Don't know how many watts the current systems use or how little power they might be able to get by with using good design - that the scale of the drone market might be able to afford. There also might be the option of not requiring the same range for low speed/low range drones, or maybe use some kind of hybrid system where the low and slow guys use FLARM, or similar that gets passed along to the ADS-B fleet if one is close to the FLARM equipped unit? Is the transmit power consumption really that much of a problem for gliders? Just a wild guess at some numbers: 5 watts transmit power for 1/10 second each one second interrogation/transmit cycle @ 50% efficiency = 10 watts. At 12 volts that = .83 amps. An 8 hour flight would use .8 hours of transmit time. For .8 hours @ .83 amps = ~1 amp hour.