2.9 GHz Intel Core i5
You need to be more specific, what model i5, how many cameras? what are their resolutions? have you enabled direct to disk recording in BI for EACH camera? its a must with high res cams.
The mac pro actually is not very efficient. A modern windows i5-haswell/skylake system like the hp elitedesk will idle at 15w and consume about 30w at 30 percent cpu consumption. Its 300 vs 4000 for the mac pro. Power consumption is largely based on the cpu...a windows pc with the same processor would be just as efficient.
What cameras? Are you using the BI demo?
I am not sure you are looking at the new mac pro's.... I was pretty sure the Mac Pro isn't too much of an energy hog... So I looked at apple's specs and they give this: https://support.apple.com/en-ca/HT201796
Something is wrong with your measurements if BI only adds 2 watts. I saw something online indicating 87w..regardless 43w idle and 205w at 100 percent is high for a modern pc. Modern i5's idle at 15 or so, i7 is similar. Dell/hp workstations/servers are way more power efficient.. Regardless the point was that a windows server would perform just as efficiently spec'd the same...except it would cost 1/5 the price. You have a different situation than the op since your system is already running and in place. It is always best to run a vms on a clean dedicated pc. For op's load, a 200/300 dollar hp elitedesk would be more than sufficient and way more efficient than the old hardware.
Have you set direct to disk? Buy a 200/300 hp/dell 4/5th generation i5..you will have tons of room to grow. The system will pay for itself in power consumption alone.
That processor is very weak. Try dropping your frame rates down to 5FPS and see what happens.
Just to clarify did you make sure EACH camera is set to direct to disk and activate the BI license on that machine? It seems like BI is too much for this machine though it should work with your light load...the power cost savings alone make it worthwhile to upgrade to a proper machine...
Toss the old box in the trash...
Did you activate BI on the machine? This is an important step as without it direct to disk will not work properly.