My past experience has been, the 45W Power Adapter was able to charge a Macbook Pro 15 inch fine. (maybe it was just slow but it gets the job done).
At the same time, the 85W one worked fine for a Macbook Air as well.
So are the 45W, 60W, 85W Power Adapters interchangeable? Sometimes, I might have the adapter in one room, and I may bring the Macbook Pro or Macbook Air in the room and use that adapter for days.
One opinion I heard was, if I use a 45W for a Macbook Pro, it'd be slow and it is just fine. But if I use a 85W for a Macbook Air, and either when it is 100% charged, or after a day or two, it is better to unplug the power adapter or else it may overcharge the battery in the Macbook Air and make the battery deteriorate faster? Then in this case, isn't it true that I can get 45W and use it throughout the whole house, so that all can charge any Macbook Pro/Air, but none will overcharge or make the battery deteriorate?