Is there any software available to monitor PSU usage? I know it probably depends on the mobo, but is there anything?
(see this quesiton as to why I would want to: Power to USB lost when computer is under load)
no, to the best of my knowledge there is nothing of accuracy.
for accurate readings you will have to monitor power consumption externally.
Given appropriate motherboard sensors, it would be possible to monitor various voltages (eg CPU, RAM, possibly PCIx bus) and amperages (USB bus comes to mind, possibly others) and then do some fuzzy math to extrapolate. But that's a really-only-slightly-educated guess.
Molly's spot-on; if you want an accurate picture of your system power usage, you need to monitor it externally.