In a simple DC (Direct Current) system as shown in Figure 1, the power flows from the source (typically the UPS battery) to the load and may be described by the following formula:

Power [Watts] = Volts x Current

Figure 1 Power Flow in a DC System.

It is important to note that in a DC system, the power always flows from the source to the load.

However, in an AC (Alternating Current) system, the power flow is a little more complicated. The voltage and current are sine-waves as shown in Figure 2. The voltage and current alternate at the frequency of the AC system. For example, in a 50 Hertz system, the voltage and current alternated 50 times per second.

In an AC system, one way power is determined is by measuring the voltage and current present at the load and multiplying these two numbers together. This product gives the volt-ampere (VA) power requirement of the load. An outline of an AC system is shown in Figure 2.

Figure 2 Power Flow in an AC System

The product of the voltage and current measured gives the “apparent power” of the load. This “apparent power” is made up of a part that does the useful work (measured in Watts) and a part does no work but does generate heat. The part that does useful work is called “real power” and the part that does no useful work is called “reactive power.”

To covert from VA to Watts in an AC system, a technical term called the power factor (p.f.) is used. Power factor is a number between 0.0 and 1.0 representing the fraction of the apparent power delivered to the load that does useful work. Power factor is defined as:

abbreviated as p.f.

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To determine the real power of the load, simply multiply the VA by the power factor.

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If the power factor of the load is 1.0, then the real power (Watts) and the apparent power (VA) are equal. That is 12 VA = 12 Watts. An example of a load with a power factor very close to 1.0 is an incandescent light bulb.

In equipment with power factors of less than 1.0, which is most equipment, the VA rating will always be higher than the Watts rating. The power factor of computer equipment is between approximately 0.6 to 0.7.

A comparison of the VA rating for two personal computers, models A and B, both having power supplies rated at 200 W, with power factors of 0.6 and 1.0 respectively, is shown in Table 1. The input voltage to the power supplies is 120 VAC.

Computer Model

Watts Rating

Power Factor

Input Voltage


Input Current


VA Rating


200 W


120 VAC

2.78 AAC

333.3 VA


200 W


120 VAC

1.67 AAC

200 VA

Table 1. Comparison of Watts And VA Ratings

Note that the VA rating of model A is much higher that its Watts rating due to a power factor of 0.6. Also note that the input current is much less for model B which has a p.f. of 1.0.

Understanding the power requirements of the load to be protected by a UPS is crucial in selecting a correctly sized UPS. Normally both the real power (Watts) and apparent power (VA) of the load are needed to correctly size the UPS. The discussion will focus on computer loads but is relevant to all other types of electrical equipment.

Switch-mode power supplies (SMPS) are widely used to power all modern computers and present a power factor to the source in the range or 0.6 to 0.7, depending on their design and rating. Switch-mode supplies installed in larger computers such as minicomputers have a power factor closer to 0.7, whereas supplies installed in smaller computers might have a power factor closer to 0.6.

Recently, new types of computer power supplies have been introduced into the market which have power factor corrected circuitry to force the real and apparent power of the load to be nearly equal which gives a power factor of close to 1.0 (unity power factor). Unity power factor is preferred by electric utility companies because it uses power more efficiently. The current situation is that these power have a higher cost and are not yet mandatory. In the future, international standards such as IEC 555 may force all SMPS to be designed to have unity power factor.

Most manufacturers specify the computer power requirements in VA: however, some specification give power requirements in Watts. If only a Watts rating is given, the VA of the power supply can be easily determined by assuming a power factor of 0.6. For example, to convert the Watts rating to VA ratings, simply divide the Watts by 0.6.

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A UPS will be overloaded if either its real power rating (in Watts) or its apparent power rating (in VA) is exceeded. Therefore, it is necessary to know both the real and apparent power requirements of the load and the capability of the UPS to supply both real and apparent power to make sure the UPS is sufficiently large enough. It is prudent to allow 20 to 25% extra capacity for future needs when finally deciding on a UPS.

If only the apparent power (VA) rating of the UPS is known, the power factor rating may be estimated using 0.6. However, it is better to ask the manufacture for the real value.

Once the power factor of the UPS is known, a quick calculation of the Watts rating of the UPS output can be done for comparison with other UPS products and to assure that sufficient real power is available from the UPS to power the load.

Calculating The Total Requirement For Complex Loads.

Often the UPS is called upon to power a load that consists of several or even dozens of computers, monitors, modems and a PABX, etc. When selecting the size of a UPS, the total power requirement of the system must be determined by summing the power requirements of the individual loads. The example that follows illustrates the simple procedure.

    Consider a small computer network system consists of the following components:
  • 1 file server and 4 computer workstations
  • 5 monitors
  • 5 modems
  • 1 PABX

all of which must be protected by a UPS. Table 2 shows the power requirements of each of the individual components, and the calculations to work out the correct size UPS required.






Power Rating of each unit

Watts or VA

Calculation of

VA Requirements

Sub Total





300 Watts
Total VA= 5 x (300 / 0.6)




200 VA
Total VA = 5 x 200




50 VA
Total VA = 5 x 50 VA




1000 VA
Total VA = 1 x 1000 VA

      UPS VA Rating Required


Table 2 Calculation Of The Total Power Requirement

It is important to take into consideration any likely future additions to the network or computer system and to plan for an increase in power requirement. The UPS rating must take into account the future power needs or the ability to upgrade, either by adding additional power modules or by adding another UPS is parallel operation must be present in the UPS design.

Another important specification to consider when deciding on the UPS is its Crest Factor rating and the Crest Factor requirement of the load. The definition of Crest Factor (Also known as Crest Ratio.) is the ratio between the instantaneous peak current and RMS (root mean square) current required by the load. The UPS must be able to supply both the peak and the RMS current required or the load may not work properly. Figure 3 graphically illustrates a typical Crest Factor for a switch-mode power supply.

Computer switch-mode power supplies require high peak currents, when supplied from the mains, which result in a Crest Factor of 3 or more. A high Crest Factors is undesirable because it causes operation at higher temperatures, which lowers the reliability and life of a power supply. A True On-Line UPS can limit the Crest Factors to about 3 because the UPS output impedance limits the peak and spreads the width of the current pulse.

Figure 3. Crest Factor Illustration

Another important point to consider is the ability of the UPS to start loads, such as computer, monitor or any device with a switch-mode power supply, which requires an extra “kick” when first energized due to a normal inrush current requirement. The time when this high inrush current is present is an overload and is usually very short in duration, but may be a few seconds or more for special loads. To handle this inrush current, a UPS has an output overload specification. The overload specification is not the same for all UPS products and a UPS is capable of supplying an overload of 150% for up to one minute is desirable.