You know Coconut battery? The MBP reads out the Ampere and Volt information of the battery. Coconut battery calculates the resulting watt for you. istat gives you the original A/V values and you can calculate yourself.Wait, how did you test that? A wattmeter wouldn't work unless it's plugged in.
Power draw should change significantly when you reach 2% battery. I've experienced very, very noticeable underclocking during extreme low power situations.
Obviously I tested with an almost full battery. I never reach 2% battery.