AmpSizerAmps to watts
Amps to watts
1,800 watts
from 15 amps at 120 V, power factor 1.00
| Also equal to | Value |
|---|---|
| Current | 15 A |
| Real power | 1,800 W = 1.80 kW |
| Apparent power | 1,800 VA = 1.80 kVA |
| Mechanical equivalent | 2.41 HP |
| In milliamps | 15,000 mA |
- Breaker for this as a continuous load
- 20 A
- Copper conductor that carries it
- 12 AWG
At a power factor below 1, watts and volt-amps split apart. The wire carries the volt-amps; the meter bills the watts.
A sizing aid from the published NEC tables, not a design — have it signed off by a licensed electrician against the code your inspector enforces. Calculations follow the US NEC; outside the US, follow your local electrical code.
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MonthlyOne thing a month worth measuring, testing or recalculating on a home electrical system — the part of sizing that happens after the install. No product pitches, and nothing that needs a live panel.
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How do you convert amps to watts?
The AmpSizer amps-to-watts result for 15 amps at 120 volts is 1,800 watts — amps times volts, at unity power factor.
With a power factor below 1 the answer splits in two. The same 15 A at 120 V is still 1,800 volt-amps of apparent power, but at a power factor of 0.8 only 1,440 watts of it does work. The conductor has to carry all 1,800 VA either way.
That gap is why generators and transformers are rated in kVA and electricity bills are in kWh. One is measuring what has to be delivered, the other what got used, and the ratio between them is the power factor.
What each circuit in a house can deliver, as watts, at the two voltages that matter.
| Circuit | At 120 V | At 240 V | Continuous ceiling at 240 V |
|---|---|---|---|
| 15 A | 1,800 W | 3,600 W | 2,880 W |
| 20 A | 2,400 W | 4,800 W | 3,840 W |
| 30 A | 3,600 W | 7,200 W | 5,760 W |
| 40 A | 4,800 W | 9,600 W | 7,680 W |
| 50 A | 6,000 W | 12,000 W | 9,600 W |
| 100 A | 12,000 W | 24,000 W | 19,200 W |
The last column is the honest design figure for anything running for hours. The middle column is where the protection operates.
The power factor, in one paragraph
Current and voltage in an alternating-current circuit do not have to peak at the same instant. In a resistive load they do, and every amp does work. In an inductive load the current lags, and part of it simply sloshes in and out of the magnetic field without doing anything useful — while still flowing through your conductor and still heating it.
Power factor is the fraction that does work: 1.0 for a heater, about 0.8 for a small motor, and worse at light load than at full load. It is the reason a lightly loaded motor is a poor citizen on a circuit, and the reason capacitors get fitted to correct it in industrial installations.
Frequently asked questions
How many watts is 15 amps?
1,800 W at 120 V, 3,120 W at 208 V and 3,600 W at 240 V. That is real power at unity power factor — for a motor at 0.8 the same 15 A is only 1,440 W of work at 120 V, while still occupying 1,800 VA of the circuit.
Is a 20 amp circuit 2,400 watts?
At 120 V, yes as a ceiling: 20 × 120 = 2,400 W is where the breaker is at its rating. The usable continuous figure is 80% of that, 1,920 W, because a circuit supplying a continuous load is sized at 125% of it. Both numbers are worth knowing and they are used for different things.
What is the difference between watts and VA?
Volt-amps are volts times amps, full stop. Watts are the part of that which becomes work rather than circulating back and forth. For resistive loads they are identical. For inductive loads — motors, transformers, older fluorescent gear — VA is larger, and the wire is sized on VA.
Do I bill on watts or on VA?
Domestic meters bill real energy in kWh, so watts. Larger commercial supplies are often billed with a power-factor penalty or on kVA demand as well, which is the utility’s way of charging for the current they had to deliver but could not sell.
How do I convert amps to horsepower?
Work out the watts first, then divide by 745.7. But the answer is the electrical input, and a motor’s horsepower rating is its shaft output — so a motor’s true input is its output divided by its efficiency. A nominal 1 HP motor at 80% efficiency draws about 930 W, not 746.
How we calculate this
volts × amps × power factor; three-phase is
1.732 × volts × amps × power factor. Apparent power in volt-amps drops the power factor from the same
expression. Horsepower divides real power by 745.7 watts per mechanical horsepower, which is shaft output
and therefore an idealisation of a motor’s electrical input — the true input is the output divided by the
motor’s efficiency. Where a device and conductor are shown for the resulting current, the current is
taken at 125% as a continuous load, the device is the next standard rating in NEC 240.6(A), and the
conductor satisfies Table 310.16 with the 240.4(D) ceiling applied.Worked example
With Amount 15, Measured in amps, Convert to watts, Voltage 120 V, Supply Single phase, Power factor 1.00, this page works out 1,800 watts. from 15 amps at 120 V, power factor 1.00
- Breaker for this as a continuous load20 A
- Copper conductor that carries it12 AWG
Sources:
Where the two numbers matter
Sizing a conductor: use VA. The copper carries current, and current comes from apparent power.
Sizing a generator: use kVA and check the watts too. A generator has both a kVA limit set by its alternator and a kW limit set by its engine, and a poor power factor can hit one long before the other.
Working out a running cost: use watts. Only real power turns into kilowatt-hours, and only kilowatt-hours appear on the bill.