Hello, Ken! Welcome to Zwift and the Zwift Forum!
First off, let me say that there are a number of potential factors at work when using a Power Up (PU), so I’m not sure I’ll get your exact situation. Hopefully, though, this will help at least a little.
First off, I would say that when using a PU you probably won’t “feel” anything with regard to your interaction with the trainer. Your wattage certainly won’t change, as that is just the amount of power you are putting out. Your speed, however, may or may not change depending on what type of PU you are using and on what type of terrain (e.g. a feather won’t make any difference going downhill and the aero helmet won’t be of much help going up).
If you do use a feather PU on a climb, though, what you should see is your watts per kilogram (wkg) increase when you’re looking at your name in the rider list on the right side of the screen. The effect of the feather is that it makes your rider 15 pounds lighter (for 15 seconds), so at the same watts output your wkg will increase for that time. All other things equal this should make your rider climb a little faster for that duration, but if the incline happens to increase at the same time you use the feather you may not see any speed change at all, or possibly even a reduction.
The effect of the aero PU may even be a bit harder to discern, unless you are able to view your avatars progress against another rider travelling at around the same speed. I generally use these on descents and definitely notice my avatar ‘rolling away’ from others with whom I have been descending. If riding on the flat the effect would be less noticeable, but you should still see some effect of your avatar catching another rider more quickly and/or your speed increasing by a bit at the same watts output.
So, in a nutshell: using a PU won’t have any effect on your wattage (all other things equal), but you might see some change in speed depending on when you use it.