[nickcw]

Power = volts * amps

Volts is as supplied by the utility company.

Amps are monitored per rack and the usual data centre response to going over an amp limit is that a fuse blows or the data centre asks you for more money!

The only way you can decrease power used by a server is by throttling the CPUs.

The normal way of throttling CPUs is via the OS which requires cooperation.

I speculate this is possible via the lights out base band controller (which doesn't need the os to be involved), but I'm pretty sure you'd see that in /sys if it was.