watts (and current) are supplied on-demand. the power supply need only be able to meet or exceed the rating to work. Voltage and frequency need to match however.
Apple's original macbooks shipped with a paltry 60w adapter. it's perfectly fine to run them off the 85w adapters. you may need to run a firmware update if the machine is behind on its updates, since the new magsafes won't be recognized by the original mobo's firmware.
Also the latest are the ones that run the cord away at 90 deg, sometimes that can cause problems with other cables or arrangements, or if the case is bent (mbp) it may not meet properly.
