The Trig mode-S seems to draw 6w (about .5 amp in a 14v aircraft), so if you wanted to have a 10 hour capacity you would need 60Whr of battery, A Lithium Ion battery would need to be about 0.4kg of battery (or about 10 digital camera batteries).
That is actually much better than I suspected. For reference, the car battery in my VW diesel is 70 Ah. Theoretically that would supply enough juice for 140 hours of operation.
Although I am a bit suspicious: The Trig brochure claims 240 watt nominal power output at the antenna connector. That power has to come from somewhere and with losses and everything suggests a power draw of 300-400W. Obviously that's burst power in response to an interrogation but it does mean that the busier the area is, radar-wise, the more responses are required (selective interrogation helps a lot here, obviously) and the more the power draw goes up. And I believe TCAS does active interrogation as well, don't they? Instead of relying on scatter from transponders in a radar environment?
It would be nice to hear what the impact of being in a busy radar environment is on power consumption. Nevertheless, I'm impressed with the numbers and I've got the feeling that power consumption therefore should not be an issue in creating a portable/rechargeable transponder of a size not much larger than, let's say, a milk carton, batteries and everything included. Make it robust enough and you could even argue that parachutists should be taking one with them.