Rooftop cafeterias would indeed be the preferred idea for designing a cafeteria in a multi-storey corporate building. It gives employees a total new ambiance from their boring cubicle structures. There is nothing more refreshing than standing on a 9th floor rooftop cafeteria and getting a glimpse of the city you live in. However I am not sure, how much power-efficient this design is.
Almost everyone from Level 1 to Level 7 use the Lift to reach the Cafeteria. Assuming there are 300-400 people per floor, and the Lift capacity being at max 20, average Lift Usage would be (400/20)x7 ~= 140 !! And this is just for moving up. And assuming not everyone uses the Lifts at the same time, we can hope that some of the downward usage of the early lunch people will coincide with the upward lift usage of late lunch goers, so total usage may not directly be double. However the total usage number would further increase, assuming there will be people frequenting the cafeteria for breakfast, mid-tea breaks, snacks, dinner etc.
I am beginning to wonder what would be the ideal location for cafeteria in a mutli-storey building design considering to economize Lift usage as well ?