Power factor is the ratio of real power (kW), the power that does useful work, to apparent power (kVA), the total power your electrical system has to deliver. It runs from 0 to 1. The further it falls below 1, the more current and infrastructure capacity you need to do the same amount of work.
That shows up in two places. On your utility bill, a low power factor can trigger penalties or inflate demand charges. Inside your facility, it eats into the usable capacity of UPS systems, transformers, generators, and circuits. Neither is visible until you measure it.
Power factor measures how effectively an AC electrical system turns supplied power into useful work. It is the ratio of real power to apparent power:
Power factor = Real power (kW) ÷ Apparent power (kVA)
It is expressed as a decimal between 0 and 1, or as a percentage. A power factor of 1.0, called unity, means all of the power delivered does useful work. At 0.90, every 100 kVA of apparent power delivers about 90 kW of real power.
The easiest way to picture the three is a freshly poured pint of beer:
In a simple AC circuit the three form a right-angled triangle. Real power runs along the bottom, reactive power up the side, and apparent power is the long side connecting them:
Apparent power² = Real power² + Reactive power² (S² = P² + Q²)
The angle between real and apparent power, called the phase angle (φ), tells you how far voltage and current are out of step. Power factor is its cosine:
Power factor = cos φ = P ÷ S
The smaller the reactive side, the closer the angle gets to zero and the closer power factor gets to 1.
In a DC system, voltage and current are steady, so power is simply volts × amps and power factor is generally a non-issue. AC is different. Voltage and current reverse direction 60 times a second in North America and 50 in much of the world, and power at any instant is the product of the two.
Two things pull power factor below 1:
True power factor, also called total power factor, combines both effects. That is why a facility full of modern electronics can still measure poorly, and why a meter that only calculates cos φ can overstate your real power factor.
A power factor of 1.0 is ideal but rarely reached in AC systems. Most facilities aim for 0.95 or higher, which clears the penalty threshold in most utility tariffs.
In data centers, circuits feeding IT equipment rarely drop below 0.9. That sounds like a narrow margin, but at the scale of a data hall it is enough to make true power measurement essential for cost allocation and for managing high-power equipment.
Not through your energy charge. Utilities meter kWh on real power, so the consumption part of your bill is unaffected. The cost shows up in demand.
Utilities have to build and maintain enough capacity to deliver apparent power, not just the real power you use. According to the U.S. Department of Energy, a 100 kW load at 0.70 power factor needs about 142 kVA of capacity behind it. Corrected to 0.95, it needs about 105 kVA, with 26% less current. Utilities pass the cost of that difference back to you in one of two ways:
Here is what that looks like for a facility with 500 kW of real demand on a kVA-billed tariff at $15 per kVA. Demand rates vary widely by utility.
At 0.85, that is almost $16,000 a year for power that never did any work.
One detail catches many facilities out: utilities often assess power factor at your peak demand, not as a monthly average. A brief dip can trigger a charge that an average would smooth away. Your bill shows the result, but not the moment that caused it.
There is a smaller indirect cost too. Higher current means more resistive losses in your own cables and transformers.
UPS systems, transformers, generators, PDUs and switchgear are often rated in kVA, which is apparent power. The real power they can support depends on power factor, which is why a 100 kVA UPS is not automatically a 100 kW UPS. To convert between the two:
kW = kVA × Power factor
kVA = kW ÷ Power factor
A device drawing 50 kVA at 0.90 power factor uses 50 × 0.90 = 45 kW of real power, but takes up the full 50 kVA of capacity upstream.
Now compare two loads that each draw 100 kVA:
Both take up the same capacity, but Load B delivers 20% less useful power. If you plan from amps or kVA alone, you can overestimate the productive capacity you have left, or misjudge how much of your contracted capacity you can actually use.
That matters most when you are deciding whether a UPS, feeder or panel can take another rack, line or tenant, and whether you can defer an upgrade. Tracking kW, kVA and power factor together keeps capacity planning grounded in what the equipment actually draws.
