Eric Greenwell wrote:

Do you have a reference to the regulation concerning power requirement?
I haven't been able to find it, and people just seem to rely on the
product descriptions when I ask about it.

Actually, I do, too. I see no reason not to believe the manufactorers.
According to Funkwerk, output requirements are higher for tansponders
which are operated in aircraft flying at altitudes above 15,000ft or
speeds above 175kt. Unfortuately (of fortunately), gliders often operate
higher than 15,000ft.

I write this with the European situation in mind where Mode S is already
mandatory. As air traffic is typically an international thing, I would
assume that the FAA and EASA have been reasonable enough to discuss this
and to agree to the same requirements.

BTW, problably not the cheapest but one of the more popular mode S
transponders in Europe is the TRT800H by Funkwerk.
http://www.funkwerk-avionics.com/cms...4&changelang=4