Batteries arenīt the problem!
Take a standard 7,2Ah battery and it will be enough for a day of non-stop transponder usage. With solar cells you can even keep the capacity of the battery if the weather is reasonably well.
I do have a Mode-S transponder in my glider and switch it on from time to time when crossing an airspace-E-final of a busy military aerodrome or when entering C for a wave flight.
As an ATCO I would not recommend a mandatory utilisation of transponders in E for all gliders since conflict alerts would cover all our screens. It might be useful in certain areas where TMZs could be implemented (as they are in Germany, but only in controlled airspace). Also they donīt have to be Mode-S transponders, the old Mode A/Cs are just fine as well. I donīt mind the registrations of the glider, itīs just good to know that someone is there.