Importance of Power Factor in Uninterrupted Power Supply (UPS) Solution

When choosing a UPS solution its power rating must be taken into account and it should match the requirement or else it could fail when it is needed the most. But unfortunately choosing the correct power rating is not as easy or straightforward as it may seem.
Power factor is a quantity which has important implications when sizing a UPS system and power distribution equipment. Power is a measure of the delivery rate of energy and in DC (direct current) electrical circuits are expressed as the mathematical product of Volts and Amps (Power = Volts x Amps). However, in AC (alternating current) power system, a complication is introduced; namely that some AC current (Amps) may flow into and back out of the load without delivering energy. This current, called reactive or harmonic current, gives rise to an “apparent” power (Volt x Amps) which is larger than the actual power consumed. This difference between the apparent power and the actual power gives rise to the power factor. The power factor is equal to the ratio of the actual power to the apparent power. The apparent power is expressed as the Volt-Amp or VA rating. Therefore, the actual power in any AC system is the VA rating multiplied by the power factor.
To size a UPS and ensure that the UPS output capacity is sufficient, both the VA rating and the Watt rating of the load are important. The watt rating of the UPS relates to the amount of power it can deliver, and the VA rating of the UPS relates to the amount of current it can deliver. Neither the Watt nor the VA rating of the UPS can be exceeded. The best approach is to size a UPS the Watt rating of the load. This is particularly true for larger IT installations where the power factors of the loads are nearly 1.
If there is confusion regarding power ratings or power factor, and it is desirable to ensure the load can be powered by the UPS, then choosing a UPS with a Watt rating greater than or equal to the VA rating of the load will always ensure a safety margin. Power factor has an important implication in the specification of UPS run time on battery. Battery run time is dictated by the watt load on the UPS. However, when many UPS manufacturers specify run time at full load they are referring to full VA load, not the full watt load.
Input Power Factor:
Input Power factor is the percentage of electricity that is being used to do useful work. It is expressed as a ratio. For example, a power factor of 0.72 would mean only 72% of your power was being used to do useful work. Perfect power factor is 1.0, (unity), meaning 100% of the power is being used for useful work.
Output Power Factor:
Output power factor rating is the percentage of electricity that is available to do useful work. For example, a power factor of 0.80 would mean only 80% of your power is available as real power to do useful work. Perfect output power factor is 1.0, (unity), meaning 100% of the power is available for useful work.

Power factor is a major consideration when selecting a UPS, but unfortunately, it remains a misunderstood subject, and ignoring or misapplying the power factor concepts could result in a number of problems. It is really important to understand that if the ups cannot handle the real power and the reactive power consumed by the load, a situation can develop due to overload and that could quickly lead to UPS damage.


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