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

[Charles Lepple]

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.