Hi Barbara =)
My AC was last year and it was divided into a personal fit interview, a case study and a role play. You will get material and some time to prepare a presentation you will give in front of the interviewers (there were consultants as well as HR recruiters). Then you will have a case study. It was related to the energy sector, but just a normal kind of case study. I didn’t particularly prepare for inhouse consulting cases or something, a general case prep should suffice. The case took about an hour and the calculations were a bit tough for me but I think it was just because I was nervous.
“inhouse” consulting is not that important for the case but for the personal fit part, I would say. You should know why you want to work for innogy and why inhouse consulting. The benefit of inhouse consulting is the focus on a specific industry, so you should be somewhat passionate about the energy sector when you wanna work for innogy. Be well-informed about innogy's current projects and problems they are facing in the energy sector.
I hope I could help you. Good luck at the AC!