If you do video calls for 7 hours a day and then ran out it means you could have maybe ~7.5 hours. Not nothing, but differences in things like screen backlight and other component efficiency still dominate battery interests over whether there is an NPU or not. If you don't spend your day on video calls it's more like a 0% increase (mic noise processing is much lower load).

Regardless if it does zilch or some minor good for you in the battery respect, the point was more NPUs don't deliver on the above reasons everyone was supposed to want AI for. Most likely, IMO, because they are far too weak to do so and making them powerful takes too much power+cost.