The European Commission, which is examining the deal, recently sent out lengthy questionnaires with more than 200 questions to competitors and customers of E.ON and Innogy. The deal has triggered criticism from numerous municipalities and small energy providers.

The deal would see E.ON acquire Innogy's prized regulated energy networks and customer operations, while RWE would take on the renewables businesses of both E.ON and Innogy to become Europe's No.3 renewables player.

EU regulators want to know whether E.ON and Innogy compete closely with each other and if there are other credible alternatives for customers, according to the questionnaire seen by Reuters.

Respondents were also asked for their views on E.ON and Innogy's commercial and pricing strategy and whether they have been able to attract customers from the two companies.

Regulators also asked if rivals were able to compete with the two companies in concession tenders and as a supplier of smart meters. Complainants say E.ON may be able to offer such metering services 50 percent cheaper than the next competitor.

The questionnaire looked into the impact of the deal on electric vehicle charging stations and if E.ON and Innogy have a competitive advantage.

The Commission's preliminary review of the deal ends on March 7. It can either clear the merger with or without concessions or open a four-month investigation if it has serious concerns.

(Reporting by Foo Yun Chee; Editing by Mark Potter)

By Foo Yun Chee