Reactance reduces the useable power (watts) that is available from the apparent power (volt-amperes). The ratio of these two numbers is called the power factor (PF). Therefore, the actual power formula for AC circuits is watts = volts x amps x power factor. Unfortunately, the ups power factor is rarely stated for most equipment, but it is always a number of 1.0 or less, and about the only thing with a 1.0 PF is a lightbulb.

Using old UPS systems when the load was sinusoidal and had a lagging power factor allowed some economies in the design. A 10kVA motor rated at 0.8PF would only take 8kW. This means that the UPS battery and charge requires only to supply 8kW + re-charging current for the autonomy time. This led to UPS products being supplied with apparently quite small chargers to save cost.

In modern UPS boxes the designer is aware of a lot of these problems and the UPS is usually designed to accommodate large peak currents which occur at every cycle in the waveform. Current limiting to protect the UPS is usually vested in switching to bypass earlier than is necessary to maintain some thermal margins. However with microprocessor controlled inverters the output stage can have several different overload checks – any one of which will activate protection. For example output switch / heatsink temperature / actual peak current / integrated current over time and many other solutions.

The nearer the ups power factor is to unity (1 pf), the closer the two waveforms are in phase with each other and the device uses power more efficiently, hence why power factor relates to UPS efficiency.

power factor relates to UPS efficiency

Convention stipulates that inductive loads are defined as positive reactive power, with capacitive loads defined as negative reactive power. But power factor is never described as positive or negative, it is either lagging or leading.

Lagging Power Factor

A lagging power factor denotes that on the phasor diagram, the current lags (is behind) the voltage, and a leading power factor denotes that the current leads (is ahead) the voltage.

For inductive loads (e.g. induction motors, coils, lamps), the current lags behind the voltage, thus having a lagging power factor.

For capacitive loads (Synchronous condensers, capacitor banks) , the current leads the voltage, thus having a leading power factor.

The lagging or leading distinction does NOT equate to an positive or negative value. Rather, lagging corresponds to an inductive load, while leading corresponds to a capacitive load.

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 power value of 1762 W (1.76 kW).

Lagging Power Factor

Unity Power Factor 

The ratio of active power to apparent power is called as power factor. When power factor is unity, or 1, it means the entire apparent power is being used for active work being done, or active power.

This is the ideal condition desirable, but not attained in practice most of the time. In power systems from utility, this generally varies between 0.9 to o.98, depending on load.

All earlier answers give nice explanation about the relationship between power factor, phase angle and reactive power. Capacitors are extensively used in systems to improve the power factor to improve to 0.90 -0.98 or even better, by improving lagging phase angle.

Unity power factor (1 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 power value of 2300 W (2.3 kW).

Leading Power Factor

A leading power factor signifies that the load is capacitive, as the load “supplies” reactive power, and therefore the reactive component is negative as reactive power is being supplied to the circuit.

Loads with a leading power factor 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 power factor of 0.766 has a real power value of 1762 W (1.76 kW). 

UPS efficiency

UPS efficiency is affected by Power Factor, whether leading, lagging or distortion. The losses (or inefficiency) in the UPS come from carrying current through the UPS. As Power Factor drops, current rises for a given kW. The UPS simply has to do more work to provide the same kW at a lower Power Factor. At Power Factor .90 the UPS with carry 10% more current and have 10% more loss. A modern UPS, such as SYPX 250 may have an efficiency of .96 with a linear load. At Power Factor .9 the efficiency will be closer to .956. calculation for losses 100 – (.04100+(.0410)) (A 100 kW load at PF=1.0 has 4kW loss, a 100 kW load at PF=.9 has 4.4 kW loss)

That being said, modern “real computer” loads are now Power factor corrected with listed Power Factors approaching 1.0.
So, a modern UPS with a modern load with a Power factor of .99 will have an efficiency of .959 calculation for losses 100 – (.04100+(.041))

What this means is that the efficiency rating of the UPS with a linear load is valid for “real computer” loads


Whilst a UPS with unity input power factor is the preferred solution to lower installation costs, the output factor must reflect the actual requirements of the load. This would mean at loads lower than 100 % Pn, the PF could be lower.

Furthermore, the fact that UPSs with an output PF of 1 are marketed at a higher price or at a lower capacity than those with a PF of 0.9 should make us reflect on the real utility of PF1.

Do we really need to compromise on UPS capacity for such a limited if not inexistent number of cases? Is an output PF of 1 really relevant for all applications?

The answer is no given that the majority of loads are nowhere near this value, and a 30% margin is usually left with respect to actual load requirements when selecting an uninterrupted power supply.


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