A 500 kVA generator cannot run a 500 kW load, and the reason is power factor. This page shows exactly how kVA, kW and kVAR relate, why machines are rated the way they are and how it affects your electricity bill.
kVA and kW look similar on nameplates, yet confusing them leads to undersized generators and higher energy bills. Learn the difference, the formulas, real sizing examples and how Indian tariffs now treat apparent power.
What Is the Difference in kVA vs kW?
In kVA vs kW, kVA is apparent power, the simple product of voltage and current, while kW is real power, the part that actually does work as heat, light or shaft output. They are linked by power factor, so kW = kVA × PF, and the two are equal only when the power factor is 1.
| Point | kVA (Apparent Power) | kW (Real Power) |
|---|---|---|
| Meaning | Total power the source must carry | Useful power converted to work |
| Formula, 3 phase | √3 × V × I ÷ 1000 | √3 × V × I × PF ÷ 1000 |
| Depends on PF | No | Yes, kW = kVA × PF |
| Measured by | Voltmeter and ammeter | Wattmeter or energy meter |
| Rated in | Transformers, DG sets, UPS | Motors (shaft), heaters, engines |
| Limits | Winding current and heating | Engine or prime mover power |
| Billing unit | kVAh, maximum demand kVA | kWh energy |
The gap in kVA vs kW is reactive power, measured in kVAR, which motors and transformers need to build magnetic fields. Our page on active, reactive and apparent power explains each part in more detail.
- kW = kVA × power factor
- kVA = kW ÷ power factor
- kVAR = √(kVA² minus kW²)
- At 0.8 PF, 100 kVA gives 80 kW and 60 kVAR
- Machines limited by current are rated in kVA
Aggreko, one of the largest temporary power suppliers, states that the average generator works at a power factor of 0.8. Its own example shows that 375 kVA × 0.8 gives 300 kW, which is the number your load list must be compared with.
kVA vs kW Formula and the Power Triangle
The three powers form a right angled triangle, with kW on the base, kVAR on the vertical side and kVA on the long side. The angle between kW and kVA is the phase angle φ, and its cosine is the power factor.
kVA² = kW² + kVAR²
PF = cos φ = kW ÷ kVA
kW = kVA × PF
kVAR = kVA × sin φ
For a deeper look at the phase angle and how inductance causes it, see our article on impedance and reactance in AC circuits. A purely resistive heater has φ = 0 and therefore kVA equals kW.
Power factor below 1 can be lagging or leading. Induction motors make it lagging, while a lightly loaded cable or an oversized capacitor bank can make it leading.
Solved Example of kVA vs kW
The kVA vs kW link is easiest to see with real numbers, and the calculator further down uses these same values. Take a 500 kVA transformer feeding a plant load at a measured power factor of 0.8.
kW = 500 × 0.8 = 400 kW
sin φ = √(1 minus 0.64) = 0.6
kVAR = 500 × 0.6 = 300 kVAR
Check: √(400² + 300²) = 500 kVA
If the power factor improves to 0.95, the same 500 kVA transformer could carry 475 kW. That extra 75 kW of capacity comes free, simply by supplying kVAR locally, as shown in our guide on calculating capacitor bank kVAR.
kVA = 300 ÷ 0.8 = 375 kVA
Current at 415 V = 375000 ÷ (1.732 × 415)
= 521.7 A
When a supplier quotes a generator in kW only, ask for the kVA and the rated power factor too. Two sets with the same kW can have very different alternator current limits.
Why Are Generators Rated in kVA Not kW?
A generator set has two separate limits, the kW of its engine and the current of its alternator winding. The Toromont Cat consultant note explains that above the rated 0.8 power factor the engine horsepower limits output, while below 0.8 the alternator amperage becomes the limit.
The same note says 0.8 lagging is the NEMA standard because average industrial loads contain many motors. That is why a DG set nameplate in India often reads something like 500 kVA, 400 kW, 0.8 PF.
Engine sets the kW limit, alternator sets the kVA limit. Rated at 0.8 PF.
Copper loss depends on current and core loss on voltage, neither on PF.
Inverter current limits kVA, while battery and rectifier limit kW. Modern units are often rated near unity PF.
Nameplate kW is shaft output, so input kVA is higher due to efficiency and PF.
Sizing a DG set therefore needs both numbers from the load list, which our article on generator sizing for industrial loads walks through. For standby duty, the AMF panel and DG auto start logic also has to match the set rating.
kVA vs kW for Motors and Motor Loads
Motors are the one place where kVA vs kW often trips up students, because the nameplate kW is shaft output, not electrical input. Input kW equals output kW divided by efficiency, and input kVA equals input kW divided by power factor.
Input kW = 37 ÷ 0.93 = 39.8 kW
Input kVA = 39.8 ÷ 0.86 = 46.3 kVA
So a 37 kW motor actually loads the supply with about 46 kVA, and that is the figure to add when sizing a transformer or DG set. Our page on motor full load current converts the same kVA vs kW result into amperes.
Why Is a Transformer Rated in kVA?
A transformer maker cannot know what power factor the customer load will have. Copper loss depends on current and core loss depends on voltage, so the heating limit is fixed by voltage times current, which is kVA.
