LOL. Yes, the devil's in the details! What I have in mind, or what I'm imagining, is some simple and inexpensive method the average OGR guy can use to get a clue about power requirements.
I was thinking by dropping out the fixed/overhead power (8 Watts just for plugging the Z-1000 into the wall), the slope of Power Out vs. Power In would be the incremental efficiency (about 80% for this Z-1000).
So how would it work? Say you add 5 lighted cars to the track and the power meter goes up 25 Watts. Because the transformer's incremental efficiency is 80%, this means only 20 Watts (80% of 25 Watts) is going to the cars. Now I have a bead on what the bulbs are drawing. Then you start the engine pulling slowly and the meter goes up, say, 50 Watts. Again, use the 80% slope and that suggests only 40 Watts (80% of 50 Watts) is going to the engine. Remove the lighted cars and again some simple math to estimate how many Watts it takes to pull the cars at different speeds...it may be that even if going to LEDs there's not enough transformer power to pull a long consist of heavy-weights. And so on.
Remember we're just talking here! 
