~~0.8 watts? Honestly, I think it gets closer than I was expecting.~~ (edit: millli/micro, messed it up. This is a tiny amount of power. Needs to get near that 1W they are aiming for to be useful). Searching around, I see estimates of 5-20 watts when fast charging, and 1-2W in standby mode. The article says they are aiming for 1W in the next couple of years, which can probably do it. However, it's not clear what peak output it. You would probably use half the space for a normal battery and half for this power source, so that the phone can charge itself but also have a higher output when it's needed.

It probably doesn't even need to provide all the power. Imagine if your phone would trickle charge wherever you were. If you're watching netflix you might run out of battery and have to charge. If you aren't using it much, even if the output of these things can't keep up, the battery could last days or a week on a charge before eventually running out.