Reviewed - Hub Power Meters
HUB based power meters have been available for over a decade. For some people it's the obvious location for power metering as it's measuring forces from inside the rear wheel hub, driving the bike. It's similar to measuring the power of a car on a rolling road.
With a hub power metering system the power output will be a little less than a pedal, crank or chainring system. A small amount of power is "leaked" through the chain and drivetrain system. Unfortunately not 100% of the power you push down on your pedal reaches the back wheel ...
A hub power meter provides a highly accurate measured TOTAL power output. Similar priced single sided power meters do provide a TOTAL power, but it's the measured power from ONE leg doubled.