SizingKitPower & current · W, VA and VAR
Volts, Amps and Watts Calculator
Free converter between volts, amps and watts on single-phase alternating current, where watts is not volts times amps unless the power factor is exactly 1. Enter any two of the three plus the load’s power factor and it returns the third along with the apparent power in VA, the reactive power in VAR, the angle between them and the impedance in ohms — no signup, and the triangle is drawn rather than described. A 1,500 VA uninterruptible supply rated 900 W is that arithmetic on the badge.
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- No signup
- W, VA and VAR
- Triangle drawn
- Impedance in ohms
Two readings and the power factor
On alternating current two figures are not enough: volts and amps give volt-amperes, and only the power factor turns that into watts. Say which two you took and what the load’s power factor is.
Default 0.85 — The midpoint of the 0.80–0.90 band NEC (NFPA 70) 2023 Table 430.250 treats as the working range in its synchronous-motor note, which raises the table's unity-power-factor currents by 10 % at 0.90 and by 25 % at 0.80. The midpoint is this site's choice of default; the band is the code's.
Loads worth trying
The three powers, the impedance and the angle between them
Real power — W
1,000 W
1 kW · 1.34 hp
Apparent power — VA
1,176 VA
1.18 kVA
Reactive power — VAR
619.5 VAR
31.8° between W and VA
| Voltage | 120 V |
|---|---|
| Current | 9.8 A |
| Impedance |Z| = V ÷ I | 12.2 Ω |
| Resistive part R = |Z| × PF | 10.4 Ω |
| Reactive part X = |Z| × sin φ | 6.45 Ω |
The ohms above are an impedance, which is what volts over amps means once the current alternates. Only at a power factor of 1.00 is that figure a resistance you could confirm with a meter on a dead circuit; below it, the resistive part carries the watts and the reactive part carries current that does no work but still has to fit down the conductor. Power factor belongs to the load, not to the supply. A resistance heater, an incandescent lamp and a resistive test load are 1.00 by definition. An induction motor runs 0.80–0.90 at full load and falls steeply as it unloads. A switch-mode supply with active correction reads 0.95–0.99, and one without it nearer 0.6. Read it off the nameplate, or off a clamp meter that reports it — nothing on this page can infer it from the other two figures.
The triangle assumes a sine wave. This power factor is the displacement kind — the cosine of the angle between voltage and current — and on a load that chops its current rather than shifting it, the true power factor of IEEE Std 1459-2010 is lower still because of the distortion term. Nothing here measures harmonic content, so on an LED driver, a variable speed drive or an uncorrected switch-mode supply, take the power factor from an instrument that reports true PF rather than from this triangle.
How to turn two AC readings into watts
A clamp meter gives you current, the panel gives you voltage, and neither of them gives you watts on its own.
Choose the pair you actually measured
Volts with amps is the clamp-meter case, volts with watts is the nameplate case, and amps with watts is a metered branch where the supply voltage is not what you assumed. Each pair fixes the triangle differently, so choosing it is not cosmetic — with volts and amps the apparent power comes first and the watts follow, and with volts and watts it goes the other way.
Enter the load's power factor, not the supply's
Power factor is a property of what is plugged in. Read it from the nameplate, from a meter that reports it, or from a controller's own display; the 0.85 prefilled here is the middle of the band the NEC treats as normal for a motor and is a placeholder for your figure, not a specification. A resistance element is 1.00 exactly.
Size the copper on VA and the bill on W
The apparent power is what the conductor, the breaker, the transformer and the generator have to carry, because it is fixed by current. The real power is what the utility meter registers and what turns into shaft work, light or heat. Where the two differ by 25 %, so does the equipment you have to buy.
What the triangle covers, and where it stops
| Input pairs | 3 — volts with amps, volts with watts, amps with watts. Power factor is a fourth input on every one of them and is never inferred from the other two. |
|---|---|
| Figures reported | 9: real power, apparent power, reactive power, phase angle, voltage, current, impedance magnitude, its resistive part and its reactive part. |
| Power factor field | 0.85 prefilled, editable between 0.01 and 1.00. Anything above 1.00 is refused, because apparent power is the hypotenuse and cannot be the shortest side. |
| Phase angle range | 0° at unity power factor to 89.4° at 0.01 — the arc-cosine of the figure you type, drawn to scale in the triangle. |
| Real power in three units | W, kW and hp together, on the mechanical horsepower of 550 ft·lbf/s exactly, because a US motor plate reads horsepower and an IEC one reads kilowatts. |
| Kind of power factor | Displacement only — the cosine of the angle between a sinusoidal voltage and a sinusoidal current. The distortion term that IEEE Std 1459-2010 adds for non-sinusoidal current is not computed here. |
| Phase configuration | Single-phase. No √3 is applied anywhere on this page; a three-phase result is a different identity with its own page. |
| Rounding | Reactive power and the phase angle to one decimal place; everything else follows the precision rule of its own unit, which prints two decimals below 10 and none above 100. |
Frequently asked questions
Why doesn't volts times amps equal the watts my meter shows?
Because on alternating current that product is the apparent power, and only the part of the current in phase with the voltage does work. Multiply 120 V by 9.8 A and you get 1,176 VA; at a power factor of 0.75 the meter registers 882 W and the other 294 VAR sloshes back and forth between the supply and the motor's magnetic field twice per cycle. The current is real — it heats the conductor and trips the breaker — but it delivers no net energy, which is exactly what the reactive side of the triangle measures.
