One of the issues with training with power is that the power you can produce varies massively depending on what you're doing:
-Most riders find they produce less power on the turbo due to different muscle recruitment patterns and the different inertial load of a trainer (you have much less help through the dead spot of the pedal stroke).
-Most riders find they produce less power in aero positions, whether that's on the drops or on aerobars
-Most riders can produce more power riding uphill (
http://cyclingtips.com.au/2013/09/climbing-and-time-trialling-how-power-outputs-are-affected/)
As I'm primarily interested in time trialling next season, I'm going to do all my turbo rides on the TT bike in position. To facilitate better tracking of this, I've done an FTP test on the TT bike on the turbo to establish what my FTP is in this scenario. That will help me more accurately track the training load I'm building, as doing it based on my road bike FTP would make it artificially low.
TLDR: not all FTPs are equal
I did an FTP test on Saturday which established my FTP as 252w, and followed that up with 2x20mins at my new FTP last night which was hard but achievable. First interval was spot on, second interval was 2w under overall. This session isn't that fun but it's very specific to the demands of a 10 mile TT so I'll be doing it (or similar sessions) pretty regularly over the next few months.
I want to add in some harder intervals to improve my ability to put out more power on the TT bike but they will be more frequent after Christmas, no need to overdo it now.