UPS 500VA = 300 Watts How come?? Please??

[[email protected]]

Hi,

I need to order a UPS for my computer system so I connected an amp
meter and under the worst load conditions it was drawing about 4.2A
for everything. I figured 4.2 x 120 = 504 so I need a 500 Watt unit.

I started looking and I find things like...
Battery load capacity: 500VA (300 Watts)

How is VA different from Watts?? I thought VA meant V x A.

Thanks for any clarification.

 

[Ian Stirling]

Hi,

I need to order a UPS for my computer system so I connected an amp
meter and under the worst load conditions it was drawing about 4.2A
for everything. I figured 4.2 x 120 = 504 so I need a 500 Watt unit.

I started looking and I find things like...
Battery load capacity: 500VA (300 Watts)

How is VA different from Watts?? I thought VA meant V x A.

Power factor.
VA and W are the same for resistive loads.
For a load with a large hase shit between the current and voltage, and
the same power as a resistive load, the absolute current is higher.

 

[Spehro Pefhany]

Power factor.
VA and W are the same for resistive loads.
For a load with a large hase shit between the current and voltage, and
the same power as a resistive load, the absolute current is higher.

Best to let your hase out in the back yard, when it feels the urge.

Best regards,
Spehro Pefhany

 

[Jeroen Vriesman]

P = 0.5 * Vmax * Imax * cos(phase) (I can give you the calculations for
that, but not needed I think).

What you read on your volt or current meter display is "effective value":

Veff = Vmax/sqrt(2)
Ieff = Imax/sqrt(2)

So for sero phase shift you get: P = Veff * Ieff

The VA is defined as only: 0.5 * Imax * Vmax, for whatever phase, that
makes sense, because the circuits can handle a certain maximum current and
voltage.

The device itself is specified for a real load, with a certain maximum
capacitance or inductance, from the above you can see that it is specified
to hande a maximum phase shift of:

cos(phase) < 300/500, =>

-0.92 < phase < 0.92 rad.

 

[[email protected]]

Hi Ian, thanks for taking the time to answer.

So where does that leave me with the UPS? With a max of 4.2A, will the
300 Watts be enough or do I need to go with a 500 Watt unit whatever
that works out in VA??

Thanks.

 

[Keith R. Williams]

Hi,

I need to order a UPS for my computer system so I connected an amp
meter and under the worst load conditions it was drawing about 4.2A
for everything. I figured 4.2 x 120 = 504 so I need a 500 Watt unit.

Thus you need a unit that can supply 504VA.

I started looking and I find things like...
Battery load capacity: 500VA (300 Watts)

Get a bigger UPS. You're right at (actually over) the edge.

How is VA different from Watts?? I thought VA meant V x A.

Your current waveform isn't going to be sinusoidal (making your current
measurement suspect too) and not in phase with the voltage waveform.
Thus you need to take the PowerFactor into consideration to convert
from VA to W. The PF, coincidentally, is equal to W/VA, making it the
perfect "fudge factor" (I.e. right answer/your answer ;-).

Thanks for any clarification.

In this case, you need to worry about the VA capacity of the UPS.
Though, again, your current reading is suspect. I'd go to at least a
750VA unit.

 

[scada]

Hi,

I need to order a UPS for my computer system so I connected an amp
meter and under the worst load conditions it was drawing about 4.2A
for everything. I figured 4.2 x 120 = 504 so I need a 500 Watt unit.

I started looking and I find things like...
Battery load capacity: 500VA (300 Watts)

How is VA different from Watts?? I thought VA meant V x A.

Thanks for any clarification.

As others have stated the difference between Watts and VA is the power
factor. However that aside, a good rule of thumb would be to double your
expected wattage, 1000Watts. You do not want to constantly run at the rated
maximum, the heat generated by inverters would do it in by itself!

 

[[email protected]]

Thanks Keith and scada. I will opt for the 750 as there is a huge
price hike to the 1000VA units. Given the 4.2A was measured when
everything was running under load which is rarely if ever the case.
Most of the stuff has "power-saver" mode too, so I am sure 750 will be
more than enough.

Thanks again for the help. Much appreciated.

 

[Ian Stirling]

Hi Ian, thanks for taking the time to answer.

So where does that leave me with the UPS? With a max of 4.2A, will the
300 Watts be enough or do I need to go with a 500 Watt unit whatever
that works out in VA??

What did you measure the 4.2A with?
I'd say you likely need the 500W unit.
It's almost certainly a good idea to go up a size unless you'r on a very
tight budget.

This will increase runtime, and it gives a little margin if you run it
below the limits.

 

[Ian Stirling]

Thanks Keith and scada. I will opt for the 750 as there is a huge
price hike to the 1000VA units. Given the 4.2A was measured when
everything was running under load which is rarely if ever the case.
Most of the stuff has "power-saver" mode too, so I am sure 750 will be
more than enough.

You might also consider what you want on the UPS.
504VA sounds like a rather high number.
Do you really care if the printer stops printing and you get an
error if power goes out, for example?

You can get by with a significantly smaller UPS if you move all but essential
loads off it onto an unsupported power-strip, then plug in loads
that you actually need.

