UPS: VA versus Watt

[Banana]

I'm not sure what the difference is between VA and wattage for a UPS. I know wattage can vary even though the VA is the same. Which figure is more important when considering a UPS? Thanks.
1YP

 

[MWink]

IMO the VA rating on a UPS is a joke. The old rule used to be 1VA=.7W. This is no longer true all the time. Some companies give their UPS's a much higher VA rating than they deserve. I look at watts when picking a UPS. That is a more constant rating.

The really stupid part is the fact that VA and W are in reality the same exact rating. VA = Volts x Amps, W = Volts x Amps.

 

[TimeKeeper]

Actually I missed out the "Anandtech class" on this issue also.
I was wondering how much power a regular system consume when it is on idle?

I am pretty dumb...can you guys translate into...something like...

1Ghz w/ 1 HD on idel = 60 or 100 Watts light bulb?..😉

 

[pm]

There is more to this that V*A = W because you are dealing with AC signals. If it's a DC signal (ie. 5A@12V) then you can easily do 5*12=60W, but when it's AC if you do 120V@3A = 360 then you have actually calculated volt-amps, not watts.

From the UPS FAQ:

Q: How are the "sizes" of UPS's determined?

A: Typically, a UPS has a VA rating. The VA rating is the maximum number of Volts * Amps it can deliver. The VA rating is not the same as the power drain (in Watts) of the equipment. Computers are notoriously non-resistive. A typical PF (power factor: Watts/VA) for workstations may be as low as 0.6, which means that if you record a drain of 100 Watts, you need a UPS with a VA rating of 167. Some literature suggests that 0.7 may be a good conversion factor, but this will depend heavily on the machine. WARNING: Don't take my word for it! Note: Some UPS's can continue to deliver power if the VA rating is exceeded, they merely can't provide above their VA rating if the power goes. Some can't provide power above their VA rating at all. Some may do something really nasty if you try. In any case, I strongly recommend not doing this under any circumstances.

The key thing here is the PF (Power Factor). I started to type up a big explanation but things are busy today, so I did a quick search on Google. Here's a blurb from PC Quest:

Once you?ve analyzed the power situation in your area, you should do power planning and decide upon the UPS capacity you really need. An important aspect here is the PF (Power Factor). In electrical terms, this is the ratio of the effective power (watts) to the apparent power (volt-amps or VA). Mathematically put:

To put it differently:

PF = Effective Power (Watts)/ Apparent Power (VA)

Watts = PFxVA

Resistive load, such as light bulbs have unity power factor, so its apparent power VA is equal to the effective power in Watts. Computer load, however, is non-linear, so its power factor is less than unity. If a lower Power Factor rating is plugged into the above equation, the Effective power consumed by the load is lower than its apparent power. Since most UPS ratings are in VA, you must know the actual power consumed by your load and its power factor to determine what UPS rating to actually go for. Usually, a Power Factor value of 0.7 is good enough for computer load.

Power Factor can be measured using a Power analyzer. Ask your UPS vendor to use one to measure it for your total load. If your projected load is 100 VA, then the actual power consumption for it is around 70 Watts. This will help you determine the actual battery ratings you need, so that you don?t end up paying extra for higher rated batteries.

Since most computer equipment specifies loading in VA, then I would stick with VA rather than doing the conversion to Watts for purposes of calculating UPS capacity - especially since most equipment doesn't have PF ratings so the Watts calculation can involve guessing.