Calculate power factor, convert between real, apparent, and reactive power, and size capacitor banks for correction. See exactly how much low power factor is costing your clients — and how to fix it.
A power factor calculator converts between real power (kW), apparent power (kVA), and reactive power (kVAr), computes cos φ, and sizes the capacitor bank needed to correct a lagging installation. Enter any two values to get instant results. The calculator below is free to use — no sign-up required.
Calculate power factor from power values or electrical parameters
Typical target: 0.95
Power factor is a measure of how efficiently an electrical installation converts the current it draws from the supply into useful work. In an ideal world, every ampere of current drawn would contribute directly to powering the load — turning a motor shaft, heating an element, or producing light. In reality, most electrical loads are not purely resistive: they contain inductive or capacitive elements that cause the current waveform to shift out of phase with the voltage waveform.
When the current and voltage are perfectly in phase, the power factor is 1.0 — also called unity power factor. All the power drawn from the supply is real power (measured in kilowatts, kW) and performs useful work. When the current lags or leads the voltage, only a portion of the apparent power (measured in kilovolt-amperes, kVA) is real power. The remainder is reactive power (measured in kilovolt-amperes reactive, kVAr), which oscillates between the supply and the load without doing any useful work but still causes current to flow through the cables and switchgear.
The power triangle is the fundamental relationship that links these three quantities. Real power (kW) forms the horizontal leg, reactive power (kVAr) forms the vertical leg, and apparent power (kVA) is the hypotenuse. The angle between real power and apparent power is called the phase angle (phi), and the cosine of this angle is the power factor: PF = cos(phi) = kW / kVA.
Understanding power factor matters because it directly affects installation sizing, energy costs, and equipment lifespan. A poor power factor means the installation draws more current than necessary, which requires larger cables (see the cable sizing calculator), larger switchgear, and a larger transformer — all of which cost more to install and maintain. It also means higher losses in the distribution system and, for commercial and industrial customers, additional charges from the electricity supplier. The maximum demand calculator accounts for power factor when sizing supplies.
Real (true) power (kW) is the power that actually performs useful work. It heats elements, turns motor shafts, and produces light. Real power is what you pay for on a domestic electricity bill and what the load actually consumes. It is measured in kilowatts (kW) or watts (W).
Apparent power (kVA) is the total power that the supply must deliver to the installation. It is the product of the RMS voltage and the RMS current: for single-phase, kVA = V x I / 1000; for three-phase, kVA = V x I x 1.732 / 1000. Apparent power is what the supply cables, switchgear, transformer, and generator must be rated for. It is always equal to or greater than the real power.
Reactive power (kVAr) is the power that oscillates between the supply and the load, sustaining the magnetic and electric fields in inductive and capacitive components. It does no useful work, but it is essential for the operation of motors, transformers, and other electromagnetic devices. Reactive power is measured in kilovolt-amperes reactive (kVAr). For inductive loads, the reactive power is positive (lagging); for capacitive loads, it is negative (leading).
kVA = sqrt(kW² + kVAr²)
The power triangle: apparent power is always the hypotenuse
The relationship between these three quantities is always governed by the power triangle. If you know any two, you can calculate the third. This is what the Elec-Mate power factor calculator does: enter any two of kW, kVA, or kVAr, and it instantly computes the third, along with the power factor, the phase angle, and the current drawn from the supply.
Low power factor has both direct and indirect costs. The direct costs are the reactive power charges levied by the electricity supplier. Most commercial and industrial tariffs in the UK include a reactive power charge — typically measured in pence per kVAr per month — that is applied when the site's average power factor falls below a threshold, usually 0.90 or 0.95. Some suppliers calculate maximum demand charges based on kVA rather than kW, which penalises low power factor even more heavily.
Example: A factory draws 200 kW at a power factor of 0.70. The apparent power is 200 / 0.70 = 285.7 kVA, and the reactive power is sqrt(285.7² - 200²) = 204.1 kVAr. If the supplier charges based on kVA, the factory is paying for 285.7 kVA of capacity when it only needs 200 kW. If the maximum demand charge is £5 per kVA per month, the factory is paying £1,428.50 per month instead of £1,000 — an excess of £428.50 per month, or £5,142 per year. Correcting the power factor to 0.95 would reduce the kVA to 210.5, cutting the excess charge to just £52.50 per month.
