The Good Relations Group agency will handle media relations, content and digital strategy, and will work closely with tech marketing firm McDonald Butler to support its sales programme.
Harvard will target environmental and technology titles, as well as the mainstream business press, widening its reach as energy efficiency and rising prices increasingly become a board-level concern for businesses.
The agency, which was appointed without a pitch, will also focus on reaching policymakers on green issues.
PowerPerfector provides technology that enables businesses to reduce energy use, saving costs and cutting carbon emissions. It is used by companies such as Tesco, Hilton and EDF, as well as several government agencies.
Harvard MD Louie St Claire said: 'PowerPerfector has so many stories to tell. It's a fantastic SME success story, a great example of a UK brand making it overseas and a brilliant technology company doing amazing things for its customers.'
PowerPerfector previously handled comms activity in-house for more than a year.