The agency won a pitch process for the brief, previously handled by a roster of McCann Birmingham, Frank and Camargue worth about £1m annually.
GolinHarris will now handle work covering Industrial and Commercial, SME, ECO/Green deal and PR for the npower brand.
The agency's hire comes after the recent arrival of Guy Esnouf from E.On as director of external comms.
Esnouf said: "I am delighted to be working with GolinHarris. We have a challenging time ahead of us and I believe the knowledge and expertise displayed by the team will be invaluable in the coming months.
"We have been accustomed to working with top-class agencies and I would like to thank McCanns, Frank PR and Camargue, who have worked alongside us over the past few years, for their invaluable contribution. We part with some real regret.'
Earlier this week npower, which came last in January’s Which? survey of customer satisfaction with energy providers, rose its prices by 10.4 per cent.
It is the latest of the big six to do so amid rising anger from politicians and the public.
At the Labour Party Conference Ed Miliband pledged to cap energy bill rises, while David Cameron has now announced that he intends to "roll back" green levies pushing up bills.