Watt in the world are volt amps?

I’m not an electrical engineer. The extent of my knowledge about electricity is that when I flip the power switch on the wall, the lights turn on or off. Oh, and something about not blow-drying your hair in the bathtub.


So what are volt amps, you say? I found an APC sponsored PDF called “Watts and Volt-Amps: Powerful Confusion” (direct PDF link) and gave it a read with the hopes that I would finally understand. It’s a quick read. Really quick. The PDF is 5 pages, but you can subtract 3 of them (front cover, back cover and author bio all get their own page) to get to only two pages of actual content.

Apparently both terms refer to power draw. While watts are referred to as the real amount of power that a piece of equipment draws, the PDF describes volt-amps as being “used for sizing wiring and circuit breakers.” No solid definition was supplied.

The VA and watt ratings for a single device can differ, with the VA number always being equal to or more than the watt rating. Never less than the watt rating. The ratio between the VA and watt rating of a device is known as the “Power Factor” or PF and can vary between 0.55 and 1 with a typical number around 0.60 (also represented as 60%) for consumer electronics devices. E.g. A 1000 VA device can pull between 550 and 1000 watts, but never more than 1000 watts.

Watts and volt amps come into play when you realize that there are two types of power supplies in common usage: Power Factor Corrected (PFC) supplies and capacitor input supplies. PFC supplies have a power factor (PF) of 0.99 to 1.0. That means if it’s rated to pull 1000VA, it pulls 1000 watts (and vice versa). Capacitor input supplies have a PF typically between 0.55 and 0.75 which means if it’s rated to pull 1000 watts, it will pull between 1550 and 1750 VA.

Great! Just buy things that only have PFC supplies and you won’t have to do any mathematical gymnastics! One problem: you can’t tell which kind of power supply is which. There are no external indicators (usually). The general rule, which seems to be pretty solid to rely on, is that all major devices such as business class networking equipment, servers, appliances, drive shelves and etc. manufactured after 1996 use PFC power supplies. All home and user equipment typically have capacitor input power supplies. That means desktops, desktop switches, scanners, printers, etc.

What this means is that when making specifications for a UPS system, if you’re working on enterprise class equipment in the server room, VA and wattage numbers can be virtually interchangeable. However, if you’re putting a UPS on a more consumer level device, possibly things like a POS system that includes a switch, printer and scan gun, you’ll need to keep those ratios in mind.

According to the APC PDF, it’s an industry standard to use 60% as the power factor when sizing a UPS. To use the above POS scenario, if you have four devices that total 500 watts, you’ll need a UPS that is rated at least for either 500 watts or 700 VA. However, if a piece of equipment is rated not in watts but in VA, you know you can use the VA number as a ceiling since it can’t pull anymore watts than it’s rated in VA. Of course, this can lead to way oversized UPS solutions. Maybe a wattage meter is in order.

Of course, the APC paper points out that they have some nifty online sizing tools that can help you make those decisions.

Reiterating my earlier question: What exactly are volt amps? The answer is: I don’t know, but at least I know how not to blow my UPS up! Seriously, I need to get edukatated in electricity. I’m ashamed to admit it, but I think “The Manga Guide to Electricity” might be a good book for me. Not because I like manga (I don’t particularly), but because I’m that stupid and need lots of pictures.

Oh sweet! The Manga Guide to Electricity is available on my Safari account!!


  1. ZethaByte

    May 26, 2010 at 5:58 am

    Oh !!! a few days ago, I asked the same question …..What exactly are volt amps?


  2. furicle

    May 26, 2010 at 6:08 am

    See http://en.wikipedia.org/wiki/Volt-ampere and http://en.wikipedia.org/wiki/Apparent_power

    One Watt is consumed when One Volt supplied causes One Amp to flow. One VA is what you’d measure with a simple volt meter and a simple amp meter. In a DC circuit, or when running a simple resistor like a heater, one watt is always going to be one VA.

    It goes off the rails when you start using AC power and more complicated loads.

    Basically, electrical loads (like a power supply) can temporarily store energy, so the voltage supplied and the current that flows aren’t directly related all the time. If you replaced that volt meter and amp meter with meters capable of showing the values at every instant in time, with time stamps, you’d see the Voltage rise or drop, then some time later the current rise or fall. The stored energy in the power supply is making up the difference. The time they are out of sync is the difference between VA and Watts.

    Make any sense?


    • Eric Sproul

      May 26, 2010 at 7:26 am

      Thanks furicle that is a clear, concise definition. Does it make sense to think of VA as incorporating the efficiency of an AC power supply into the equation? In other words, VA is the load seen on the circuit, including the current that ends up doing “real work” plus the current lost to inefficiency?


      • furicle

        May 26, 2010 at 8:56 am

        Well, I’m no electrician, but I see VA as kinda the stress on the wire rather than the ‘load’ the hydro company sees.


    • Wesley.Nonapeptide

      May 26, 2010 at 8:20 am

      Thanks furicle! It’s as clear as mud. =)

      I don’t know what it is about the topic of electricity that is hard for me to grasp. Possibly it’s because I’ve never sat down and tried to grok it in earnest. I’ve only picked up fragments here and there. Part of it may be some dyslexic symptoms that I’ve never had diagnosed. I really don’t know.

      Can you recommend any good learning tools? That Manga Guide to Electricity is really looking good about now…


      • furicle

        May 26, 2010 at 9:00 am

        Sounds to me more like a mental block than a real issue. Whatever I picked up was high school electricity plus construction experience plus A/V sales installation experience. I never met that handy primer reference, but basic electricity is more straight forward than say SNMP 🙂 Quit thinking it’s tough and you’ll get there faster.


        • Wesley.Nonapeptide

          May 26, 2010 at 9:03 am

          Point taken. I do tend to build undue tension behind topics that are actually quite simple.

          And SNMP is as easy as falling off a log. =)


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