175 or 250 watt transponder?

Needing a replacment, my transponder shop told me that above 15k' the
FAR's require a 250 watt transponder (125w below 15k').

True?

I'm pretty sure that sailplane owners are mostly installing 175 watt
units and flying them to 18k' (and higher in wave windows). Right?

If this 15k' rule is real, I'm supposing that, since gliders aren't
required to have a transponder except in certain airspace (this
airspace is all below 15k' -- except Class A...) that we're generally
free to install anything we want (except 18k' in Class A?).

Bottom line -- 175 watts or 250 watts?

Eric Rupp
ER
(My Terra TR250D transponder is inop and I'm having trouble getting
parts - anybody have a dead one that we might be able to salvage from?)