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Watts Calculator

Enter any two of watts, amps and volts and get the third, with power factor and three-phase handled properly — then see what the load costs to run for a day, a month and a year.

Where are you?

AWG, NEC, 120/240 V, miles and gallons — we remember this on every calculator.

What are you trying to find?
Supply type

DC is batteries and solar. Single-phase AC is every household outlet in North America. Three-phase is commercial and industrial, and the calculation picks up a factor of the square root of 3.

Volts

The supply voltage. On a three-phase supply this is the line-to-line voltage — the number on the nameplate — not the 120 V or 230 V you measure from one line to neutral.

V
Amps

The current drawn. Look for the "A" figure on the rating plate, or clamp a meter round one conductor.

A
Power factor

How much of the current actually does work. Heaters, kettles and incandescent bulbs are 1.0. Motors, compressors and transformers are 0.7–0.9. If you do not know, leave it at 1.0 for anything that only makes heat and 0.85 for anything with a motor.

1
Heater, kettle, filament lamp
1.00
LED lighting, modern electronics
0.90–0.95
Induction motor at full load
0.85
Induction motor lightly loaded
0.60–0.70
Generator nameplate convention
0.80
Energy and running cost

Used only for the energy and running-cost figures. A fridge cycles rather than runs, so use about 8 hours a day rather than 24.

h/day

The per-kWh figure from your bill, including delivery charges. The US average is about 17 cents.

per kWh

Your result

Watts

1,200 W

Watts
1,200 W
Kilowatts
1.20 kW
Amps
10 A
Volts
120 V
Apparent power
1,200 VA
Energy per day
4.80 kWh

🪿 The goose says: 120 V × 10 A is 1,200 W — 1.20 kW. Left on for 4 hours a day that is $24.84 a month.

  • NEC sizes a continuous load at 125% of its current, so 10 A wants a 15 A breaker and conductors rated for it. A 15 A household circuit is limited to 12 A continuous, which is 1,440 W at 120 V.

The same load on a different supply

How many amps 1,200 W draws at each common voltage, at the power factor you set.

12 V DC
100 A
120 V
10 A your supply
240 V
5 A
480 V three-phase
1.44 A

Doubling the voltage halves the current, which is why the same appliance needs half the copper on 240 V that it needs on 120 V.

What this costs to run

Energy per day
4.80 kWh 4 h a day
Energy per month
146 kWh
Energy per year
1,752 kWh
Cost per day
$0.82
Cost per month
$24.84
Cost per year
$297.84

At $0.17 per kWh. A month is 30.44 days, the average across a year.

How this watts calculator works

Watts, volts and amps are three views of the same thing. Volts is the pressure pushing electrons, amps is how many are moving, and watts is the work they do — pressure times flow. That gives one equation, W = V × A, which you rearrange depending on which of the three you are missing.

On direct current that is the whole story. On alternating current it is not, because voltage and current do not necessarily peak at the same instant. Anything with a coil in it — a motor, a transformer, a fluorescent ballast — pulls current slightly out of step with the voltage, and the portion that is out of step does no useful work. Power factor is the fraction that does.

Three-phase adds one more term. Three-phase equipment is rated by its line-to-line voltage, and each of the three lines carries current, so the total works out to √3 (about 1.732) times line volts times line amps times power factor.

  1. Step 1 DC, or resistive AC watts = volts × amps
  2. Step 2 Single-phase AC watts = volts × amps × power_factor Power factor is 1.0 for anything that only makes heat, and 0.7–0.9 for anything with a motor.
  3. Step 3 Three-phase AC watts = √3 × line_volts × line_amps × power_factor Line volts means the 208 V, 240 V or 480 V between two phases, not the 120 V to neutral.
  4. Step 4 Rearranged for current amps = watts ÷ (volts × factor)
  5. Step 5 Rearranged for voltage volts = watts ÷ (amps × factor)
  6. Step 6 Energy and money kWh = watts ÷ 1000 × hours; cost = kWh × price_per_kWh Watts is a rate. Kilowatt-hours is the quantity you actually buy.

Example: a 1,500 W space heater on a 120 V outlet draws 1,500 ÷ 120 = 12.5 A. That is already above the 12 A continuous limit of a 15 A circuit, which is why heaters trip breakers when anything else shares the outlet. Run it four hours a day and it uses 6 kWh — about $31 a month at 17 cents.

Assumptions & caveats

Everything this calculator quietly assumes on your behalf. If one of these does not match your situation, the answer will be off.

  • Power factor is applied to AC only. On DC, voltage and current are always in step, so watts is simply volts times amps.
  • Three-phase is assumed balanced — all three lines carrying the same current. An unbalanced load has to be worked out phase by phase.
  • The voltage entered for three-phase is line-to-line. Entering 120 V for a three-phase supply gives a nonsense answer, because no three-phase system is rated on its line-to-neutral voltage.
  • Nameplate figures are maximums. A 1,000 W microwave is rated on its cooking output and typically draws around 1,500 W from the outlet; a 3-ton air conditioner draws its full current only in hot weather.
  • Breaker suggestions apply the NEC 125% continuous-load factor to the calculated current. They do not account for conductor derating, ambient temperature, grouping, or what else is already on the circuit.
  • Running cost assumes a flat rate per kWh and the same hours every day. Time-of-use tariffs, demand charges and tiered rates all change the answer.
  • Standby draw is ignored. Anything with a clock, a remote sensor or a wall wart keeps using a watt or two even when it is off.

Frequently asked questions

How do I calculate watts?

Multiply volts by amps: a 120 V outlet supplying 10 A is delivering 1,200 W. On alternating current with a motor involved, multiply by the power factor as well, which is typically 0.85. On a three-phase supply, multiply by √3 (1.732) on top of that.

How many watts is 1 amp?

It depends entirely on the voltage, which is why the question has no single answer. One amp is 120 W at 120 V, 240 W at 240 V, and only 12 W at 12 V. Amps measure flow; watts measure flow multiplied by pressure.

How do I convert watts to amps?

Divide watts by volts. A 1,500 W heater on a 120 V circuit draws 12.5 A, and the same heater on 240 V would draw 6.25 A. If the load has a motor, divide by the power factor as well, which increases the current.

What is the difference between watts and kilowatt-hours?

Watts is a rate, like miles per hour; kilowatt-hours is a quantity, like miles travelled. A 1,000 W appliance running for one hour uses 1 kWh. Your electricity bill charges for kilowatt-hours, which is why running time matters as much as wattage.

What is power factor and do I need to worry about it?

Power factor is the fraction of the current that does useful work, and it matters whenever there is a motor, transformer or compressor in the circuit. A pump drawing 10 A at 120 V with a power factor of 0.8 delivers 960 W, not 1,200 W. For heaters, kettles, ovens and filament bulbs the power factor is 1.0 and you can ignore it entirely.

How many watts can a 15 amp circuit handle?

A 15 A circuit at 120 V can carry 1,800 W momentarily but only 1,440 W continuously, because the NEC limits a continuous load to 80% of the breaker rating. A 20 A circuit gives you 1,920 W continuous. That 1,440 W limit is why a space heater and a hair dryer on the same circuit trip the breaker.

Why does three-phase use the square root of 3?

Because the three phases peak at different moments, 120 degrees apart, so the total power is not simply three times the single-phase figure. Working through the trigonometry gives √3, about 1.732, when the voltage is measured line-to-line. It is the reason a 480 V three-phase supply carries far more power per amp than its voltage alone suggests.

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