Watts to Amps Calculator

What Is the Watts to Amps Calculator?

How to use the watts to amps calculator

1. Pick the supply. Press DC, Single-phase AC or Three-phase AC above the boxes. DC hides the power factor box, because the idea has no meaning for a steady current, and three-phase renames the voltage box Line-to-line voltage.
2. Enter the power. Type the load's power in W or kW, the figure on its rating label. For a motor, enter the electrical input power: the kW on a motor nameplate is the shaft output, and the input is that divided by the efficiency from the data sheet.
3. Enter the voltage. On single-phase this is the rms supply voltage, such as 230 V (UK/Europe nominal) or 120 V (North America nominal). On three-phase it is the voltage between two lines, such as 400 V or 480 V.
4. Enter the power factor. Use 1 for a kettle, a heater or a filament lamp, and the nameplate figure for a motor. Anything above 1, or 0 and below, is refused.
5. Read the result and the extras. The headline is the current. On AC the extras give the Apparent power in VA and the Reactive power in var; Load resistance appears on DC and on single-phase at a power factor of 1, Phase voltage on three-phase, and Power in kW on every supply.
6. Solve the other way. The Solve for menu turns the tool into an amps to watts converter (Power) or finds the voltage a given power and current imply (Voltage). Open Show working to see the formula for the supply you picked, with your numbers in it.

The headline figure trims trailing zeros, so a 1500 W heater on 120 V shows 12.5 A while the working line, which always carries four significant figures, ends in 12.50 A. They are the same number. The same happens with the headlamp, which shows 5 A in the headline and 5.000 A in the working.

Three slips account for most wrong answers. On three-phase, typing the phase voltage of 230 V instead of the line-to-line 400 V turns the motor preset's 16.98 A into 29.53 A. Leaving the power factor at 1 for a motor under-reads the current, and typing 2000 with kW selected, when watts were meant, asks for 2000 kW and returns 8696 A.

Worked example: change one thing at a time

Start from the kettle and move as little as possible per row, except at row 7, which jumps to a new load. Every figure in the table is the string the calculator itself displays for those inputs, so if the table and the tool ever disagree, the tool is right. A dash means the calculator shows no such extra for that supply.

Rows 1 to 3 hold the watts and change the voltage, then the watts. Keeping 2000 W in the power box while the voltage box drops from 230 V to 120 V multiplies the current by 230/120 = 1.917, to 16.67 A, and that describes a second kettle whose element was designed for 120 V. The resistance extra shows it, falling from 26.45 Ω to 7.200 Ω, and the item Two kettles, not one further down gives what the first kettle would really do on 120 V.

Rows 4 to 6 show what the power factor does. At 1000 W and 230 V, lowering it from 1 to 0.8 lifts the current by 1/0.8 = 1.25, from 4.348 A to 5.435 A, while the watts stay put and the apparent power rises to 1250 VA. The resistance extra disappears, because below a power factor of 1 the load is no longer a plain resistor.

Rows 7 to 9 are the motor preset reached one change at a time. Treating 10 kW at 400 V as single-phase gives 29.41 A; pressing Three-phase AC divides that by sqrt(3) = 1.732, to 16.98 A in each line, and the phase voltage extra reads 230.9 V. Raising the power factor to 1 brings it to 14.43 A, smaller by the factor 1/0.85 = 1.176.

Row 10 is the headlamp on DC. The power factor box is hidden, the extras shrink to the load resistance and the power, and 60 W on 12 V comes out at 5 A, ten times the 0.5 A that the same 60 W needs on 120 V.

Formula and symbol reference

The calculator uses three forms of one idea: I = P / V for DC, I = P / (V × PF) for single-phase AC and I = P / (sqrt(3) × V × PF) for balanced three-phase AC. The extras come from S = P / PF, Q = S × sqrt(1 - PF²) and R = V / I, and the reverse modes simply rearrange the same equations, P = V × I × PF and V = P / (I × PF), with sqrt(3) added on three-phase.

The physics: why the supply type changes the current

A volt is a joule per coulomb and an amp is a coulomb per second, so volts times amps is joules per second, which is watts. That is the whole of the DC case, and it is why the calculator refuses to answer without a voltage: the same power can arrive as many coulombs at a low voltage or a few at a high one.

