How much power does the PiZero consume on it's lowest possible cpu clock setting (underclocked)?
(Let's say the only usb add on is a wifi dongle.)
And... is it worth it?
Underclocking modern ARM cores usually brings little benefit. While lower clock frequency (and perhaps lower core voltage) do reduce power consumption, a core clocked to the nominal frequency runs faster, meaning it spends less time in active state and more time in sleep, which is a more efficient* way of saving energy than permanently running underclocked.
I remember playing with clock settings on a Banana Pi under moderate load (video playback) and checking current consumption via
/sys/class/power_supply/. Differences I observed were smaller than the measurement fluctuations.
(*) efficient in the sense that you still have full CPU performance available when you need it.
• power measurement chart of the Pi-Zero: less than the RaspPi A version. https://plus.google.com/post/z13wuzkavtb1f3hnc224ifgq4oehi1mmg