I thought 1 V⋅A = 1 W so why are UPSs rated for example 600VA 320W?
Watt vs VA
deme on 04/08/2021 - 11:32
Comments
Cheers, this is great advice.
The answer is actually more complicated than just power factor. A UPS will use an AC source to charge a battery (ie. convert AC to DC) and when the mains fail, use the battery to supply connected devices (convert DC back to AC). Power factor will have some effect on the in/out ratings, but the main factors are battery & inverter sizing.
Just something to keep in mind … the amount of time you want connected devices to run is as important as total power draw when sizing a UPS. Refer to the manufacturer runtime curves/charts
VA (kVa) v's kW (W)
Depending on a few factors, basic calc is.
VA *0.8 = kW rating.
E.g. 2kVa = 1.6kW
If you are trying to work out how much many amps you need for a solution, then you want to match the volts and have a greater current (amp) than what is required.
E.g. notebook PC charger. It says it uses 19.5V then match 19.5V to the power supply.
It states it wants a current of 6.9 amp then ensure the supply feed is of 6.9 amps or greater.The system will only draw what it needs, providing the minimum is being delivered. If the volts are too high or low, you run the risk of burning something out. Same with the current being too low.
For UPS, you need to calc all the devices you are planning to run from it.
They should be a single-phase solution for a home device - standard outlets etc.
Then work out the expected run time.Most UPS provider's websites will have a calc to help you with this.
Otherwise, not to sure what your question is, or what info you are wanting.
Hope this helps.
Where did you pull 0.8 from?
Is this a serious question?
The standard conversion factor for kVa/kW is .8
Sure, there are other factors depending on what you are trying to work out. However, as a standard conversion, all other things being the same - it's .8Google will help you also my friend. :)
"The kW to kVA Formula:
Apparent power (kVA) x power factor (pf) = actual power (kW) e.g. 100 kVA x 0.8 = 80 kW. The formula for converting kW into kVA is: Actual power (kW) / power factor (pf) = apparent power (kVA)"That’s a terrible way of citing your sources - “google is your friend” Is that what tertiary education has come to?
Saying Power Factor is 0.8 is an assumption, not standard conversion
The standard conversion factor for kVa/kW is .8
Dude you need to read up on this rather than just spout misinformation. Google "real power and reactive power" for more information.
@fantombloo: I think if each of you re-read my comment you will find I also stated that it 'depends on what you are trying to work out'.
My point still stands.
Standard comparison of kVa and kW is 1 to .8 when talking about real power being converted.
Google reference was for the above comment, so others can do research. Shall we discuss the signal phase v's 3 phase, phase angle, real power v's reactive power and so on?
For those that want to get 'overly technical':
P(kW) = S(kVA) × PF
kW = kVA × PF
P = 3kVA × 0.8 = 2.4kW
With that, well educated on this thank you, and no need to 'read up' on anything further. The love of social and forums. I suspect those above may wish to check their calcs.
Let's get the popcorn out! :)
Have a good evening.
@fantombloo: eye roll
Actually… I can't be bothered having this discussion.
I'd recommend you read my previous comments in detail before you respond any further. Either way, if it makes you feel better to attempt to show one up, please feel free. I really can't be bothered. I'm very well aware of my basis of comment.
If it makes you feel better - and you wish to discuss particular UPS's in this case, rather than properly read my response on 'real power'; then yes, there is some UPS's like those from Eaton's 9PX range that is said to offer 1:1 kVa/kW, however, it's not a direct 1:1 or real reference as much as it is marketing and carries a higher rating for kVa.
I think the point is that you are missing my reference as to the conversion / real power and power factor.
But I'm not here to both schooling you on this. The comment still stands!
Not sure if you are Mr Tesla and Mr Westinghouse in this debate. ha! :)
All the best for your evening.
@fantombloo: Can you clarify, as I too am under the belief that 0.8 lagging is a good assumption to use as a basis for calculations involving power factor, when you're talking about consumer mains.
@Chandler: 0.8 (lagging or leading, doesn't matter) is sometimes a permissible aggregated power factor for purchasing energy - example here from the first duckduckgo hit I got on "minimum permissible power factor."
This is a limit to keep the power companies happy so they are getting properly paid for the power they provide - they charge for energy (not power), based on delivery of real power, a product of voltage and current, which is diminished when they are not in phase (power factor less than unity).
@fantombloo: From what I'm gathing:
This a rule of thumb for grid -> home?Rather than UPS Battery -> Devices? Or is it always 0.8?
I'm guessing those negging you are doing so because 0.8 doesn't appear to be cited nor helps calculating VA -> W for UPS?
@deme: 0.8 is a number that is considered but in the end essentially pulled out of someone's ar5e. The owner of that ar5e, the power company, has every right to demand so for connection to their own facilities for their own reasons, because it's their ar5e. Considering most homes need to connect to the power company it's a good limit to consider. (Most people won't need to worry about this - good suppliers of electrical products will already have this as a concern in designing their products.)
0.8 has nothing to do with the OP.
@fantombloo: That's right - I had forgotten it was that the supply authorities specified 0.8 as your minimum power factor.
I wouldn't say that has nothing to do with the OP however. A UPS will not be able to output the same number of kW as it's VA rating, depending on the power factor of it's output. At unity it would, but at 0.95 you'd be getting 95% of it's VA capacity in kW. And the power factor of it's output depends on the load you're putting on it
I would say most homes would be primarily resistive loads and it wouldnt matter; but most homes these days have mostly fluorescent or LED lighting and lots of AC/DC power supplies, so there is some reactive power there.
Is there enough to matter, in the scheme of things? Probably not. Especially given you're probably only connecting a few key devices to the UPS.
OP here is a few calcs for you.
Hope it helps…https://upsselector.eaton.com/Calculator
VA is a measure of power supplied.
Watt is a measure of power consumed.
The Watt rating will always be lower than the VA rating due to: https://en.wikipedia.org/wiki/Power_factor
You need to always use the Watt rating to find a suitable UPS. Ignore the VA.