The account covers Powergen's consumer and corporate mailings. Powergen has 9m gas and electricity customers and is owned by German energy company E.ON.
In a statement, Powergen said it had worked with Publicis Dialog since November 2004, during "a time of significant competition in the sector", but was now looking to extend its existing relationship with Tequila's sister, above-the-line agency TBWA\London.
Last month, Powergen launched its TBWA\London-created press, poster and ITV weather sponsorship campaign, featuring its "Wind of change" strapline, which aims to highlight the company's investment in offshore wind farms and alternative sources of energy.
A spokesman for Powergen said: "Tequila\London has been appointed on a project basis -- we are looking forward to working with them to maximise the opportunities and synergies created by our relationship with advertising agency TBWA\London."
A spokeswoman for Publicis Dialog confirmed the account would move to Tequila\London once its four-month notice period had been served, and that the decision would allow them to "pursue a larger account in the utilities market" after handling what it described as a relatively small amount of Powergen's direct marketing business.