Power factor used to be a slow-moving number in a traditional data center. That is changing quickly. The International Energy Agency expects data center electricity use to roughly double from 485 TWh in 2025 to 950 TWh in 2030, with AI-focused facilities growing much faster.
The loads themselves are harder to manage too. According to the IEA, the power density of AI servers rose 11 times between 2020 and 2025, and AI data centers see repeated swings in server load of more than 50% of rated capacity within a second. Servers, UPS rectifiers and variable-speed drives all draw nonlinear current, and the resulting harmonics raise current and losses across transformers, cables, switchgear and busway.
Rapid load swings, reactive power and harmonic distortion are three different problems that often appear together. Treating them as one leads to the wrong correction equipment. What they share is that you cannot size a fix, or prove it worked, without knowing what each circuit in your high-density racks is actually doing.
Amps tell you current is flowing, but not how much useful work a load is doing. Current-only monitoring is useful for spotting overloaded circuits, but it cannot calculate real power or power factor. To do that, a monitor has to sample voltage and current at exactly the same instant, thousands of times a second, and multiply them across the AC waveform.
A practical approach:
Once you know where power factor is low and why, you can choose the right fix. The Department of Energy lists the common ones: capacitors to offset reactive power, energy-efficient motors, and cutting the time motors spend idling or lightly loaded. Where harmonics are the cause, active harmonic filters are usually the better choice. Either way, measure first so you size the fix correctly, and measure again to prove it worked.
Power factor is easy to ignore because it hides inside numbers you already receive. It stays quiet until it shows up as a surcharge or a capacity shortfall.
Packet Power monitors measure volts, amps, VA, watts, power factor, watt-hours and frequency on every circuit, so real power, apparent power and power factor are measured rather than estimated. Our High-Density Power Monitor tracks up to 120 circuits from a unit smaller than the inside of a standard light switch. It installs without new network cabling and sends data to your existing BMS or DCIM, or to EMX, where phasor diagrams show how far voltage and current are out of step on each phase.
Circuit-level data also keeps energy cost allocation fair. If you bill tenants or departments on apparent power while they consume real power, someone is paying for the foam.
Packet Power has more than 250,000 monitoring devices deployed in over 40 countries. Speak with our team to see what your true power looks like.
Divide real power by apparent power: PF = kW ÷ kVA. For a single-phase circuit, apparent power is volts × amps. A 208V circuit drawing 10A has 2,080 VA of apparent power; if it uses 1,872 W, its power factor is 1,872 ÷ 2,080 = 0.90. For three-phase circuits, apparent power is 1.73 × volts × amps.
Above 0.9 is typical for IT circuits with modern power supplies. Aim for 0.95 or higher to stay clear of most utility penalty thresholds, especially since penalties are often assessed at peak demand rather than on a monthly average.
Inductive loads such as lightly loaded or oversized motors, transformers, variable frequency drives, welders and older lighting ballasts cause phase shift. Nonlinear loads such as UPS rectifiers and server power supplies cause harmonic distortion. In data centers, the nonlinear loads are usually the ones to watch.
Multiply kVA by the power factor. A 100 kVA load at 0.80 power factor uses 80 kW of real power. To go the other way, divide kW by the power factor.
Not the energy charge, because utilities meter kWh on real power. It raises demand-related costs, either through a power factor penalty or through kVA-based demand billing. Higher current also adds resistive losses in your own cables and transformers.
VA is apparent power, what the circuit has to carry. Watts are real power, what does useful work. The ratio between them is the power factor, and because it is usually below 1, the watt rating is lower than the VA rating.
Many commercial utility bills print it, but that figure is a facility-wide average. It will not tell you which circuits or equipment are pulling it down. Circuit-level monitoring that reports power factor alongside volts, amps, watts and VA is the only way to find the source.