[Nut-upsuser] Newbie question: real-time power usage monitoring?

Christian Convey christian.convey at gmail.com
Wed Oct 19 12:40:40 UTC 2011

Thanks for the ideas.

You're right - I *am* looking for total energy expenditure (Joules,
Watt-hours, etc.)  I have no specific desire to do that integration in
my own code.

As far as sampling frequency, here's the deal:  I'm working on a
system to estimate the power draw associated with a run (or set of
runs) of an arbitrary program on my Linux computer.  Some of the
program runs will be brief, but I'm not sure how brief we're talking.
I'm no statistics expert, but I believe my power-draw estimates will
get pretty inaccurate unless the sampling rate is markedly higher than
the program's running time.  I *can* limit my studies to programs that
run for a long time (so as to reduce the relative error stemming from
a slow-sampling power meter), but that would put an unwelcome
limitation on my research.

On Wed, Oct 19, 2011 at 8:24 AM, Charles Lepple <clepple at gmail.com> wrote:
> On Oct 18, 2011, at 10:18 PM, Christian Convey wrote:
>> I'd like to write a Linux app that will know, in approximate
>> real-time, how much power is being drawn by a computer plugged into an
>> outlet.
> Can you narrow down your definition of "approximate real-time"?
> The default poll interval of NUT drivers is 2 seconds, but usbhid-ups has
> longer intervals for some values since retrieving values over low-speed USB
> takes a non-trivial amount of time.
> If you are ultimately trying to integrate power over time to get energy
> consumption, you probably want a device that does this measurement for you
> (a watt-hour meter, basically). Then, the exact update rate doesn't matter
> as much - if you poll less frequently, the energy per interval will simply
> be proportionally larger. You can still divide out the time to get average
> power, but of course, that won't show surges as effectively.
>> Is NUT well-suited to this usage?
>> And if so, can anyone recommend relatively cheap monitoring hardware
>> (ideally less than $200 US) that will meet my needs?
> I haven't gone shopping for an UPS in a while, but I'd think you would need
> to spend at least USD $300-400 to get an UPS with decent current-monitoring
> hardware. (The voltage-monitoring side usually is good enough even in
> low-end UPSes.)
> Do you need backup power as well as monitoring capability? What about power
> control?
> If you just want monitoring, you might consider one of the Watts Up
> products:
>   https://www.wattsupmeters.com/secure/products.php
> (Disclaimer: I haven't personally tried any of them, and I can't vouch for
> any of their software, either.)
> You could also look at power distribution units (PDUs) but I have a feeling
> that the monitoring capability is also not available in the under-$200 price
> range.

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