What Is Power Factor and Why Do Utilities Penalise You for a Low One?

Power factor is one of those concepts that intimidates beginners but is actually straightforward once you see the right analogy. Consider ordering a pint of beer:

• The liquid beer is what you actually want to drink — this is real power (kW), also called active power. It does useful work: turning motor shafts, heating elements, producing light.
• The foam on top takes up space in the glass but you can’t drink it — this is reactive power (kVAr). It doesn’t do useful work, but it is required to maintain the magnetic fields in motors, transformers, and fluorescent lamp ballasts. It sloshes back and forth between the source and the load 100 times per second (on a 50 Hz system) without being consumed.
• The total glass — beer plus foam — is apparent power (kVA). This is what the utility’s generators, transformers, and cables must deliver. The glass has to be big enough for both the beer and the foam.

Power factor is simply the ratio of what you actually use (kW) to what the system must deliver (kVA):

Power Factor = kW / kVA = cos φ

A power factor of 1.0 means all the power is doing useful work — the glass is 100% beer, no foam. A power factor of 0.7 means only 70% of the apparent power is doing useful work — the rest is reactive power sloshing back and forth, consuming conductor capacity without contributing to output.

Here is the practical problem with low power factor. A cable does not care whether the current flowing through it is doing useful work or not. It heats up either way. The cable’s temperature — and therefore its current-carrying capacity — depends on total current, which is determined by apparent power (kVA), not real power (kW).

Consider a factory drawing 200 kW of real power:

The same 200 kW of useful load requires a cable 60% larger at 0.70 power factor compared to unity. The transformer must also be rated for the higher kVA — a 200 kVA transformer at PF 1.0 versus a 300 kVA transformer at PF 0.70. Every conductor, busbar, switchboard, and protective device in the path must be sized for the higher current. This is wasted infrastructure capacity.

The reactive current also increases voltage drop. The voltage drop formula includes both resistive and reactive components: ΔV = I × (R cosφ + X sinφ) × L. At low power factor, the reactive term (X sinφ) contributes significantly, making voltage drop worse than it would be for a purely resistive load of the same real power.

The direction of power factor matters because it tells you what type of load is causing the reactive power:

• Lagging power factor (the most common) is caused by inductive loads: motors, transformers, solenoid valves, fluorescent lamp ballasts, and any device with a coil. The current waveform lags behind the voltage waveform. Most industrial and commercial installations have lagging power factor because motors dominate the load.
• Leading power factor is caused by capacitive loads: power factor correction capacitors, long unloaded cables, and some electronic power supplies. The current waveform leads the voltage waveform. Leading power factor is less common and can actually cause problems — overvoltage, generator instability, and resonance — if excessive.

The physics is symmetrical: both leading and lagging reactive power occupy conductor capacity without doing useful work. However, since most installations are naturally lagging, the corrective action is almost always to add capacitors (which produce leading reactive power) to cancel out the lagging reactive power from motors and transformers.

This cancellation is not magic. The capacitor supplies the reactive current locally, so it no longer has to flow from the supply transformer through all the cables. The total current in the upstream cables drops, freeing capacity and reducing losses. The motor still draws the same reactive current — it just comes from the capacitor next to it instead of from the grid.

Utilities have a strong economic incentive to penalise low power factor. Their generators, transmission lines, and transformers must handle the full apparent power (kVA), even though they only bill for real power (kWh) in basic tariffs. Low power factor means their infrastructure is underutilised — they are delivering capacity that generates no revenue.

There are three common mechanisms for power factor penalties:

1. kVAr demand charges: The utility bills directly for reactive power consumed, typically measured as the peak kVAr demand in a billing period. Rates vary but A$5–15 per kVAr per month is typical in Australian commercial tariffs.
2. kVA-based demand charges: Instead of billing demand in kW, the utility bills in kVA. Since kVA = kW / PF, a lower power factor directly increases the demand charge. If your demand charge is $15 per kVA and your peak demand is 200 kW at 0.75 PF, you pay for 267 kVA instead of 200 kVA — an extra $1000 per month.
3. Power factor surcharges: A percentage penalty applied to the entire bill when the average power factor falls below a threshold (typically 0.90 or 0.85). For example, a 1% surcharge for each 0.01 below 0.90 — so a 0.80 power factor incurs a 10% surcharge on the entire electricity bill.

Worked example: A manufacturing facility consumes 150,000 kWh per month with a peak demand of 400 kW at 0.78 power factor. The tariff charges $12/kVA demand.

• Current demand: 400 / 0.78 = 513 kVA → demand charge = 513 × $12 = $6,156/month
• Corrected to 0.95 PF: 400 / 0.95 = 421 kVA → demand charge = 421 × $12 = $5,052/month
• Monthly saving: $1,104, or $13,248 per year

Power factor correction (PFC) involves installing capacitor banks to supply reactive power locally, reducing the reactive current drawn from the supply. The economic case is usually compelling because the equipment is relatively cheap and the savings are immediate.

Continuing the example above: to correct 400 kW from 0.78 to 0.95 power factor, we need to cancel a portion of the reactive power:

• At PF 0.78: kVAr = 400 × tan(cos⁻¹(0.78)) = 400 × 0.802 = 321 kVAr
• At PF 0.95: kVAr = 400 × tan(cos⁻¹(0.95)) = 400 × 0.329 = 132 kVAr
• Capacitor bank required: 321 − 132 = 189 kVAr

A 200 kVAr automatically-switched capacitor bank (with detuning reactors to avoid harmonic resonance) costs approximately $15,000–25,000 installed, depending on the market and whether harmonic filtering is required. At $13,248 annual savings, the payback period is 14–23 months.

Additional benefits beyond the direct tariff saving include:

• Reduced cable losses: Lower current means lower I²R losses in all upstream cables. For the example above, current drops from 513 A equivalent to 421 A — losses reduce by (421/513)² = 0.67, a 33% reduction.
• Released transformer capacity: The existing transformer now has 92 kVA of spare capacity (513 − 421) that can supply additional loads without upgrading the transformer.
• Improved voltage regulation: Less reactive current flowing through cables means less voltage drop, improving voltage levels at the equipment.
• Deferred infrastructure upgrades: By reducing the apparent power, existing switchboards, cables, and transformers can serve the same real load with headroom for growth.

Understanding typical power factors helps you estimate whether a facility is likely to have a problem:

The worst offenders are lightly loaded motors and old magnetic-ballast fluorescent lighting. A factory running ten large motors at 50% load will have a significantly lower power factor than the same factory at full production, because motor power factor drops sharply at part load.

Power factor affects cable sizing, voltage drop, transformer loading, and electricity costs. Understanding it is essential for anyone designing or managing electrical installations.

To see how power factor interacts with cable sizing, try the Cable Sizing Calculator — change the power factor input and watch how the required cable size and voltage drop change. For power factor correction sizing, the Power Factor Calculator determines the capacitor bank size needed to achieve a target power factor.

For a deeper understanding of how power factor affects voltage drop specifically, read the Understanding Voltage Drop article in the Learn section.