Calculating PC watts from ac true rms current ?

[digitaldreams]

Yes thinking of seeing how much our pc's use during peak gaming and hence choosing future power supplies as a result.

I have a fluke 'true rms' meter and was going to attach in line with the mains cable. Yet how do I calculate watts from that ?. Yes watts is volts X amps but rms must give a different value to a non-rms ac amp meter

Going deeper we then have the power supply efficiency. I always buy titanium now hence subtract 3% lost in the supply itself ? (assuming 50% load)

 

[Mr Evil]

You can't calculate AC power from current and voltage alone due to power factor. You need to know the phase angle between the current and voltage too, which you can't measure with a normal multimeter. You can get dedicated plug-in power meters that will do the job fairly cheap.

 

[Ormy1]

Any modern good quality PSU will have active power factor correction, and therefore have a power factor approaching 0.99 so volts x amps (both rms) would give a good estimate. But yeah just easier to buy one of these and they are useful to have around.

https://www.electronics-notes.com/a...gin-power-meter-electricity-usage-monitor.php

 

[gpuerrilla]

As above, I have two of these wall meters now. If you shop about they can be had for under £15 and will give enough accuracy. If you spend long sessions with the computer on, or like me have it on 24/7 then a better quality power supply will shave off some watts overall.

It has to be said though if the platinum or top end unit is way more expensive than say a gold, I wouldn't panic about the pennies in difference it will save and just get a gold.

 

[Mr Evil]

Any modern good quality PSU will have active power factor correction, and therefore have a power factor approaching 0.99...

At high output yes, but it can be <<1 at lower ouput. For instance I just did a quick test of a good 1kW PSU, and the power factor is 0.97 at 300W, and only 0.8 at 100W.

 

[Ormy1]

At high output yes, but it can be <<1 at lower ouput. For instance I just did a quick test of a good 1kW PSU, and the power factor is 0.97 at 300W, and only 0.8 at 100W.

Hmm. 0.97 at 300W sounds right but I would expect more like 0.9-0.95 at 100W, can't argue with empirical data though.

 

[EsaT]

I have a fluke 'true rms' meter and was going to attach in line with the mains cable.

Won't help anything with moments of peak power draw, which need very high sampling rate.
50ms resolution max/min measurements of Brymen BM869 would start getting somewhere closer.

And true load transients, which are insane on current high end GPUs (esp. Nvidias) are masked completely.
https://www.igorslab.de/wp-content/uploads/2020/09/8a-Gaming-Zoom-Power-1.png

I always buy titanium now hence subtract 3% lost in the supply itself ? (assuming 50% load)

Titanium PSU has 5% losses, not 3%.