search for the several threads on this type of thing that gives the guide. Varies from:

1) Do some testing to understand what's happening, then you can decide which to use.
2) Just use the powermeter as that's what you use outside
3) It's because you're using disk brakes and tubeless....

In many cases the lack of a warmup of the trainer and calibration at that point is at least partly to blame. With the amount (lack of) information you've provided that's all, hence please look back for the many many threads on 'my trainer is different to my powermeter' - normally linked with 'please justify my belief that the lower one is the incorrect one'.

Make sure the pedals are installed right and use those. I've had some pretty terrible accuracy issues with kickrs before. Besides you might as well use the same power meter you'll be using inside and out

Always go with the higher number;).

I think it's fair to say that neither is correct. Both sources are subject to error. As the old saying goes; 'A man with one watch always knows what time it is, but a man with two watches is never sure'.

When you say 40watts higher, what is your point of comparison? Is this an eyeball comparison? Is this at a steady state of 100w, 200w, 300w,...? Was it just a momentary glance at two readouts, or did you compare over several minutes?

I just went through a comparison process myself using a Stages Dual PM, Kickr V1, and Neo OG. I used Zwift to record the trainer measurements and my Garmin to record the Stages PM. I used zwiftpower.com to compare power readings from the devices (Stages vs Kickr, and Stages vs Neo). It took a little be of time to align and trim the data, but wasn't to difficult to do in the end. Like you saw, my Kickr reported higher power than my crank (11% higher average power). The Neo also reported higher, but was within 3% (average power).

Your topic title is backwards from this post. Title says the Kickr reads lower, which you typically do lose something in drivetrain to the trainer. The forum post quoted here says opposite, that the Kickr reads higher.

Could you clarify which way it is?

Also, is your P1 pedals dual sided (both pedals) or just one side?

Next, you could ask a buddy to come try your smart trainer and validate what the delta is from their on-bike power meter to the Kickr. That would be a great data point.

I own three bikes with on-bike meters, so a plethora of options to see what's going on if I have an issue. Give that "test" a try and off your buddy a beer for the task of stopping by.

Great point I’ll fix the heading. Yes the dual sided PM.

I am thinking just using the power off the P1s to keep everything consistent.?