The difference between volt-amperes (VA) and watts (W) simply explained

Until recently, I always thought that multiplying voltage in volts and amperage in amperes only resulted in watts. But that's not the case. And even if, in simplified form, volt-amperes and watts seem to be the same, it is not necessarily so. So I did a little reading, and now I'm smarter. What I have found, I would like to pass on to you as simply as possible. 

Topic finding: Confusion due to power consumption meter

That came recently Voltcraft Energy Logger 4000 Power consumption meter at me. When I tried it, I was a bit confused about the separate display of watts and volt-amps. Also because both values ​​were different for one and the same measurement. Sure, if they were the same, they wouldn't have to be listed separately from each other. But at first glance I didn't really understand this breakdown.

The power consumption meter Voltcraft Energy Logger 4000 outputs not only the power in watts but also the apparent power in volt amperes. In this guide I have put together for you what the difference between volt-amperes and watts is.

The power consumption meter Voltcraft Energy Logger 4000 outputs not only the power in watts but also the apparent power in volt amperes. In this guide I have put together for you what the difference between volt-amperes and watts is.

So far I always thought that volts x amperes = watts, but it's not that simple. Because in addition to watts (W) as a unit for power, volt-amperes (VA) as a unit for apparent power can also be used to estimate electronic devices. In addition to the bare figures and the theoretical calculation of the performance (e.g. by the manufacturer), this also takes into account losses due to the stress on components, heat dissipation and the like.

The difference between volt-amps and watts

Voltampere is the unit of measurement for the apparent power S, which is used in electrical power engineering for devices and machines that are associated with alternating quantities (alternating current, alternating voltage, etc.). In addition to the volt and ampere values, the apparent power also includes losses due to stressed components. The active power P and the reactive power Q can therefore also be used for the calculationtot be used. 

Watt is the unit of measurement for power P, which can be calculated purely from the values ​​volts (voltage U) and amperes (current I) and does not include any loss or interference factors for the electrical area. The power only indicates which energy consumption is realized per time period. In other areas of physics, other values ​​and units can therefore be used to calculate the power in watts, such as J / s.

Further details and formulas can be found in the Wikipedia articles on apparent power (here.) and performance (here.).

What is reactive power?

Above I brought the term reactive power into play. This describes the power that “blindly” switches back and forth between the power source (power plant, generator, UPS, etc.) and the consumer, i.e. the electrical device. It does not play a role in the electricity bill, since it means no consumption due to the commuting between source and consumer, but it must be able to be transmitted by components and lines / cables, in addition to the active power.

Reactive power does not occur if active power and apparent power are equal. This is the case with consumers where the electrical current used is completely converted. This is the case, for example, with classic light bulbs that emit light and heat. As shown above, the addition of the reactive power to the active power gives the apparent power. Therefore it is 0 when S = P.

So what do we understand by performance?

The power in watts is required on the computer, for example, to operate components such as hard drives, processors, RAM, fans and the like and is given off as heat. The electrical energy that comes in here is partly converted into thermal energy and is lost for the power grid or the specific connection between power source and consumer. If the power is multiplied by the time, then the work results, for example in watt hours (Wh) or kilowatt hours (kWh). 

Power in watts and apparent power in volt-amperes are also not entirely unimportant when comparing UPS devices for uninterruptible power supply. The UPSs are connected to the power grid and the consumer (e.g. the Apple Mac) and provide the power for bridging operation in the event of a power failure, for example. In this way, open files can be saved in peace and the computer can be shut down. More about this and examples of good devices can be found here: UPS for the Mac - Which models and manufacturers are possible?

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5 comments

  1. Dirk says:

    So volts x amps is still / just watts and that's why it's so easy! Only VA cannot (necessarily) be derived from this. ????

  2. John Dau says:

    As a teacher, I can tell you that you still have a lot to learn when it comes to explaining. I didn't understand what volt-amperes are now. You don't make laypeople understand anything with formulas. The "reactive power", I believe, is dispensable if one wants to explain volt-amperes. And you don't have to explain what a UPS is to anyone who reads such an article at all... Basically you write like the electrical engineers in Wikipedia - there all these articles are also largely illegible for laypeople. But if I have to research further to understand your article and the wiki articles, then they are bad articles.

    • Jen Kleinholz says:

      I'm biased, of course, but I (as a graduate educator) just re-read Johannes's article and think it's understandable for laypeople, as it summarizes the two main differences just above the formula:

      In addition to the volt and ampere values, the apparent power also includes losses due to stressed components.

      and

      Watt is the unit of measurement for power P, which can be calculated purely from the values ​​volt (voltage U) and ampere (current intensity I) and does not include any loss or interference factors for the electrical area.

      And if you want to know more about it, you can have a look at the other explanations. I mean, we're not talking about the show with the mouse here, but rather a page where you can assume that someone has already heard a little bit about Watt and Co. But if you still have some catching up to do, you can look here at the mouse. By the way, it's a great explanation!

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