will work on fully integrated campaigns for the company, which is one of the "big six" energy suppliers.
Bartle Bogle Hegarty, Karmarama, and Mother all took part in the process, which was handled by ISBA.
Npower, owned by German holding company RWE, had considered holding an e-auction after BBH and Karmarama dropped out of the process, but later rejected the idea.
Edward Madden, the head of marketing at Npower, said: "We’re excited to have FCB Inferno on board and are looking forward to working with them to develop a new creative strategy that meets the challenges faced by our business.
"They demonstrated great strategic focus and creative ability throughout the pitch process and I am confident that we will benefit from their expertise, ideas and fresh perspective."