Amps to Watts Calculator — Real & Apparent Power
Amps times volts gives you power — but on AC there are two answers, and mixing them up is the single most common error in load estimating. Watts is real power, what the utility bills. Volt-amperes is apparent power, what the conductor actually carries. They diverge as soon as power factor drops below 1.0. This calculator returns both, plus the reactive component that explains the gap.
Convert amps to watts
1.0 resistive · 0.9–0.95 modern electronics · 0.8–0.9 motors at load
Real power
At unity power factor, watts equals volt-amperes — a purely resistive load.
Kilowatts
2.40 kW
Apparent power
2,400 VA
See the breakdown
Size conductors, breakers, transformers, and generators from the VA figure — the conductor carries the full current whether or not it does useful work.
The formula, explained in plain English
One multiplication for apparent power, and the same multiplication times power factor for real power. The difference between them is the whole subject of power factor.
Size from VA, bill from watts
The conductor and breaker have to carry the full current, so they follow VA. Your utility meter measures real work, so it follows watts. Two different numbers for the same load.
The power triangle
Real, reactive, and apparent power form a right triangle. Power factor is the cosine of its angle. At PF 1.0 the triangle collapses to a line and VA = W.
Why UPS ratings are in VA
A 1,000 VA UPS at PF 0.8 delivers 800 W. Comparing a load's watts to a UPS's VA over-promises capacity — always compare like with like.
Breaker rating isn't load
A 20 A circuit is capacity, not consumption. Use the actual current for load estimating and the breaker rating only when you want the circuit's maximum.
Worked examples
A resistive circuit where both answers agree, a motor where they don't, and a three-phase feeder.
20 A at 120 V, resistive
Power factor 1.0 — a fully loaded 20 A kitchen circuit. The defaults above.
S = 120 × 20 = 2,400 VA · Q = 0 VAR
continuous limit: 20 × 0.8 = 16 A → 1,920 W
Result: watts and VA are identical because nothing is reactive. Note the continuous ceiling — a load running three hours or more may only use 1,920 W of that 2,400 W.
15 A at 240 V motor, PF 0.8
Single-phase inductive load.
S = 240 × 15 = 3,600 VA
Q = √(3,600² − 2,880²) = 2,160 VAR
Result: the conductor carries 3,600 VA but only 2,880 W does work — 720 VA of the apparent power is pure overhead. Size the wire for 15 A regardless; the power factor changes the bill, not the current.
100 A at 480 V three-phase, PF 0.9
A commercial feeder.
S = 1.732 × 480 × 100 = 83,139 VA ≈ 83.1 kVA
Q = √(83.1² − 74.8²) = 36.2 kVAR
Result: 83.1 kVA is the number that sizes the transformer; 74.8 kW is the number on the energy bill. If the utility also charges a kVA demand rate, correcting that 36.2 kVAR is worth money — see the Power Factor Correction Calculator.
Circuit capacity in watts
What each circuit rating can carry at unity power factor, and the reduced figure once the load is continuous (three hours or more, so limited to 80% per NEC 210.20(A)).
| Circuit | 120 V max | 120 V continuous | 240 V max | 240 V continuous |
|---|---|---|---|---|
| 15 A | 1,800 W | 1,440 W | 3,600 W | 2,880 W |
| 20 A | 2,400 W | 1,920 W | 4,800 W | 3,840 W |
| 30 A | 3,600 W | 2,880 W | 7,200 W | 5,760 W |
| 40 A | 4,800 W | 3,840 W | 9,600 W | 7,680 W |
| 50 A | 6,000 W | 4,800 W | 12,000 W | 9,600 W |
| 60 A | 7,200 W | 5,760 W | 14,400 W | 11,520 W |
| 100 A | 12,000 W | 9,600 W | 24,000 W | 19,200 W |
Sources & standards: NEC (NFPA 70) 2023 — 210.20(A) continuous load and the 125%/80% relationship, Article 100 definition of continuous load, 240.6(A) standard overcurrent ratings, Table 310.16 conductor ampacities. Local amendments override the model code, and a licensed electrician plus the AHJ have final say on anything installed.
Frequently asked questions
Common questions about amps, watts, volt-amperes, and power factor.
How do I convert amps to watts?
Multiply amps by volts, then by power factor for AC. DC: P = V × I. Single-phase AC: P = V × I × PF. Three-phase AC: P = √3 × V × I × PF. So 20 A at 120 V with unity power factor is 20 × 120 = 2,400 watts.
How many watts is 20 amps?
At 120 V, 20 A is 2,400 W. At 240 V the same 20 A is 4,800 W — twice the power for identical current, because watts depend on both. Note that a 20 A circuit should only carry 2,400 W briefly: for a continuous load the 80% rule limits you to 1,920 W at 120 V.
What's the difference between watts and volt-amperes?
Watts are real power — energy actually converted to heat, light, or motion, and what your utility bills you for. Volt-amperes are apparent power — the product of voltage and current regardless of whether that current does useful work. They are equal only at unity power factor. Conductors, breakers, transformers, and generators are all sized in VA, because they have to carry the full current either way.
Why does my UPS or generator say VA instead of watts?
Because its limit is current-driven, not work-driven. A 1,000 VA UPS at 0.8 power factor delivers only 800 W of real power — the remaining 200 VA is reactive current the equipment must still carry. Manufacturers publish VA because that is the honest capacity figure; the watt rating depends on what you plug in. Always compare a load's VA against a UPS or generator's VA rating, not its watts.
Can I use the breaker rating as the amps?
You can, but you will over-estimate. A breaker rating is the circuit's capacity, not the load on it. A 20 A circuit with a single 60 W lamp is carrying 0.5 A, not 20 A. Use the breaker rating only when you deliberately want the circuit's maximum — for instance when checking whether a panel can supply a fully loaded circuit. For continuous loads the usable figure is 80% of the rating; see the Breaker Size Calculator.
What power factor should I assume?
1.0 for resistive loads — heaters, resistance elements, incandescent lamps. 0.9 to 0.95 for modern LED drivers and electronics with power-factor correction. 0.8 to 0.9 for motors near full load, dropping toward 0.5 on a lightly loaded motor. If a nameplate gives both watts and VA, divide them to get the actual power factor.
Does three-phase really give more watts for the same amps?
Yes — about 73% more, because of the √3 factor. 20 A at 480 V single-phase is 9,600 VA; the same 20 A at 480 V three-phase is 16,628 VA. That is three conductors each carrying 20 A rather than a pair, so more total power moves for the same per-conductor current. It is the reason three-phase equipment of a given rating uses smaller wire.
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