He will star in a campaign focusing on the utility supplier's long-term commitment to responsible use of energy resources.
The campaign, created by Rainey Kelly Campbell Roalfe/Y&R, launches on October 27 and will feature Day as a character called 'Bob', a cross between a handyman and a new age spiritualist.
The campaign continues the theme of previous Powergen TV work, in which it positioned itself as an innovative and responsible company.
Its endline of 'Power in good hands' remains in this campaign. The latest ads bring humour to a serious message, according to Powergen. There will also be a radio campaign.
Powergen agreed this week to buy power distributor Midland Electricity for £1.15bn, a deal that will leapfrog the company to the number one spot in the market above British Gas.