Using an UPS to measure my system power usage

Ned Slider · Post by **Ned Slider** » Thu Sep 23, 2004 4:44 am

Hi,

Just wondering if this is a sensible way to measure my systems power usage or if it's technically flawed.

I have an APC 420VA UPS (300 Watts). The software records % load being placed on the UPS. It shows my system placing a 46% load on the UPS when the CPU is idle and 50% when the CPU is fully loaded with the SETI@Home program. Only the system PSU is plugged into the UPS and the monitor is plugged directly into the wall outlet so as to not affect measurements.

So at load that's 150W load being placed on the UPS but my PSU is approx 75% efficient so my system is drawing 112.5 Watts from my PSU at full load and the PSU is drawing 150 Wats from the UPS (wall effectively) to deliver that power.

Sounds about right to me. Have I missed anything here or is this a good way to measure power usage?

Ned

sthayashi · Post by **sthayashi** » Thu Sep 23, 2004 8:10 am

It could be the % load could be a function of VA and not Watts. Do you know whether or not you have an Active PFC PSU?

Ned Slider · Post by **Ned Slider** » Thu Sep 23, 2004 8:25 am

sthayashi wrote:It could be the % load could be a function of VA and not Watts. Do you know whether or not you have an Active PFC PSU?

I just checked the supply used and I don't know if it's active or passive. It doesn't mention PFC at all, but I assume it must be one or the other because I'm in the UK and PFC is a requirement here by law.

I don't really understand the concept of VA too well, but isn't VA directly proportional to Watts and since I did a VA to Watts conversion at the start of my calculations wouldn't the end result be the same?

Ned

sthayashi · Post by **sthayashi** » Thu Sep 23, 2004 10:07 am

Watts = VA * Power Factor.

On Active PFC PSUs, PF is usually around 0.97-0.99. On passive PFC PSUs, it's closer to about 0.6-0.7. It's not clear though whether the UPS measures its load in terms of VAs or Watts though.

And if you DO have an Active PFC PSU, then it really doesn't matter either

Edit: Emoticon never showed up....

hvengel · Post by **hvengel** » Fri Sep 24, 2004 9:15 am

Depending on what UPS and software you have soem of these will also tell you how much AC power you are using. i know that I was able to do this with the UPS that used to be on my computer that is now on my stepson's computer.

fractal · Post by **fractal** » Fri Sep 24, 2004 7:32 pm

You can get them for 25-30 bucks pretty much anywhere online. I bought my last one off ebay.

They measure v,a,va,kw,pf, and kwhr. Great units, well worth the price. Essential for finding "phantom loads" and dealing with "you run your computer 24 hours a day, IT is the cause of our electric bill" arguments when you can use the kwhr mode to show (to yourself, wives are immune to logic) that the tv costs more to run than your computer.

Manufacturers link: http://www.p3international.com/products ... 00-CE.html

Using an UPS to measure my system power usage

Using an UPS to measure my system power usage

Try the kill-a-watt meter