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eeeaddict

join:2010-02-14
reply to idaggerpwn

Re: UPS - APC

I never understood if 500va is 300 watts and the modem/router take 30 watts shouldn't it be 10 hours not 2 of runtume?



Davesnothere
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said by eeeaddict:

I never understood if 500va is 300 watts and the modem/router take 30 watts shouldn't it be 10 hours not 2 of runtume?

 
Ah (Ampere hour) rating of battery is the main factor influencing runtime, not the W or VA of the electronics of the power inverter in the unit, which instead only tells you how hard you can load it without damage.

Even THEN, calcs are not as easy as we would like them to be, as how heavy the load is (% of max) makes a disproportionate diff to the answer.

APC's website used to have a pulldown menu-driven estimator for that, for the Smart UPS series at least, mayhaps for some of the others.

You select the load, and it says how long that each model of that group ought to run.

BTW, 500VA is approx 300W.

Fraoch

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said by Davesnothere:

BTW, 500VA is approx 300W.

How do you figure that?

Isn't 500 VA = 500 W?

Or is it best to always assume a PF less than 1 for computer equipment?

Not trying to point out an error as you seem to know more about UPSes than I do, just curious.


sbrook
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Technical ...

That applies to DC supplies as in from a battery, or from perfectly sinusoidal supplies with no power factor (purely resistive load)

The reality is that UPS output is not a perfect sinusoid and the loads today on electronic devices do present a power factor.

To make a UPS rating sound good they quote the VA rating just like audio amplifiers quote some exaggerated power rating instead of RMS Watts

The reality of course is that in this case, it's a problem, since if you believe that a 500VA UPS will deliver 500 W, it will have a shortened lifespan!



Davesnothere
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4 edits
reply to Fraoch

said by Fraoch:

said by Davesnothere:

BTW, 500VA is approx 300W.

How do you figure that ? ....

 
Actually, AFAIK, there is some rounding being done to say that 500VA is approx 300W.

As I understand it, it's a rendering of that old Peak vs RMS comparison formula - just another way for UPS manufacturers to misrepresent/optimize the specs of their units when advertising them.

Speaker and audio amplifier makers do the same (or worse), and have since Christ was a cowboy.

RMS is about .707 of peak, and conversely, peak is about 1.41 times RMS.

If we were to apply that formula, then 300W would be more like 424VA, or 500VA would be more like 353W.

For one of their common larger units (an older model of Smart UPS 1000), APC states that 1000 VA = 670 Watts.

If you apply the same formulae as I did above, the result is closer to agreeing with APC's published specs for THAT particular beast.

Quick rules of thumb for this :

(1) The smaller power number is the more honest (and SAFER) one, when deciding how much stuff to try to protect with a UPS.

(2) RMS vs Peak ratios are pretty close if estimating, but so is a ratio of 2:3 or 3:2.

(3) The size of a UPS's battery (expressed in Ah) is often more important to a user than the VA or Watt rating of the electronics of a UPS, as the battery size/capacity determines the runtime.

However, Watts and VA are easier to sell.

Cheers !

InvalidError

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said by Davesnothere:

Actually, AFAIK, there is some rounding being done to say that 500VA is approx 300W.

As I understand it, it's a rendering of that old Peak vs RMS comparison formula - just another way for UPS manufacturers to misrepresent/optimize the specs of their units when advertising them.

The difference between the VA rating and Watts rating has nothing to do with RMS.

Volt-Amperes are the "apparent power" of a capacitive or inductive (reactive) load rated in VARs while "real" watts is the power of a dissipative load. In complex numbers, watts are the real component and VARs are on the imaginary axis. The load the UPS sees is the magnitude of this W+jVAR vector.

If you have an UPS rated for 500VA which can handle up to 300W, it means it can handle up to 400VARs at 300W.

Since nearly everything uses SMPS these days and SMPS usually have power factors better than 80% even without PFC circuitry, the maximum VAs UPSes would likely see at 300W is 375VA.

It is likely possible to push a 500VA UPS to 500VA but you need inductive loads like AC motors and transformers. Not many people put their fridge or freezer on UPS but 500VA might be enough for that.


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1 edit

said by InvalidError:

....Volt-Amperes are the "apparent power" of a capacitive or inductive (reactive) load rated in VARs while "real" watts is the power of a dissipative load. In complex numbers, watts are the real component and VARs are on the imaginary axis. The load the UPS sees is the magnitude of this W+jVAR vector.

If you have an UPS rated for 500VA which can handle up to 300W, it means it can handle up to 400VARs at 300W.

Since nearly everything uses SMPS these days and SMPS usually have power factors better than 80% even without PFC circuitry, the maximum VAs UPSes would likely see at 300W is 375VA.

It is likely possible to push a 500VA UPS to 500VA but you need inductive loads like AC motors and transformers. Not many people put their fridge or freezer on UPS but 500VA might be enough for that.

 
SO, regardless of which explanation we believe and accept for doing so, yours or mine (and the Peak vs RMS one does look like it could work in some cases), what remains is that we would be safest, when shopping and deciding the load for a UPS, to believe the Watts figure and ignore the VA one, though the marketing gurus would rather that we did the opposite.

InvalidError

join:2008-02-03
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said by Davesnothere:

SO, regardless of which explanation we believe and accept for doing so, yours or mine

The difference is that mine is correct engineering-wise. VAs represent the UPS' ability to drive a combined dissipative and reactive loads.

The relation between VAs, VARs and watts is: VA rating = sqrt((VAR rating)^2 + (wattage rating)^2)

It is only a coincidence that watts = VA/(sqrt(2)) on SOME UPS and that would be when their VAR rating = wattage rating. In that specific case, the VA rating would be 1.4x the wattage rating. Of course, no UPS manufacturer advertises VAR ratings since they are redundant and high VARs is something devices try to minimize anyway.

The reason only watts matter in most modern cases is because SMPS have relatively little reactive power and technically, this is a good thing since reactive power does not contribute to driving actual useful loads; it only contributes to draining batteries faster, requiring thicker wire gauges and higher rating fuses, breakers, PCB traces, etc. That's also why industrial and commercial power feeds either have penalties for excessive reactive loading or are billed based on VAs rather than watts like they usually do for residential.