In the 30-second ad, by Beattie McGuinness Bungay, Wallace takes a shower, with Gromit working furiously in the basement stoking a boiler to keep the water warm.
But the clever hound then gets on the phone and calls out npower's hometeam to install a modern new boiler.
Bil Bungay, a founder of BMB, said: "Wallace was superb to work with, he admitted to having laid off the Wensleydale for a couple of weeks prior to the shoot and it showed.
"He looked pretty buffed for his age."
The ad, which airs on the evening of Saturday, aims to highlight how the company is aiming to make energy efficiency easy and fun.
United Kingdom
Wallace and Gromit npower ad - watch it here
LONDON - Wallace and Gromit star in the latest npower ad, with viewers getting the chance to see Wallace in the nude for the first time.
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