There is a thread on a (mainly American) soaring webite - "ras" - on transponders. They are fitted to some gliders in the USA in regions where flights are habitually near airline routes. One recent posting included reference to an 18AH battery, which is far larger than any I have seen (or seen room for) in any UK-based gliders. It refers to a motor glider (Schleicher ASW 26E) with a foldaway engine having doors on top of the fuselage (behind the cockpit and wings) and a retractable undercarriage. I don't know how the modifications for installing the transponder would get approved in the UK, though I expect it could be done on the same model in this country, at a cost, if the CAA would permit it:
"I have the 175 watt Becker with ACK A30 encoder in my 26E. No clue how long
the ship's 18 AH avionics battery would power it as there's solar panels on
the engine bay doors helping out. If I turned off WinPilot I could probably
sell power to the utility. In any case, I suspect it'll last way longer than
me.
AFAIK the 250 watt Becker is only recommended if you spend a lot of time
in the upper flight levels.
"Antenna is mounted on the belly just aft of the gear door. The thinking is
to get the antenna as far as possible, within reason, away from the pilot's
body. No sense testing the long term effects of high frequency radiation."
This sort of thing certainly could not be done at present on the glider I normally fly, nor on many others, because of weight and space limitations.
Chris N.