On AC the supply has to carry the Apparent power extra, not just the watts. When voltage and current fall out of step, as they do in a motor, part of the current surges back and forth doing no net work, and the Reactive power extra measures that part, while the number in the power factor box is simply watts over volt-amperes. Dividing by it is what makes the current rise when the power factor falls.

A three-phase supply shares the load between three live conductors whose voltages are staggered evenly through each cycle. The voltage between two lines is sqrt(3) times the voltage from each line to neutral, so the calculator divides by sqrt(3) as well and reports the current in each line, with the line-to-neutral figure as the Phase voltage extra.

Where the conversion breaks down

The arithmetic is exact. What goes wrong is the number typed into the power box, the power factor assumed for the load, or the supply the formula takes for granted.

A rating is not a measurement

The watts on a label are what the appliance is rated for, and the current the tool returns is the current at that rating. Real draw moves around it: a thermostat switches a heater on and off, and a motor running light takes far less than its full-load figure.

Symptom: a clamp meter reading well below the calculated current on a lightly loaded machine.

A motor's kW is its output

Motor nameplates rate the mechanical power at the shaft, and the motor takes more than that from the supply. What belongs in the power box is the electrical input, shaft kW over efficiency; that efficiency belongs to the particular motor, so take it from its data sheet and verify before use rather than guessing one.

The first moment after switch-on

The result is the current once the load has settled into steady running. For a moment after switch-on, a motor that has not yet spun up or a lamp whose filament is still cold takes far more, and nothing in the boxes can show that surge.

Resistance that changes with temperature

The Load resistance extra is V / I at the rated power, which for a lamp is the hot filament. A multimeter on the cold lamp reads lower, because a metal filament's resistance rises as it heats.

Loads that distort the current

Switch-mode power supplies, chargers and LED drivers draw current in short pulses rather than a smooth sine wave. The formula still holds if the power factor you enter is the true one, which includes that distortion, but a figure that describes only the phase shift under-reads the current.

Unbalanced three-phase

The three-phase formula assumes the same current in all three lines. If the phases are loaded unequally, work each one as a single-phase load on the phase voltage the three-phase extra reports, 230.9 V for a 400 V supply, and expect a different current in each line.

Two kettles, not one

The two kettle presets describe a 2000 W kettle built for each voltage. Plug the 26.45 Ω element of a 230 V kettle into 120 V and, if its resistance stayed constant, it would draw 4.537 A and give only 544.4 W. The watts on a label hold only at the voltage the appliance was built for.

None of this is installation advice. Cable sizes, fuses and circuit breakers are chosen under local wiring regulations, by a qualified electrician, and the current this tool returns is only the starting point of that work.

Where it is actually used

Three-phase workshop machines

Press Single-phase AC with the motor preset loaded and the headline climbs from 16.98 A to 29.41 A, the same 10 kW of electrical input squeezed through one pair of conductors. That factor of sqrt(3) less current per conductor is part of why larger machines are wired for three-phase.

Generators and UPS units rated in VA

The VA figure on a backup supply caps the current it can deliver, and a separate watt figure, where one is given, caps the power. Run each load through the tool, add up the Apparent power extras and compare the total with the VA figure; simple addition errs on the safe side.

12 V battery and solar systems

Low voltage means high current for the same power. The headlamp preset needs 5 A at 12 V, and on DC a 1000 W load would need 83.33 A, which is why low-voltage systems use heavy cables and keep their loads small.

Extension leads and adaptors

A lead or adaptor is rated for a maximum current, and the rating printed on the lead is the figure to compare against. Add the watts of everything plugged into it, enter the total, and set the result beside that printed rating.

Reading a rating label

Most appliances print their watts and their supply voltage, but not their current. For a heater, kettle or other resistive appliance, type the two figures into the boxes with a power factor of 1 and the headline is the current at full power.

Checking a circuit with Ohm's law

On DC, or single-phase at a power factor of 1, the resistance extra gives you the load as a plain resistor, so the result can go straight into an Ohm's law check. The 12 V headlamp behaves as 2.400 Ω, and 12 V across 2.400 Ω gives back the same 5 A.

Where to go next

For the reasoning behind the three formulas, with worked problems that go step by step, read the guide Watts to Amps: Conversion and Formula, and for the relation the resistance extra rests on, the article on Ohm's law. Take the load resistance to the Ohm's law calculator, or the same watts to the electricity cost calculator to see what the load costs to run. Or browse the library of physics simulations.