Why VA is different from Watt in UPS

[JesseFrank]

Watt = Voltage times the current

VA = Voltage times the current

So theoretical Watt = VA

No why on a UPS is it that VA = 1.4285 times the Wattage ?

[lopburi3]

That is only true for DC or AC heater type circuits where there is no reactance involved. Believe this explains well in simple language:

http://whatis.techtarget.com/definition/volt-ampere-VA

In an AC circuit, power and VA mean the same thing only when there is no reactance . Reactance is introduced when a circuit contains an inductor or capacitor . Because most AC circuits contain reactance, the VA figure is greater than the actual dissipated or delivered power in watts. This can cause confusion in specifications for power supplies. For example, a supply might be rated at 600 VA. This does not mean it can deliver 600 watts, unless the equipment is reactance-free. In real life, the true wattage rating of a power supply is 1/2 to 2/3 of the VA rating.

When purchasing a power source such as an uninterruptible power supply ( UPS ) for use with electronic equipment (including computers, monitors, and other peripherals), be sure the VA specifications for the equipment are used when determining the minimum ratings for the power supply. The VA figure is nominally 1.67 times (167 percent of) the power consumption in watts. Alternatively, you can multiply the VA rating of the power supply by 0.6 (60 percent) to get a good idea of its power-delivering capability in watts.

Also see current , voltage , power , reactance , and watt .

This was last updated in March 2011

Contributor(s): Guillermo Cardenas

Posted by: Margaret Rouse

[JesseFrank]

To me was told that I have to multiply Watt exactly with 1.4285 to reach the VA .

I looked at a few UPS and they also use that same exact number.

Must be a reason for .

[Satcommlee]

Oh dear.. please don't bring Power Factor Correction into this... If google says it's 1.42 then Google is right! Honest

[Crossy]

You multiply VA (that's Volts x Amps) by Power Factor to get Watts.

PC power supplies can have a Power Factor of anything between 0.5 (cheap Chinese) to 0.95 or better (expensive with active PF correction).

The UPS manufacturers use worst case in order that the user doesn't overload the unit accidentally.

EDIT The measured PF of my computer with three 23" monitors is 0.77 (measured with my Kill-A-Watt work-alike)

[JesseFrank]

Is there a way to find out the power factor from a certain UPS before you have purchased it, as in listed in the spec or so ?

[Crossy]

Is there a way to find out the power factor from a certain UPS before you have purchased it, as in listed in the spec or so ?

The better quality units, particularly those with Active Power Factor Correction invariably state it in the specs, cheapies don't admit to having a carp PF.