Simple question, complex answer.
In short, the number of watts your computer reads doesn’t matter. Divide it by 3.14 and call it apple pies, it’s not about getting that number exactly correct, it’s about getting that number consistent.
If you suspect that there is a discrepancy between your trainer and your pedals, a good way to test that is to record a workout via a head unit recording from your pedals, and a computer running TrainerRoad and recording from the trainer. If you do something like South Twin -3, you should get a good series of results, although my preference is to do a Ramp Test, which gets you lots and lots of discrete power points.
You can then compare the files using something like DC Rainmaker’s Analyser tool (although that costs money), and look at the exact time points and see the difference in data.
Now, as DCR will tell you, you can’t tell whether either or both of your power meters are accurate from 2 data files! You need a third, reliable reference to do that.
OK, so you’ve measured your two power meters (pedals and trainer) and you’ve realised there is an offset. What can you do with that data? Let’s say that you’ve run a ramp test, and your FTP via TR and the Trainer was 225W, but your highest minute on the pedals x.75 gave you a 250W result. That’s a significant difference.
If most of your training is on the trainer, use the 225W result, and do your workouts indoors based on that. But remember when you are riding outside, that what looks like 250W on the head unit, is actually more like 225W. You may need to adjust your TSS if that’s something you care about.
My offset is the other way around. My trainer gives me an exaggerated number compared to my pedals. So, I just have to remember that when I’m doing threshold intervals outside, I need to chase a lower number than what i’m looking for indoors.
Powermatch can solve that issue entirely if you always have your pedals on the same bike which you use for the trainer. It means that TR is taking the power from your pedals, and adjusting the resistance of your trainer to get the desired result, as opposed to using the internal strain gauge in your trainer.
If you’ve got a standalone trainer bike, and an outdoor bike with the pedals, you’re screwed though, because switching pedals across from bike to bike all the time defeats the purpose!