Out of curiosity and because I'm too lazy to google:
I was reading the PZ test of the 18-55/2.8-4. There's not a single word about the behavior of the lens in terms of noise (which means "power-consumption"): As soon as I switch on the camera, I hear a little motor continuously spinning, no matter if OIS is ON or OFF. Okay, it's louder and more irregular, when OIS is ON, but I was expecting OIS OFF means "no power consumption". That's what I'm used to from the DSLR lenses.
This behavior is also shown by the 100-400, so the source can be the OIS or the electrical actuation of the aperture, which both lenses share in common.
Be it what it wants to be, but it sucks the battery empty quickly. Not as fast as a Sigma Merrill, but still, "spare-battery" became a must even for short walks.