Power factor (pf) is the difference between actual energy consumed and the apparent powerin an AC circuit. It will calculate as a decimal or percentage between 0-1 pf and 0-100% i.e. 0.9 pF = 90%.

UPS input power factor

The UPS input power factor is the ratio of UPS input active power to input power. UPS is not only a part of power supply, but also a load on the utility power source. It means the UPS power supply not only obtains active power from the mains, but also obtains reactive power.

The UPS rectifier has reactive and capacitive components, so will also have a factor power and this will
have to be accounted for when making the upstream electrical connection. The UPS input is a characteristic of the design and is usually declared by the manufacturer in the technical specification.
With modern IGBT front-end rectifier technology, the input is typically close to unity, 0.99, at 100% nominal load.

UPS rated output pf is a UPS design factor

The rated output pf describes the maximum active and apparent loading the UPS can tolerate by design. For example, a 100 kVA UPS pf 1.0 can handle loads up to 100 kW load. If the pf is 0.8, these loads become 80 kW and 100 kVA, respectively. To correctly select and size the UPS, you should know the total active and apparent power of the load. A UPS with rated pf of as example 0.8 can handle loads of higher factor as well – and vice versa.

Lagging Power Factor

These are loads where the current waveform lags behind the voltage by a factor equal to the load’s reactance, typically between 0.5 and 0.95. 

In the below image, a 2300 VA load with a lagging 0.766 pf would have a real value of 1762W (1.76 kW). 

Lagging Power Factor

Unity Power Factor

Unity pf loads have the current and voltage waveforms in phase with each other. In the example below, a 2300 VA load with 1 pf has a real value of 2300 W (2.3 kW).

Unity Power Factor

Leading Power Factor

Loads with a leading pf have a current waveform that leads the voltage by a factor equal to the load’s reactance, usually between 0.8 and 0.95.

Using the same 2300 VA as in previous examples, a leading pf of 0.766 has a real value of 1762 W (1.76 kW).

Leading Power Factor

A UPS will call upon to support different types of load equipment and these may have nonlinear
input stages. The compatibility of the UPS output stage (ie, the inverter) with different types of nonlinear load equipment is usually stated in the product documentation. Note that:

  • In cases with high inrush currents or peak currents – eg, motor loads – even the stated range of UPS output one does not provide enough detail to determine if the UPS can support the load.
  • With static loads, the UPS specification can use to determine compliance.

How to choose UPS?

A UPS with a rated factor of 1.0 means that you get equal amounts of kVA and kW out of the unit. With much modern equipment this is a good match as usually the load consumption kVA and kW are similar. However, if the load pf differs from unity, the dominating limit is the kVA rating, not the kW rating. For example, for a load with a rated capacity of 80 kW / 100 kVA (pf 80/100 = 0.8):

  • If the UPS rated output pf is 1.0, then the UPS rated capacity needs to be a minimum of 100 kW /100 kVA (100 = 100 * 1.0).
  • If the UPS rated output pf is 0.8, then the UPS rated capacity needs to be a minimum of 80 kW/ 100 kVA (80 = 100 * 0.8).

Power factor is a major consideration when selecting a UPS. it is very important to understand that if the UPS cannot handle the real capacity and the reactive consumed by the load an overload situation will quickly develop that could lead to UPS damage or outage and the critical load becoming unprotected.

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