kW to VA Calculator

Why a watt and a volt-amp are different numbers

Watts measure work delivered. Volt-amps measure what the circuit has to push: volts times amps, before any phase shift between them is counted. Power factor ties the two together, and it is the ratio of real watts to apparent volt-amps. The Department of Energy power factor fact sheet walks a 100 kW load that needs 142 kVA of apparent power, so 100 ÷ 142 = 0.70, and only 70 percent of the current the utility delivers is doing useful work.

That gap is not a rounding error. It is current flowing in the wire that never turns into heat, light, torque or data, and the wire, the breaker and the transformer all have to carry it anyway. The same DOE fact sheet notes that many utilities charge a penalty once the power factor drops below 0.95. In a dwelling the pain shows up as a breaker that trips on a load the wattage said should fit.

A volt-amp is the unit that accounts for all of the current. One VA is one volt times one amp. Multiply by 1,000 for a kVA. The everyday labels on a UPS, a power strip or a generator box lean on VA ratings for exactly this reason, which is why a kW to VA conversion is a shopping step, not a homework step.

The formula, and what power factor does to it

Three lines cover the whole family:

• VA = kW × 1,000 ÷ pf. The only formula this page needs.
• kVA = kW ÷ pf. The same thing at kilo scale, handy for generators.
• kW = VA × pf ÷ 1,000. Run in reverse to see what a UPS can feed.

The power factor does all the damage. Take one kilowatt of real power and watch the apparent power grow as the factor drops:

Check two rows by hand: 1,000 ÷ 0.9 = 1,111 and 1,000 ÷ 0.6 = 1,667. At a power factor of 0.6 the same kilowatt needs 67 percent more current than the wattage alone suggests, and the amps column is where breakers and extension cords live.

Worked example: a 1.8 kW home office on a UPS

Suppose the desk draws 1.8 kilowatts: a desktop, two monitors, a network box and a printer that wakes up grumpy. Modern switch-mode electronics usually run between 0.9 and 1.0 power factor, so take 0.9 as the planning number: 1,800 W ÷ 0.9 = 2,000 VA. That is the UPS capacity the label needs to show.

Current follows: 2,000 VA ÷ 120 V = 16.67 A. A standard 15 amp branch circuit carries 1,800 VA at face value, and only 1,440 VA continuously under the 80 percent rule used for loads that run three hours or more. This office load clears both limits with no room to spare, which is why it belongs on its own 20 amp circuit if the printer cycles often.

If the same gear were perfectly resistive at a power factor of 1.0, the answer would be 1,800 VA and 15 A, and it would still be over the continuous limit. The power factor added 200 VA; the runtime added the rest of the trouble. Both checks matter, and the calculator shows the amps either way.

Worked example: a 5 kW motor at 0.85

Motors are where the VA number earns its keep. Take a 5 kW motor. The Department of Energy motor systems handbook shows a NEMA Design B machine carrying a full-load power factor of 0.85 at 208 volts, so that is the honest planning factor: 5,000 ÷ 0.85 = 5,882 VA.

On a 240 volt circuit, 5,882 ÷ 240 = 24.51 A. At a perfect 1.0 factor the same motor would be 5,000 VA and 5,000 ÷ 240 = 20.83 A. The 0.85 factor costs 17.6 percent more current for the same shaft power, and the wires carry all of it.

The continuous-load rule makes the margin tight: a 30 amp breaker at 240 volts carries 7,200 VA at face value but 0.8 × 30 = 24 A for three hours or more, which is 24 × 240 = 5,760 VA, just under this motor. Anything that runs nonstop needs a larger feed or a smaller machine.

The reactive share of that apparent power is the square root of the difference between 5,882 squared and 5,000 squared, about 3.10 kVAR. No consumer device bills in kVAR directly, but utilities bill it on commercial rates, and correcting it is why power factor capacitors exist.

Reading the VA label on a UPS, power strip or generator

Consumer gear prints both numbers for a reason. A 1,500 VA UPS with a 900 W output rating implies 900 ÷ 1,500 = 0.6. The VA cap is the current limit of the inverter; the W cap is what the electronics inside can sustain without cooking. Load the box past the W rating, even with VA to spare, and it shuts down on heat.

That 0.6 figure is a label convention for cheap stepped-wave units, not a law of physics, and it is wrong for every other load. A resistive heater on the same UPS runs at a power factor near 1.0 and would hit the 900 W ceiling at 900 VA, leaving 600 VA of the label unusable. This is why you convert your own load rather than trusting the box pairing.

