If a UPS (power supply) is rated "1200 VA" and "640 watts" is the efficiency 640/1200 and would it be expected to drive a 64 watt AC load for 10 hrs??? Thanks. - RM

# ot: UPS rating

Started by ●November 9, 2004

Reply by ●November 9, 20042004-11-09

"Rick Merrill" <RickMerrill@comTHROW.net> wrote in message news:pl4kd.317569$wV.17210@attbi_s54...> If a UPS (power supply) is rated "1200 VA" and "640 watts" is > the efficiency 640/1200 and would it be expected to drive a 64 watt AC > load for 10 hrs??? Thanks. - RMNo. 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 minutes. 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. Meindert

Reply by ●November 9, 20042004-11-09

"Rick Merrill" <RickMerrill@comTHROW.net> schreef in bericht news:pl4kd.317569$wV.17210@attbi_s54...> If a UPS (power supply) is rated "1200 VA" and "640 watts" is > the efficiency 640/1200 and would it be expected to drive a 64 watt AC > load for 10 hrs??? Thanks. - RMVA 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 minutes. 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: 64x10= 640W. 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. Cheers, Jack

Reply by ●November 10, 20042004-11-10

Rick Merrill <RickMerrill@comTHROW.net> wrote in message news:<pl4kd.317569$wV.17210@attbi_s54>...> If a UPS (power supply) is rated "1200 VA" and "640 watts" is > the efficiency 640/1200 and would it be expected to drive a 64 watt AC > load for 10 hrs??? Thanks. - RMNo. 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 rated). 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.