Why does my Kill-a-Watt read Power of only half the Voltage times Current?
P = V × I only holds if I and V include phase information (i.e. are complex numbers), whereas the "V" and "I" displayed are actually the magnitudes of these variables.
The value we want to use is the P that is displayed, since it includes phase information.
So, in this case, the USB output current should be 10 W / 5 V × [efficiency].