Educated guess...how much am I losing with a worn out drivetrain?

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My Powertap currently lives on my backup bike, which has been retired to trainer duty. The drivetrain's shot from overuse: crunchy botom bracket, stretched chain, the cassette and chainrings are probably worn out too. Until I get a second PT on my new bike and/or get a Computrainer, the PT stays where it is. I was wondering as I was doing a MAP workout today and reviewing the data just how much power I was losing as a result of drivetrain loss. Perhaps this has been covered, but I wonder what anyone's best guess is.