I have a meter (from Maplin) which measures power in both W and VA, one
assuming that current and voltage are in phase and the other taking into
account the fact that they are not. Real power [W] = apparent power [VA] *
I presume it is the real power that an electricity meter registers and that
you want a plug-in meter to read.
You would like the meter to measure W and VA, so that
when picking a UPS (uninterruptable power supply), you
measure both quantities and pick a UPS which deals
with the more taxing one.
Sometimes the loads have more W, sometimes more VA
(relative to the ratio of W to VA the UPS offers).
(Amazon.com product link shortened)
1500VA / 900W UPS <=== example of a typical ratio of capacities
You can hang a power bar off the front of the meter,
and put your entire computer room on the meter, to get
total W and total VA, then the numbers can be used when
picking a UPS (which has both a W and a VA spec).
On a modern computer room, W is approx equal to VA,
due to the use of Active PFC on the ATX supplies. On
older computer rooms, the PF is 0.7. There was no Active PFC
in the old days, and more heat also came out of the supply
cooling hole. The new supplies are much better on
On Friday, 31 July 2020 15:10:51 UTC+1, NY wrote:
No. Power is Watts, and VA is not Watts.
Power is the rate of delivering energy, and one Watt is needed to accelerate one kilogram by one metre per second per second.
Electrically, average Power W is the average of instantaneous (Voltage x Current); and VA is Root-Mean-Square Voltage x Root-Mean-Square Current. Power Factor is the ratio of those two, W / VA
That is NOT what it registers. It registers real Energy, in real kilowatt hours.
(c) John Stockton, near London, UK. Using Google Groups. |
Mail: J.R.""""""""@physics.org - or as Reply-To, if any. |
I can remember when I worked at a factory which made tvs back in the valve
days, they used an autotransformer, so chassis was live and a half wave
rectifier. They ran soak tests on the sets in huge banks, and got a visit
from the Electric company about them squashing the half phase they wre on
they had to have a three phase supply fitted and run some off of each phase.
What I never quite got was that most makers at the time did the same thing
so out in homes these sets were all doing this, surely?
This newsgroup posting comes to you directly from...
I don't think they are particularly sophisticated (and I have a couple
that look exactly like that) and may have difficulty reading much
other than a near resistive load with any real / absolute accuracy.
However, I find them fine as a general indicator of power consumption,
especially when comparing like with like (SMPSU powering a Raspberry
Pi or a PC etc).
I can't think if I've ever measured something that was actually rated
that didn't also have variables / additions that could have impacted
the result or if I have, I didn't note that it was far away from what
was stated (suggesting they were accurate and the meter also
I think the last things I measured / compared were a RPi NAS, a
slimline Shuttle PC running as a NAS, a Synology NAS and a Buffalo
Terastation NAS and the results were about what I expected. As
mentioned though, you often had to take several readings with the HDD
idle, unit sleeping etc.
Cheers, T i m
I guess what I meant was 'I don't think they are particularly
sophisticated' ... compared with professional and more sophisticated
kit that costs many times more and probably comes with a 100 page
instruction book and a certificate of calibration ...'. ;-)
If they were as good for that price that would be nice to know.
Because the 'cheap stuff' is made down to a price rather than up to a
spec (or a much higher price <g>), they often fail / become less
accurate at the extremes, or when measuring the more awkward stuff.
All down to the laws of diminishing returns on the cheaper gear.
Cheers, T i m
They typically use dual sigma-delta converters, with maths
in firmware. And the maths in firmware, have been slightly
tweaked over the years for better accuracy. (This is what
happens when your privately held maths are exposed to
Yes, the meter is good, and fit for intended purpose.
With better quality analog components used inside,
the accuracy might be improved slightly. But not in
a way that affects the gross improvement these meters
bring (7W reading on the above meter, versus 100W
approximate reading with separate regular meters).
Using an ordinary digital meter to do volts,
a digital meter to do amps, the calculation of VA resulting,
the 100W, is dead wrong. You can validate the 7W number,
against the known loads inside the (sleeping) PC at the time.
Vetting a PC in S0 run state, that's harder to do.
That's why I recommend checking PC in sleep state,
then compare to any other method you want to cook up
for sleep state, to do the same task.
The refresh power for DRAM chips is known and on the
datasheet. You can compare that load, to what the
You can also apply DC loads to USB ports, then figure out
roughly what kind of efficiency exists in the +5VSB converter
(a separate converter, separated from the main converter).
In the old days, +5VSB was only 50% efficient (terrible).
Very useful. Thank you.
Clive says at the end: "It's actually really well made. It's accurate and
works surprisingly well". (25 min) That's all I need to know.
Mind you I did once follow Clive recipe for porridge which wasn't a total
success. Using raisins was okay but his suggestion to add Coffee Mate didn't
work for me.
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