Difference between Volt-amperes and Watts

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Power statistics in watts and volt-amperes (VA) are used when determining the requirements for uninterruptible power supplies (UPSs). This article will explain the differences between watts and VA. In addition, it will prove that both the maximum effective power (in watts) as well as the maximum apparent power (in VAs) must be taken into consideration when determining the requirements for UPSs.

Power Consumption

The maximum power consumption for a computer is specified in watts and volts.

  • Real power in watts:
    • This power is actually consumed for running the CPUs, RAM, hard disks, fans and so forth and will be converted into heat.
    • The maximum work in terms of watt-hours results from multiplication by the amount of time (such as one hour). Generally, kilowatts are used instead of watts, which results in kilowatt-hours (kWh). Energy utility companies charge for these kilowatt-hours. Note: because the power consumed changes according to the load on the CPU, the actual work is generally less than the maximum work.
  • Apparent power in terms of volt-amperes:
    • Apparent power is the total of the workload and the reactive power.
      • Reactive power represents the power that is continuously transferred between the power utility (such as a power plant) and a consumer (such as a server).
      • Because reactive power transfers back and forth, the power utility does not charge for it.
      • However, the ability to transfer reactive power to all components in addition to effective power must exist.
    • Thereby, cables and UPSs must have the ability to transfer apparent power (the total of reactive power and effective power).
    • Some electrical consumers do not use any reactive power, such as for a light bulb. In such cases, the apparent power is equal to the effective power.
    • Capacitor power supplies will consume some reactive power. Therefore, the apparent power consumption is greater than the effective power consumption. The power factor indicates the relationship between effective power consumption and apparent power consumption. With capacitor power supplies, this factor is generally between 0.55 and 0.75 (55 to 75 percent, see [1]).

UPS Design

A UPS is designed for a specific maximum effective power consumption (in watts) and a specific maximum apparent power consumption (VA).

The following considerations apply to the design of a UPS.

  • The total of the effective power consumption (in watts) of all connected devices must be less than the maximum effective power consumption for the UPS.
  • The total of the apparent power consumption (in VAs) of all connected devices must be less than the maximum apparent power consumption for the UPS.

Note: both conditions must be fulfilled for proper design.


The Product not available anymore provides:

  • a maximum apparent power of 2,000 VA
  • a maximum effective power of 1,340 watts

Table of References

  1. The Serious Difference between Watts and Volt-amperes APC White Paper Nr. 15

Foto Werner Fischer.jpg

Author: Werner Fischer

Werner Fischer, working in the Web Operations & Knowledge Transfer team at Thomas-Krenn, completed his studies of Computer and Media Security at FH Hagenberg in Austria. He is a regular speaker at many conferences like LinuxTag, OSMC, OSDC, LinuxCon, and author for various IT magazines. In his spare time he enjoys playing the piano and training for a good result at the annual Linz marathon relay.

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