Power Usage via IPMI or BIOS or?

I've seen support via IPMI, but they are via extensions to IPMI and depend on whether there is support in the motherboard for it. Here's an example on the Dell Poweredge R610 using FreeIPMI:

> /usr/sbin/ipmi-oem dell get-power-consumption-data
Cumulative Energy Start Time : 09/10/09 - 16:07:49
Cumulative Energy            : 199.93
kWh Peak Amp Time            : 04/27/11 - 19:18:27
Peak Amp                     : 0.90 A
Peak Watt Time               : 04/27/11 - 23:10:10
Peak Watt                    : 197 W

and this is on an Intel motherboard (not sure of rev) using Intel Node Manager extensions.

> /usr/sbin/ipmi-oem intelnm get-node-manager-statistics
Current Power                                 : 490 Watts
Minimum Power                                 : 36 Watts
Maximum Power                                 : 681 Watts
Average Power                                 : 497 Watts
Power Statistics Reporting Period             : 1 seconds
Power Global Administrative State             : Enabled
Power Measurements State                      : In Progress

Current Inlet Temperature                     : 18 Celsius
Minimum Inlet Temperature                     : 16 Celsius
Maximum Inlet Temperature                     : 25 Celsius
Average Inlet Temperature                     : 20 Celsius
Inlet Temperature Statistics Reporting Period : 3 seconds
Inlet Temperature Global Administrative State : Disabled
Inlet Temperature Measurements State          : In Progress

The Intel Node Manager extensions are supported by a decent number of motherboards.

There is an additional standard out there called the Data Center Management Interface (DCMI) that some motherboards support. If your motherboard supports it, similar IPMI extensions can be used to measure system power (FreeIPMI's ipmi-dcmi or ipmitool's 'dcmi' command are two I'm aware of).

Update on your Supermicro comment:

I can't speak for your specific motherboard, but some Supermicro motherboards in this "generation" have DCMI support. Here's what I got on a X8DTG-D.

> /usr/sbin/ipmi-dcmi --get-system-power-statistics
Current Power                        : 246 Watts
Minimum Power over sampling duration : 246 watts
Maximum Power over sampling duration : 246 watts
Average Power over sampling duration : 246 watts
Time Stamp                           : 05/15/2012 - 18:31:24
Statistics reporting time period     : 69380 milliseconds
Power Measurement                    : Not Available

ipmitool sdr will pull the available sensor data on the system. You may see a wattage level, depending on the type of hardware you're using.

Intelligent Rack PDU's with individual outlet-level metering can show per server or per server power supply power consumption, per branch circuit within the rack PDU, and at the input feed. There more widely available now vs. just a few years ago. You can typically get current, voltage, and in some cases, billing grade energy (kWh) data.

Also, if you're just looking for something for planning purposes, here's a site that provides server power consumption vs. workload (or utilization) by vendor make and model. http://spec.org/power_ssj2008/results/