Power Cranks

Another Newbie!

I found this while searching for Training with Power and I am glad I did.  I'm in!  

But I have a question.  My trainer is an old Cannondale equiped with Power Cranks with a Power Tap meter on a stationary trainer.  I also have a wireless Power Tap on my long distance bike (without Power Cranks).

Comparing my wattage between the two leads me to believe that my CP is about 20 watts less on the Power Crank bike.  

Is this what would be expected?  That my CP would be lower on the Power Crank bike?

My hope is to do the vast majority of the training sessions indoors on the Power Crank bike, but when weather permits, ride my long distance bike out of doors.  I would adjust the training plan accordingly.