Hi there For me and my girlfriend the indoor season started 3 months ago, which means BKool all Winther. Both of us uses Powertap P1s and have been using them since preseason on the BKool. Back then the watt difference was around 5 watts (5W higher on P1s) 3 months ago when we started using our BKool it measures 20+ Watts less than both mine and my GFs p1s, eventhough we kalibrate the P1s before every ride. Any ideas what the cause could be? Thanks
Some part of that difference is that you are measuring at two very different points. One, at the pedals where you are getting direct power input and two, at the rear wheel where you have lost power in transmission. I don't have the numbers off the top of my head, but power at the crank (like with a SRM) is always a few percent higher than at the hub (like a Power Tap hub) because of the losses in the drive train. I'd guess that measuring at the tire interface loses a bit more. 20 watts might be on the high side though depending on what kind of power you're putting out. At 300 watts on the pedals that would be 6-7% lower at the tire. You should also look at the % accuracy claimed for the pedals and the trainer and see if those error bars close the difference.
Okay, looking at DC Rain Maker's review, the accuracy of the BKool trainer is +/-10%. That's a lot. https://www.dcrainmaker.com/2012/12/bkool-wireless-ant-trainer-in-depth-review.html
I guess you get what you pay for... On the other hand as long as it's consistent it will still be a valuable tool as long as you take that 20 watts into account.
Kevin http://kevinmetcalfe.dreamhosters.com My Strava