We just got a new Server. In the sensors panel there are not only temperatures and fan speeds but also "watt usage"...
Is this value somehow calculated or is it a real sensor on the supermicro main board?
144 watts seems quite low for a system with 4 disks and 2 quad core CPUs.
The others are spot on about the sensors but here's a short answer to what you're really asking:
144 watts seems quite low for a system with 4 disks and 2 quad core CPUs.
Yes, 144 watts represents only the via-the-motherboard draw, not the draw of anything else connected to the PSU directly. It won't include, amongst other things, disks.
144W sounds about right for two CPUs, the system RAM, cooling fans plugged in to the board and the various other chips. If you're after the real draw of the system for colo requirement checks I recommend picking up a cheap inline power meter (something like a Kill A Watt meter) and sticking and plugging the server in to it. Peak load during cold boot should give you a suitable baseline.
Edit: Actually, I need to put a caveat on this ... some PSUs have a SMbus connector on them that allows for more detailed monitoring. I can't remember ever encountering one of these PSUs when building a server, however that doesn't mean your server doesn't have one.
The mainboard has a super IO chip that in addition to doing prehistoric stuff like handling floppy drives (even though the board itself doesn't even have a connector soldered for it) has several analog inputs that can measure voltage.
The rest is just some resistor magic that's used to lower the voltage range (the chip I linked can only accept up to 2V), and some similar magic to convert the current flowing into the board to a 0-2V voltage range. These 0-2V values are then converted to more meaningful values by the system's firmware.
Note that this only measures the power used by the mainboard and all its components, so this excludes the hard drives and anything directly connected to the PSU, though I'm sure many server PSUs have similar hardware in them to measure the total current flowing through them (and then report back to the host via a low-bandwidth serial line).
First, it is unclear, what type of server do you have.
It is also unclear, that it measures the power on the cpu(s) or on the whole mainboard.
But the answer for your question is nearly the same in all cases.
From the power supply is coming a constant voltage, but variable current. There is a chip somewhere around the power connectors on the mainboard (or around the CPU, if it is a CPU voltage measurement).
On most printed circuits there is a chip on the power source which reshapes and "cleans" the input current, this measurement functionality is integrated probably into that. Next to the measurement, it can communicate the actual current data on an i2c bus with the other sensors of the computer.
The power usage is calculated probably by your bios based on Ohm's law.
The exact details would need detailed informations, but this is which seems relative clear from your question.
144 Watt isn't too few, if your computer doesn't do anything, then your CPU cores stay in HLT in most time, your hard disk heads doesn't move, etc. Start to recompile something on all of your cpu cores while your are doing a filesystem reindexing, you will see on the spot what happen :-)
