kVA Calculator
Enter current and voltage - or kilowatts and power factor - to calculate apparent power in kilovolt-amperes (kVA) for single-phase or three-phase AC circuits. The calculator also returns real power (kW), reactive power (kVAR), and the line current in amps. Results update as you type.
Formula
Worked example
A 20 A load on a 240 V single-phase circuit: S = 20 A x 240 V / 1,000 = 4.8 kVA. If the power factor is 0.8, real power P = 4.8 x 0.8 = 3.84 kW, and reactive power Q = sqrt(4.8^2 - 3.84^2) = 2.88 kVAR.
What is kVA and why does it matter?
Kilovolt-amperes (kVA) is a unit of apparent power - the total power drawn from a supply by an AC circuit. It differs from kilowatts (kW), which measures only the real power that does useful work. The difference arises because inductive loads such as motors, transformers and fluorescent ballasts store and release energy on every AC cycle, drawing current that never converts to useful output. This reactive current still flows through conductors, generators and transformers, causing heat and limiting the capacity available for other loads. Utilities often bill commercial customers on their kVA demand rather than their kW consumption, so understanding and managing kVA is an important part of energy cost management.
kVA formulas for single-phase and three-phase systems
For a single-phase AC circuit the apparent power in kVA equals the current in amps multiplied by the voltage in volts, divided by 1,000. For a balanced three-phase system using the line-to-line voltage, multiply by the square root of 3 (approximately 1.732) before dividing by 1,000. When the line-to-neutral voltage is used instead, multiply by 3. You can also start from real power: divide kilowatts by the power factor to get kVA. The relationship between the three power quantities forms the power triangle: kVA is the hypotenuse, kW is the adjacent side (real power), and kVAR is the opposite side (reactive power). The power factor equals kW divided by kVA.
How to size a generator or UPS in kVA
Generator and uninterruptible power supply (UPS) ratings are given in kVA, not kW, because the apparent power determines the current capacity of the windings, cables and switchgear. To size a generator, add up the kVA of every connected load, then apply a safety margin - a common rule is to select a generator rated at 125% of total connected kVA to allow for motor startup surges, which can momentarily demand three to seven times the running current. If a load list gives wattage rather than kVA, divide by the expected power factor (0.8 is a common assumption) to estimate the kVA equivalent.
Power factor correction and its effect on kVA
Improving a poor power factor reduces kVA without changing the real power (kW) delivered to loads. This lowers the current drawn from the supply, reducing I2R losses in cables, freeing switchgear capacity, and cutting utility demand charges. The standard method is to install power factor correction capacitors in parallel with inductive loads. For example, a 10 kW load at a power factor of 0.7 draws 10 / 0.7 = 14.3 kVA and 10.2 kVAR of reactive power. Raising the power factor to 0.95 reduces the apparent power to 10 / 0.95 = 10.5 kVA, cutting the reactive demand by about 60%.
Common power factor values by load type
| Load type | Typical power factor | Category |
|---|---|---|
| Incandescent lighting | 1.00 | Resistive |
| Electric heaters / ovens | 1.00 | Resistive |
| LED drivers (corrected) | 0.95-0.99 | Resistive |
| Synchronous motors (at rated load) | 0.90-1.00 | Inductive |
| Large induction motors (full load) | 0.85-0.90 | Inductive |
| Medium induction motors (half load) | 0.70-0.80 | Inductive |
| Fluorescent lighting (ballast) | 0.50-0.80 | Inductive |
| Arc furnaces | 0.70-0.80 | Inductive |
| Small induction motors | 0.60-0.75 | Inductive |
| Arc welders | 0.35-0.60 | Inductive |
Typical power factor ranges for common electrical loads. Purely resistive loads have a power factor of 1.0; inductive loads (motors, transformers) are lower.
Frequently asked questions
What is the difference between kVA and kW?
kW (kilowatts) measures real power - the energy per second actually consumed by a load as heat, light or mechanical work. kVA (kilovolt-amperes) measures apparent power - the total current-voltage product that flows through the circuit, including the reactive portion that does no net work but still loads conductors and generators. They are related by the power factor: kW = kVA x power factor. For purely resistive loads such as incandescent bulbs and electric heaters, kVA and kW are equal because the power factor is 1.0. For inductive loads like motors and transformers, kVA is always larger than kW.
How do I convert kVA to amps?
For a single-phase circuit: amps = (kVA x 1,000) / voltage. For a three-phase circuit using the line-to-line voltage: amps = (kVA x 1,000) / (1.732 x V_LL). For a three-phase circuit using the line-to-neutral voltage: amps = (kVA x 1,000) / (3 x V_LN). For example, a 5 kVA load on a 240 V single-phase circuit draws 5,000 / 240 = 20.8 amps.
How do I convert kW to kVA?
Divide the real power in kW by the power factor: kVA = kW / PF. If you do not know the power factor, 0.8 is a common assumption for general industrial loads. For example, a 4 kW motor running at a power factor of 0.8 draws 4 / 0.8 = 5 kVA of apparent power.
What is reactive power (kVAR)?
Reactive power (kVAR - kilovolt-amperes reactive) is the portion of apparent power that oscillates between the supply and inductive or capacitive elements in the load, doing no net work. It is calculated from the power triangle: kVAR = sqrt(kVA^2 - kW^2). Although reactive power is not consumed, it still flows through cables and switchgear, limiting their useful capacity and increasing losses. Power factor correction capacitors supply reactive power locally so it does not have to travel from the utility transformer.
Why are generators and transformers rated in kVA rather than kW?
The thermal limit of a generator or transformer winding is set by the current flowing through it, not by the real power delivered. Because current is determined by apparent power (kVA) and voltage, not by real power, the kVA rating defines how much load can be safely connected. A 10 kVA transformer can supply 10 kVA regardless of the power factor of the load, but the real power it delivers could be anywhere from near-zero (at a very poor power factor) to 10 kW (at unity power factor).
What is a good power factor for an electrical system?
A power factor of 0.95 or above is generally considered excellent and is the target set by many utilities and energy codes. Values between 0.85 and 0.95 are good; 0.70-0.85 is fair and usually warrants correction; below 0.70 is poor and often attracts demand penalty charges from the utility. Purely resistive loads (heaters, incandescent lamps) have a power factor of 1.0, while lightly loaded induction motors can drop to 0.3-0.5.
How much kVA do I need for a house?
A typical single-family home in the US is served by a 100 A or 200 A single-phase, 240 V service. A 200 A / 240 V service provides a maximum of 200 x 240 / 1,000 = 48 kVA. Average household consumption is far lower - the actual demand at any moment rarely exceeds 5-15 kVA for a medium-sized home - but the service must be sized for the connected peak load including air conditioning, electric vehicles and other high-draw appliances running simultaneously.