The indirect costs are equally significant. Higher current flow means higher I²R losses in the distribution cables, which appear as wasted heat. The cables, switchgear, and transformer run hotter, reducing their lifespan and increasing the risk of failure. The available capacity of the installation is reduced: a 500 kVA transformer supplying a load at 0.70 power factor can only deliver 350 kW of useful power, whereas the same transformer at 0.95 PF can deliver 475 kW. Improving power factor effectively increases the available capacity of existing infrastructure without any physical upgrade.
For electricians working on commercial and industrial sites, understanding power factor and being able to recommend correction solutions is a valuable skill that can generate additional revenue. A power factor survey followed by a correction installation is a high-value service that pays for itself through energy savings, often within 12 to 18 months.
BS 7671:2018+A4:2026 Regulation 331.1(l) places a direct obligation on designers: power factor shall be assessed as a characteristic of equipment likely to have harmful effects upon other electrical equipment, services, or the supply. Where assessment reveals a poor power factor, designers shall consider power-factor correction as part of the installation design. This regulatory duty applies to new installations and significant alterations alike — making power factor analysis a mandatory part of any thorough electrical design, not an optional extra.
The most common method of power factor correction is the installation of capacitor banks. Capacitors generate leading reactive power (negative kVAr), which cancels out the lagging reactive power drawn by inductive loads. The net reactive power at the point of supply is reduced, bringing the power factor closer to unity.
Fixed capacitor banks are used where the load is constant or near-constant — for example, a single large motor that runs continuously. A fixed capacitor is wired in parallel with the load and provides a constant amount of kVAr correction. The capacitor size is calculated to correct the load's specific power factor to the target value.
Automatic capacitor banks are used where the load varies throughout the day. An automatic unit contains multiple capacitor stages (steps), each with its own contactor, controlled by a power factor controller. The controller continuously monitors the power factor at the incoming supply and switches capacitor stages in and out to maintain the target power factor. This prevents over-correction when the load drops and under-correction when the load increases.
Detuned capacitor banks include series reactors that shift the resonant frequency of the capacitor bank away from common harmonic frequencies. This prevents harmonic resonance — a dangerous condition where the capacitor bank amplifies harmonic currents instead of correcting the power factor. Detuned banks are essential on any site with significant harmonic-producing loads such as variable speed drives, LED lighting, IT equipment, or UPS systems.
The sizing formula is: kVAr required = kW x (tan(arccos(existing PF)) - tan(arccos(target PF))). The Elec-Mate calculator performs this calculation automatically and recommends the nearest standard capacitor bank size.
BS 7671:2018+A4:2026 Regulation 125.8 notes that using the tabulated mV/A/m value directly always overstates the actual voltage drop on an AC circuit, because the tabulated figure assumes unity power factor. For conductors of cross-sectional area 16 mm² or less, the design mV/A/m is obtained by multiplying the tabulated value by the load power factor (cos φ). A load with cos φ = 0.85 therefore produces only 85% of the voltage drop that a naive mV/A/m calculation would suggest — meaning a smaller cable section can sometimes satisfy the voltage-drop limit than first appears. Use the cable sizing calculator to apply this correction automatically.
UK electricity suppliers use several mechanisms to penalise sites with poor power factor. Understanding these mechanisms allows electricians to calculate the payback period for correction equipment and present a compelling financial case to their clients.
Reactive power charges: Many half-hourly metered tariffs include a charge for reactive power consumption above a threshold. The threshold is usually defined as a power factor below 0.95 or a reactive power exceeding 33% of the real power (which corresponds to a power factor of 0.95). The charge is typically between 0.3p and 1.5p per kVAr per half-hour period, which can add up to several thousand pounds per year for large sites.
Maximum demand charges based on kVA: Some tariffs calculate the monthly maximum demand charge based on kVA rather than kW. Since kVA is always higher than kW when the power factor is below unity, the customer pays a premium for every kVA above the kW value. The excess kVA represents the reactive power component and is entirely avoidable with correction.
Capacity charges: The agreed supply capacity (ASC) determines the maximum power a site can draw. If the ASC is defined in kVA, a site with poor power factor reaches its capacity limit at a lower kW level, potentially requiring a costly supply upgrade. Improving power factor increases the usable kW capacity within the existing ASC.
A 30 kW three-phase induction motor has a power factor of 0.78 at full load. Calculate the capacitor size needed to correct to 0.95.