 

[[email protected]]

Thanks Ian, I guess I should have explained better. That 4.2A was
measured with a digital multi-meter inline on the 10A circuit rather
than a clip on.

I could get by with less, but the cost difference is not that much so
figured I'd have it all on the one plug. Providing the 500 will do,
I'll save some bucks and use it.

The reason that prompted this was the 280 which came with the computer
now needs a new battery as it failed during a brown out the other day.
The cost of battery is about the same as a new UPS. :-O

Since I will be buying a new one I thought I'd get something that
would certainly cover the load. As of the last six months, the old one
often peeped when I first turned the computer on so I guess that was
the battery struggling to keep up.

Thanks for the help.

 

[Nico Coesel]

As others have stated the difference between Watts and VA is the power
factor. However that aside, a good rule of thumb would be to double your
expected wattage, 1000Watts. You do not want to constantly run at the rated
maximum, the heat generated by inverters would do it in by itself!

Wether the UPS gets hot depends on wether it's an on-line or off-line
UPS. The -true- on-line UPS converts the input to DC, forms a
redundant power supply together with the batteries to feed an inverter
which turns DC back into AC. These have 3 major draw-backs: They run
hot and often have a fan running constantly, they don't like rush-in
currents and they are expensive.

An off-line UPS uses a relay to switch between line supply and
inverter supply. They stay cool, can deal with high rush-in currents
(like the degaussing coil from a monitor) and are dirt cheap.

 

[Tim Shoppa]

Thanks Keith and scada. I will opt for the 750 as there is a huge
price hike to the 1000VA units. Given the 4.2A was measured when
everything was running under load which is rarely if ever the case.
Most of the stuff has "power-saver" mode too, so I am sure 750 will be
more than enough.

I have nothing against conservatively rating UPS's, and you've taken
that route.

But a different route is to lower the consumption of the necessary
equipment. This can be done by either getting more efficient equipment
or consolidating functions onto fewer boxes. Examples:

1. LCD screen instead of big CRT.
2. Going from two servers to one.
3. Segregating workstation-type activities from server-type activities.

etc. There are costs to improve the efficiency, but over time I think the
payback (less UPS maintenance, less $ to the electric company, etc.) are
worthwhile. Realistically you have to figure that the UPS batteries will
be replaced every couple of years; factor that cost in too.

Tim.

 

[N. Thornton]

For a simple answer, your UPS will need to be rated at:

in watts, whatever the puter setup takes in watts, and
in VA, whatever the puter uses in VA.

Your meter won't tell you those. 500VA would normally do.

The one possible problem area is that if you switch the monitor power
on while running on battery, the monitor takes a large initial surge
which could take your power out and crash the computer. But as long as
your kit was on when the mains went off, and you dont switch the
monitor off then on while of mains, all should be fine.

What is the UPS's run time at 500VA?

Regards, NT

 

[[email protected]]

Thanks NT, the run time is quoted at 21-35 minutes but they do not
state whether that is at full load or not. My guess is it is a bit of
marketing hype invloved with a bit of truth. How big the "bit" is in
each case is unknown.

I emailed the company and they said, "it depends on the type of load".

 

[N. Thornton]

Thanks NT, the run time is quoted at 21-35 minutes but they do not
state whether that is at full load or not. My guess is it is a bit of
marketing hype invloved with a bit of truth. How big the "bit" is in
each case is unknown.

I emailed the company and they said, "it depends on the type of load".

Hi

Call me a cynic but this may be the time with no load at all - or just
a tiny neon bulb or something. If you ask what the run time is at max
output power you'll know what your minimum possible time allowance is,
and in practice it should be around double that for a desktop. If they
cant tell you that rating, I guess there's a reason why.

Depending on your app, you will need at least enough time to respond
and get to the puter, plus a couple of minutes to shut everything
down.


Regards, NT

 

[John Popelish]

Hi,

I need to order a UPS for my computer system so I connected an amp
meter and under the worst load conditions it was drawing about 4.2A
for everything. I figured 4.2 x 120 = 504 so I need a 500 Watt unit.

I started looking and I find things like...
Battery load capacity: 500VA (300 Watts)

How is VA different from Watts?? I thought VA meant V x A.

Thanks for any clarification.

I have been away for a few days, but saw no one give you a good
answer.

If all loads were resistors, then all load currents would be
instantaneously proportional to applied voltage, and V*A would equal
watts. But if either the phase of the current does not match the
phase of the voltage (as with inductive or capacitive loads) then
there are times each cycle that the voltage and current have opposite
signs. Those times have energy being returned from storage in the
load back to the source.

This current must be handled by the inverter, even if energy is just
sloshing back and forth between the battery and the inductance or
capacitance. The inverter is usually rated for a bit higher VA than
the battery is rated for watts for this reason.

This power factor current that does not deliver one way energy flow
can also take the form of a distorted current that has lots of
harmonic frequencies in it, compared to the line voltage. Rectifiers
that charge caps at the peaks of the line voltage waveform do this,
and are common as dirt in electronic equipment. This wave shape also
puts big demands on an inverter that exceeds the battery average power
delivery.