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- Posted on
- ot: UPS rating
- Rick Merrill
November 9, 2004, 2:19 pm
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Re: UPS rating
No. 1200VA only means that the inverter is capable of delivering 1200VA. It
says nothing about the time. The time depends on the internal batteries and
judging from their 'average' size, such a UPS will only run for a few
However, if the specs clearly say that 1200VA can be delivered for, say 5
minutes, then a load of 120VA would theoretically run for 50 minutes.
Re: UPS rating
VA is needed if Voltage and Current are not in phase.
You need the "watts", thats the energy.
Like Rick said, if 640 runs 5 minutes, you would expect 64 watt to run 50
In he real world of UPS the amount of time will increase with around a
factor 2, so still not 10 hours.
Do the calculation the other way around:
Efficiency is less then 50% so around 1.2kWH is needed.
If the internal batteries are 24 volt, you will need 50AH.
This means a very heavy UPS as most UPS's have around 12 or 20AH onboard.
Re: ot: UPS rating
No. The VA and watts rating are both related to the output capacity
of the UPS. If you were running a simple resistive load (eg. an
incandescent light bulb), the VA and watt rating of that device would
be the same. For something like a PC with a switching power supply,
where it does not draw current consistently over the entire AC cycle,
you have to apply a "power factor" to convert between VA and watts.
Crudely, what happens is that the device is drawing a higher level of
power for part of the cycle, and none for the rest. The "higher"
level relates to the VA rating, the net or average load over the cycle
is the watts rating of the device. Typically for PC class switching
power supplies the PF is between .6 and .7.
Anyway, you need to size your UPS to be big enough to cover both
requirements. If you have a 500VA PC with a PF of .7, you need a UPS
with an output of at least 500VA and 350 watts. If you had a 350 watt
incandescent light bulb (with a PF of 1.0), you need a 350VA/350W UPS.
Be forewarned that many UPS vendors use unrealistically small PF
estimates (often below .55) so they can give their UPS' bigger ratings
on the box. And this has gotten worse over the years. Ten years ago
you'd buy the "Model 1000" UPS and you'd generally get a 1000VA/700W
UPS. In the last couple of years I've seen UPS ratings as skewed as
1000VA/500W. Obviously the latter is much less of a UPS than the
former, although it's still sold as the "Model 1000." At least one
vendor even manages to use different schemes for their "basic" and
"professional" lines of UPS (the former getting a more exaggerated
The runtime is not directly related to either number, and tends to
vary non-linearly with the total load on the UPS, and with the size of
the battery in it. The UPS vendor will have a chart showing run times
at various load levels, and it'll be something like "5 minutes at
100%, 10 minutes at 75%, 18 minutes at 50%, 40 minutes at 25%". And
if they offer different sized batteries for the UPS (or add-on battery
packs - same thing), they'll have charts for those options as well.