I ran a test.
I have 5 servos, with nothing attached to them, going through some preprogrammed rotations. This requires an average 10W. This is the energy required for five servos to do zero useful work.
Now when I attach my robot stuff to the servos, they require a sum average of 13W. So only 3W are for real work - thats about 23% efficiency, horrible! And I suspect a good percentage of that 3W are lost in servo gearing inefficiencies, so servo efficiency is surely below 20%.
I'm trying to calculate the exact losses of energy in my robot, and as the majority is wasted in the servos, I need to figure out what to change to fix it. Going to buy some brushless servos to see if I can get a better efficiency. I'm also considering scaling, so I'd need to understand algorithmically the energy stored as inductance in the motor.