Generators lean the other way and mostly advertise watts, because the engine sets the kilowatt ceiling. The apparent power the alternator can pass still matters when the load is motor-heavy: a 5,000 W genset feeding motors at 0.85 is carrying 5,882 VA on a 5,000 VA-ish alternator, and it shows up as voltage sag when the compressor kicks in. Motors also pull several times their running current at start, so leave headroom.

Motor starting is the number that lives outside the steady state arithmetic. A compressor, a well pump or a refrigerator pulls a starting current several times its running current for a fraction of a second, and the volt-ampere burden at that instant sits far above the running figure. That surge is why a circuit that looks fine on paper trips on a hot afternoon, why soft starters and variable frequency drives exist, and why a generator sized on running kilowatts bogs down when a pump kicks in. Size the branch for the surge, then confirm the steady state still fits.

A standby generator adds a transfer switch to the same arithmetic. That switch decides which circuits the generator feeds, and the sum of those circuits in VA is the figure that has to fit under the generator rating. A portable unit on an extension lead has no transfer switch, so the load walks from circuit to circuit and the cord rating becomes the weak link.

Amps come from VA, not from kW

Breakers, wires and plugs are rated in amps, and amps come straight off the volt-amp figure: A = VA ÷ V. The kW number sits one power factor away from that, so convert first, then divide. The units behind the division are SI: the ampere is a base unit and the volt is defined as one watt per ampere in the NIST guide to the SI.

The 80 percent continuous rule shrinks those circuits for anything running three hours or more. A 20 amp, 120 volt circuit carries 2,400 VA at face value, but 0.8 × 20 = 16 A, and 16 × 120 = 1,920 VA continuously. The 2,000 VA office from the earlier example just misses the cut, which is the kind of near miss this calculator exists to catch.

Cord length and conductor size set a second ceiling. A long, thin cord drops voltage along its run, so the appliance at the far end sees fewer volts and pulls more amps for the same job, which heats the cord further. That is why a heavy appliance belongs on a short, thick cord, and why a coiled extension cord warms up under a load it would carry when laid out straight.

Picking a power factor when the nameplate is silent

Most everyday products never print a power factor. These defaults cover the everyday cases, and each one has a source or a definition behind it:

Two habits keep this honest. First, use the worst case that will run, not the best case: the idling motor figure is real, but a sump pump that mostly runs loaded should use 0.85. Second, when a number decides money or safety, measure it. A plug-in watt meter reads watts and volt-amps together and gives you the true factor for that device in that outlet.

Mistakes that turn a sizing job into a return trip

Five mistakes cover most bad conversions in this family:

• Treating 1 kVA as 1 kW. Equal only at a power factor of 1.0. At 0.85, 1 kVA is 0.85 kW.
• Dropping the factor of 1,000. 1.8 kW is 1,800 W, but 2,000 VA at a power factor of 0.9. The kilo applies to both the watts and the volt-amps.
• Reusing amps from the wrong voltage. 2,000 VA is 16.67 A at 120 V and 8.33 A at 240 V. Same power, half the current.
• Reading a motor shaft rating as electrical input. The nameplate kilowatts are mechanical output; the electrical input is higher once motor losses are counted.
• Accepting a power factor above 1.0. Impossible by definition; if a calculation produces it, a number was typed backwards.

And one that never gets caught until the warranty claim: sizing the UPS on watts alone. The W rating gets you through the purchase, the VA rating decides whether the box hums or trips the first August afternoon when everything starts at once.

The consequence shows up as heat in the cord and a voltage that arrives low at the far end. An appliance that expects 120 V and receives a lower number pulls more current for the same job, which pushes the cord harder and shows up on your electricity bill as well. The current the strip carries is the current the load asks for, which is the VA figure divided by the volts at the outlet.

The practical habit is to keep hungry appliances on their own short cord, keep the run short, and replace a strip or cord that has been cut, crushed or run hot. Read the amp rating printed on the strip, divide the VA of the load by that rating, and stay under it. If the arithmetic leaves no margin, the fix is a shorter cord or a different circuit, not a bigger strip.

The receptacle and the breaker behind it belong to the same story. A 15 amp receptacle on a 20 amp circuit is limited by the smaller number, a GFCI receptacle protects people rather than equipment, and a wire gauge too small for the run adds heat that no label warns about. When a plug or faceplate feels warm, the circuit is telling you the load and the wiring disagree, and the answer is a shorter run, a thicker conductor or a different outlet, not a bigger fuse.