Adam Scorer joined the body in February (PRWeek, 8 February) after a stint as senior policy adviser at the Consumers' Association.
He is understood to be leaving to become a househusband following the birth of his child over the summer.
The organisation - which campaigns for tougher laws to stamp out high-pressure sales tactics by gas and electricity companies - is now on the hunt for a successor.
Scorer will remain in post until December to handover to his successor.
The role reports to Energywatch CEO Stephen Reid.
The post has proved troublesome for the organisation: before Scorer's short tenure in the role, it was filled on an interim basis by Positive Profile director Jonathan Rush, for five months, from October 2001 until Scorer took the job.