This is also why the first step in calculating transformer kVA rating is converting every load from kW to kVA. The basic working of the transformer itself is covered in this guide to how a transformer works.
- Independent of the customer load power factor
- Directly sets the current that heats windings and cables
- Matches breaker and cable selection
- Common standard for transformers and generators
- Matches what the load list adds up to
- Shows the useful output clearly
- Hides the extra current at low power factor
- Can lead to undersized sources if PF is ignored
kVA vs kW in Electricity Billing
Domestic consumers in India are billed on kWh, but most industrial tariffs also charge maximum demand in kVA. The AISSMS IOIT power quality cell in Pune summarises that MSEDCL moved its HT consumers to kVAh billing from 1 April 2020.
In kVA vs kW billing terms, poor power factor raises the energy units directly under kVAh billing, because kVAh is larger than kWh whenever PF is below 1. Our page on electrical energy consumption calculation shows how kWh itself is worked out.
At PF 0.90: kVAh = 10000 ÷ 0.90 = 11111 kVAh
At PF 0.99: kVAh = 10000 ÷ 0.99 = 10101 kVAh
Saving = 1010 units
That is why many plants now keep power factor close to unity with an APFC panel. The basics of correction are covered in power factor correction and in this power factor guide from our sister site.
kVA vs kW Calculator
Enter the apparent power in kVA and the power factor. The calculator returns the real power in kW and the reactive power in kVAR.
The defaults give 400 kW and 300 kVAR, which match the solved example above. Change PF to 0.95 and watch the kVAR fall sharply while kW rises.
Field Tips on kVA and kW
- Read both kW and kVA on the panel multifunction meter, not just one
- Size DG sets on kVA at 0.8 PF and check engine kW separately
- Add capacitor kVAR in steps to avoid leading PF at light load
- Watch harmonics, because THD lowers true power factor
- Check transformer loading in kVA, never in kW alone
- Log monthly kVAh and kWh to track power factor penalties
Harmonic currents increase kVA without adding kW, so a high THD load can show poor true power factor even with capacitors installed. Our article on power harmonics explains this effect, and a power quality analyzer is the right tool to measure it.
Displacement power factor ignores harmonics, while true power factor includes them. A VFD can show 0.97 displacement PF but a much lower true PF on its input side.
For daily work, the engineering formulas sheet keeps the kW, kVA and kVAR relations in one place. UPS buyers should also read our UPS capacity kVA calculation before ordering.
On a UPS datasheet, check the output power factor rating. A 10 kVA unit at 0.9 PF delivers 9 kW, while one at 1.0 PF delivers the full 10 kW.
kVA vs kW FAQ
kVA is apparent power, the product of voltage and current, while kW is the real power that does work. The two are linked by the power factor of the connected load.
When the power factor is one they are equal, and otherwise kW is always smaller. The gap between them is reactive power, which is measured in kilovolt amperes reactive.
Multiply the kVA value by the power factor of the load to get kilowatts. For example, 100 kVA at a power factor of 0.8 gives 80 kW of real power.
If the power factor is unknown, 0.8 is a common assumption for generator sizing. For accurate work, measure the actual power factor with a meter on site.
A generator has an engine limit in kilowatts and an alternator current limit in kilovolt amperes. The rating in kVA protects the alternator winding from overheating at low power factor.
Most sets are rated at 0.8 lagging power factor, following the NEMA convention for motor loads. So a 500 kVA set normally carries a 400 kW engine rating as well.
One kVA equals one kW only when the power factor of the load is exactly one. That happens with purely resistive loads such as heaters and incandescent lamps.
At a power factor of 0.8, one kVA delivers only 0.8 kW of useful power. Motor loads, transformers and fluorescent lighting all draw more kVA than kW from the supply.
For any real load kVA is equal to or bigger than kW, because power factor cannot exceed one. kVA covers both the working power and the reactive part.
The bigger the reactive part, the larger the gap becomes between the two numbers. Capacitor banks reduce this gap by supplying reactive power close to the load.
kVAh billing charges energy in kilovolt ampere hours instead of kilowatt hours on the monthly bill. Since kVAh equals kWh divided by power factor, poor power factor directly raises the units billed.
Several Indian distribution companies now use this method for high tension consumers. Keeping power factor close to unity with capacitor banks is the simplest way to lower the bill.
Related Articles
- Power Factor, kW, kVA and kVAR
- Active, Reactive and Apparent Power
- Power Factor Improvement Calculation
- How to Calculate Generator Size for an Industrial Load
- Capacitor Bank Sizing for Power Factor Correction
External References
- Toromont Cat Consultants Corner: Generator Power Factor (PDF)
- Aggreko: What Is the Difference Between kW and kVA
- Wikipedia: Power factor
What We Learn Today
- In kVA vs kW, kVA is apparent power from voltage and current, while kW is real power found by multiplying kVA by power factor.
- Generators and transformers are rated in kVA because winding current sets their heating limit, regardless of the power factor of the load.
- Under kVAh billing, poor power factor raises billed units, so keeping power factor near unity with capacitors directly lowers industrial energy bills.