Existing angle = arccos(0.78) = 38.74 degrees, tan(38.74) = 0.8028
Target angle = arccos(0.95) = 18.19 degrees, tan(18.19) = 0.3287
kVAr required = 30 x (0.8028 - 0.3287) = 30 x 0.4741 = 14.2 kVAr
Result: Install a 15 kVAr capacitor at the motor terminals. The corrected apparent power drops from 38.5 kVA to 31.6 kVA, reducing the current by 18%.
A factory draws 250 kW at a power factor of 0.72 from a 400 kVA transformer. The supplier charges £4.50 per kVA per month for maximum demand. Calculate the savings from correcting to 0.95.
Before correction: kVA = 250 / 0.72 = 347.2 kVA
After correction: kVA = 250 / 0.95 = 263.2 kVA
Monthly saving = (347.2 - 263.2) x £4.50 = 84 x £4.50 = £378 per month
Annual saving = £378 x 12 = £4,536 per year
kVAr required = 250 x (tan(43.95) - tan(18.19)) = 250 x (0.9646 - 0.3287) = 158.9 kVAr. Install a 150 kVAr automatic capacitor bank. Typical cost: £4,000 to £6,000 installed. Payback period: 10 to 16 months.
A three-phase supply reads 400 V (the UK nominal three-phase line voltage per BS EN 60038 / BS 7671) and 120 A per phase. The real power measured is 72 kW. Calculate the apparent power, reactive power, and power factor.
Apparent power = 400 x 120 x 1.732 / 1000 = 83.1 kVA
Reactive power = sqrt(83.1² - 72²) = sqrt(6905.61 - 5184) = sqrt(1721.61) = 41.5 kVAr
Power factor = 72 / 83.1 = 0.867 lagging
Result: The power factor is below 0.90 — this site would attract reactive power penalties. Correction to 0.95 would require 72 x (0.5774 - 0.3287) = 17.9 kVAr of capacitors.
Use a power quality analyser or clamp meter with power factor measurement capability to measure the existing power factor of the installation or individual loads. Alternatively, read the power factor from the electricity bill — many commercial and industrial bills show the average power factor or the ratio of kW to kVA. If you only have voltage and current readings, calculate apparent power (kVA) and compare with the known real power (kW) of the load.
Identify the total real power consumption of the installation in kilowatts. This is the actual useful power that performs work — running motors, heating elements, lighting, and so on. You can find this from the electricity meter readings, the supply agreement, or by measuring with a power analyser. For a single load, the nameplate rating in kW is often sufficient.
The typical target is 0.95 lagging. This provides a good balance between cost savings and safety margin. Some suppliers require 0.95 or above to avoid reactive power charges; others set the threshold at 0.90. Never target unity (1.0) or leading power factor, as this risks over-correction. A target of 0.95 to 0.98 lagging is standard practice in the UK.
Use the formula: kVAr required = kW x (tan(arccos(existing PF)) - tan(arccos(target PF))). For example, 150 kW load at 0.78 PF corrected to 0.95: kVAr = 150 x (tan(38.74) - tan(18.19)) = 150 x (0.8028 - 0.3287) = 150 x 0.4741 = 71.1 kVAr. Select the nearest standard capacitor bank size — in this case, 75 kVAr.
Choose between fixed capacitor banks (for constant loads), automatic capacitor banks with stepped switching (for variable loads), or active power factor correction units (for installations with high harmonic content). For most commercial and light industrial sites, an automatic stepped capacitor bank is the best choice. The Elec-Mate calculator recommends the appropriate type based on your load profile.
Purpose-built for UK electricians working on commercial and industrial installations. Calculate, correct, and save your clients money.
Enter any two values and the calculator computes the third. Convert between real power, apparent power…
Calculate the exact kVAr capacity needed to correct from your existing power factor to your target.
Enter your electricity tariff details and the calculator shows the annual savings from power factor correction.
See the power triangle graphically — real power, reactive power, and apparent power shown to scale.
Answer a few questions about your connected loads, and the calculator flags whether harmonic resonance is a risk.
All calculations align with BS 7671:2018+A4:2026 and the IET Code of Practice for energy efficiency.
Reviewed by Chris Dawson, qualified electrician (City & Guilds 2382, 18th Edition) · Last reviewed May 2026 · All calculations verified against BS 7671:2018+A4:2026
Free power factor calculator: convert kW to kVA and kVAr, size the correction capacitor, and fix a poor power factor. Single and three-phase.
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