On Saturday, September 23, 2017 at 6:20:38 PM UTC-7, wrote:
My mistake- the 175 watt units are good to 15,000 MSL, 200+ watts necessary above that. So, Moriarty and most high altitude western US sites with potential flight altitudes to 17,999 MSL (and higher in ARTCC authorized airspace such as wave windows or SAGE II clearance in the Sierra Nevada Wave etc.) would imply utilization of the higher transmit power of the TT22 over the TT21.

Yes but nobody really cares about this, and lots of lower power transponders have been flying in gliders for years and provide good visibility to ATC and TCAS etc.

The pressing reason to use a TT22 over a TT21 is for ADS-B Out compliance, especially in a certified glider the TT21 cannot be installed or used for ADS-B Out for any purpose.You will not get installation approval for it, and I don't want folks having tears when they discover that.

The important things are focusing in safety and installing and using a transponder at all if you are near busy airspace. Please install and use whatever transponder you have/can get/afford. For ADS-B future use reasons it is a good idea to install a TT-22.