As pointed out by bremner the saving will be minimal energy wise. I replaced mine with LEDs as I was fed up with the flickering and replacing tubes or starters Did the same with my main kitchen flourescents in a suspended ceiling for the same reason.
The real saving comes if you start costing in the time spent on replacing spent bulbs.
I wont go through the whole rigmarole but, basically. One worker to get the bulb and insert it, another worker to get the ladder and hold it while Worker 1 climbs it and inserts the bulb, and then Worker 2 puts the ladder away. This may happen several times during the life if an LED, if you're using regular bulbs