Can I convert VA to watts without knowing the power factor?
No, and any converter that does it has silently assumed 1.00. The two are different quantities joined by a number that belongs to the load, so a 5 kVA figure is anywhere between 3 kW and 5 kW depending on what is connected. This is why transformers, generators and uninterruptible supplies are all rated in kVA: the manufacturer knows what the iron and the copper can carry and does not know what you will plug into it.
Does reactive power cost me money?
On a residential meter, no — it registers real energy only. On a commercial or industrial tariff, usually yes, through a power factor penalty or a demand charge based on kVA rather than kW, and the threshold is commonly written into the tariff at 0.90 or 0.95. The utility's cost is real even though the energy is not: reactive current occupies capacity in every transformer, conductor and switchgear bus between the generator and your service, and correction capacitors at the load are how it is usually bought back.
Can power factor be greater than 1?
No — it is the ratio of one side of a right triangle to its hypotenuse, so 1.00 is the ceiling by geometry. A meter reading above unity is a sign convention or an instrument error, and a reading of 1.00 exactly on a motor circuit usually means the meter is computing watts from a current transformer and assuming what it is meant to be measuring. A leading power factor — current ahead of voltage rather than behind it, which happens on an over-corrected capacitor bank or a lightly loaded long cable — is still a number at or below 1.00, and it is reported as leading rather than as more than one.
Does a resistance heater have a power factor?
Yes, and it is 1.00 by definition rather than by measurement. Current through a pure resistance is in phase with the voltage across it, so the triangle collapses to a straight line: the reactive side is zero, apparent power equals real power, and the impedance the page reports is a resistance you could confirm with a meter on the dead element. Baseboard heat, an element in a water heater and an incandescent lamp all behave this way; a heater with a fan in it does not, because the fan motor is a separate reactive load in parallel.
My LED driver says 0.42 A at 120 V but 25 W — which figure is wrong?
None of them: 0.42 A at 120 V is 50 VA, so the driver runs at a power factor near 0.5, and that is normal for a small supply without power factor correction. What matters is that the low figure here is mostly distortion rather than displacement — the driver draws current in narrow spikes at the peak of the sine wave instead of drawing a shifted sine — so the triangle on this page understates the situation. That distinction is the reason the page names the displacement power factor explicitly and points at an instrument that reports true PF.
Do I size the conductor and the breaker on watts or on VA?
On VA, every time, because ampacity is about current and the conductor cannot tell which part of the current is doing work. Take the apparent power the page returns, divide by the system voltage to get the amps, and take that figure into a conductor and overcurrent selection. Sizing on watts at a power factor of 0.75 undersizes the current by a third, which on a 40 A calculation is the difference between two conductor sizes.
About watts, volt-amperes and the angle between them
Three quantities share the same dimensions and are measured in three different unit names precisely so that nobody confuses them. Apparent power, in volt-amperes, is the product of the voltage and the current a meter reads, and it is what every piece of iron and copper in the path has to be built for. Real power, in watts, is the part that becomes shaft torque, light or heat, and it is what the revenue meter integrates into kilowatt-hours. Reactive power, in volt-amperes reactive, is the remainder — energy that travels out to the magnetic field of a motor winding or the electric field of a capacitor and comes back a hundred and twenty times a second on a 60 Hz supply. The three form a right triangle because the reactive current is a quarter cycle out of phase with the real one, which is also why the two are added as squares rather than arithmetically, and why a 0.8 power factor costs you 25 % more current rather than 20 %.
The failure this page exists to prevent is a converter that assumes unity and says nothing. That assumption is invisible and it is wrong on almost every load that is not an element: it undersizes generators, it makes a battery inverter look bigger than it is, and it is the reason a 1,500 VA uninterruptible supply shuts down under 1,400 W of servers while its own display still shows headroom — the badge is a VA rating and the useful figure is the 900 W on the spec sheet beside it. A second failure is subtler and newer. The cosine-of-the-angle definition assumes both waveforms are sinusoids, and a switch-mode supply, an LED driver or a variable frequency drive draws current in pulses, so its true power factor as IEEE Std 1459-2010 defines it includes a distortion term this triangle has no way to see. Where the load is electronic rather than magnetic, take the figure from an instrument that measures it. If you would rather work from the nameplate kilowatts straight to the breaker, the kW to amps calculator does that crossing with the same power factor input, and the kVA to amps calculator does it from the transformer’s side, where no power factor is involved at all.
Direct current has none of this. There is no angle, no reactive term and no distinction between the two kinds of power, which is why the Ohm’s law calculator can hand back four figures from two and never mention a power factor — and why sizing a battery bank starts in watt-hours and only meets volt-amperes again at the inverter, whose own output power factor is what caps the load it will carry. Going the other way, adding two more phases does not change the triangle but does change the multiplier in front of it, and that √3 belongs to the three phase power calculator. For the five relations run one at a time with the substituted arithmetic printed rather than a triangle drawn, the electrical calculator is the console.
What happens to the readings you enter
Every number on this page is worked out by JavaScript running in the tab you are reading it in. Nothing you type — loads, lengths, nameplate ratings, the rates your utility charges you — is uploaded, logged or kept, which is also why the calculators carry on working in a mechanical room with no signal.
The triangle is drawn from the two figures in the fields and redrawn as you type, in the page itself. No reading, nameplate value or power factor is transmitted, so a meter walk round a customer’s plant leaves nothing behind on this site.