The 30-second ad, by agency Beattie McGuinness Bungay, shows the pair attempting to insulate a loft with sheep. It aims to highlight the benefits of insulating a loft properly.
It will break on 23 July during the Home Show on Channel 4.
Npower marketing director Kevin Peake said: ‘Wallace and Gromit are a family favourite. We're hoping their enthusiastic approach to energy efficiency will help people see how simple it is to make small changes.'
Npower's first TV ad campaign featuring Wallace and Gromit appeared in March.
Npower signed up the characters as brand ambassadors to a two-year deal last December, ahead of the launch of the latest Wallace and Gromit, A Matter of Loaf and Death.