At the moment the mode S transponders on the market are not practical for gliders , microlights and older aircraft without electrical systems , The CAA know this and have commissioned research program for a low cost low power consumption mode S transponder.
I hate to say it, but there
IS a cheap(er) (under £2k) Mode S available that is designed for gliders etc and is low power consumption (0.1A) so can run off a battery. It also has the height encoder built-in.