Watts to Amps Calculator

A watts to amps calculator converts power to current by dividing watts by volts: amps = watts ÷ volts. A 1,500-watt heater at 120 volts draws 1500 ÷ 120 = 12.5 amps. For AC, divide by volts × power factor; for three-phase, by √3 × volts × PF. Enter your figures above for an instant result across DC, single-phase, and three-phase systems.

The formula behind watts to amps

Everything starts from the electrical power law, often written as P = V × I, which rearranges to the basic conversion:

For direct current (DC) that is the whole story. Alternating current (AC) adds a power factor (PF), and three-phase AC adds a √3 (about 1.732) factor:

Here P is real power in watts, V is voltage in volts, I is current in amps, and PF is the power factor (1 for DC and resistive loads, below 1 for motors and reactive loads). The three-phase formula above uses the line-to-line voltage; if you know the line-to-neutral (phase) voltage instead, the equivalent is I = P / (3 × V × PF).

A worked example: 1,500 W heater at 120 V

Suppose you have a 1,500-watt space heater plugged into a standard US 120-volt outlet. A resistive heater has a power factor of essentially 1.0, so the single-phase formula simplifies to I = P / V:

So the heater draws 12.5 amps. A standard household circuit is rated for 15 A, and the National Electrical Code treats a heater as a continuous load, meaning the circuit should be sized to 125% of the load - 12.5 A × 1.25 = 15.6 A. That already exceeds a 15 A breaker, which is exactly why running a 1,500 W heater plus other devices on the same 15 A circuit so often trips the breaker. On a 20 A circuit (sized to 16 A continuous) the same heater is comfortably within limits.

How many amps is X watts at 120 volts? Appliance chart

This is the most-searched version of the question, so here it is worked out for common 120 V household devices (resistive, power factor 1.0). The last two columns flag the NEC continuous-load limit - 80% of the breaker rating, i.e. 12 A on a 15 A circuit and 16 A on a 20 A circuit.

A single high-wattage device rarely trips a breaker on its own, but two of them - say a space heater plus a hair dryer - on the same 15 A circuit add up to 27.5 A and will trip it immediately.

A three-phase example: 10 kW at 480 V

Now take a commercial load of 10,000 W on a 480 V three-phase supply (line-to-line) with a power factor of 0.9. Using the three-phase formula:

Each of the three lines carries about 13.4 amps. Notice how three-phase distribution keeps the per-line current low for a large power: the same 10 kW on single-phase 120 V (power factor 1.0) would draw 10000 / 120 ≈ 83 A, which is why factories and large equipment use three-phase power.

Three-phase watts to amps, step by step

Here is the full calculation for a 25,000 W (25 kW) load on a 480 V three-phase supply (line-to-line) at a 0.9 power factor, broken into the exact steps the calculator runs.

The result is about 33.4 amps per line. Drop the power factor to 0.8 and the current rises to 37.6 A for the same 25 kW - the reactive load makes each conductor carry more current.

How to use this watts to amps calculator

You only need two or three numbers. Work through the fields in order:

1. Current type: choose DC, single-phase AC, or three-phase AC. This switches the formula automatically.
2. Power (watts): enter the device's rated wattage, found on the nameplate, label, or spec sheet.
3. Voltage (volts): enter the supply voltage - 120 V or 240 V for most US homes, 208 V/480 V for commercial three-phase.
4. Power factor: for AC, enter the load's power factor. Use 1.0 for heaters and resistive loads, or the motor/equipment rating (often 0.8 to 0.95) for reactive loads.
5. Three-phase reference: if you picked three-phase, tell the calculator whether your voltage is line-to-line or line-to-neutral so it applies the right factor.

The amps appear instantly, along with the exact formula used so you can check the math by hand.

Who this calculator is for

Converting watts to amps comes up constantly across home, hobby, and professional electrical work. This tool helps:

• Homeowners and renters checking whether an appliance, heater, or microwave will overload a 15 A or 20 A circuit.
• RV, van, and boat owners sizing inverters, shore-power hookups, and 12 V DC loads.
• Solar and battery hobbyists converting panel and inverter wattage into the DC and AC currents their wiring and fuses must handle.
• Electricians and apprentices doing a quick running-current sanity check before a formal NEC load calculation.
• Students and makers learning Ohm's power law and how AC, DC, and three-phase systems differ.

Watts, amps, volts, and power factor explained

• Watt (W): the unit of real power - the rate at which energy is actually used or converted to heat, light, or motion.
• Amp (A): the unit of current - the rate of electric charge flow. Current is what determines wire heating and breaker trips.
• Volt (V): the unit of electric potential, or "pressure," that pushes current through a circuit.
• Power factor (PF): in AC, the ratio of real power (watts) to apparent power (volt-amps). A PF below 1 means the circuit carries more current than the watts alone suggest.
• Volt-amp (VA): apparent power, equal to volts × amps. For resistive loads VA equals watts; for reactive loads VA is larger than watts.

Common voltages and what they draw

The same wattage gives a very different current depending on voltage, which is the single most important thing to remember. Here is how a 1,200 W load compares (power factor 1.0):

• 12 V DC (car/RV battery): 1200 / 12 = 100 A - huge current, which is why low-voltage DC needs thick cables.
• 120 V AC (US outlet): 1200 / 120 = 10 A - a typical household appliance draw.
• 240 V AC (US large appliance): 1200 / 240 = 5 A - half the current of 120 V for the same power.
• 480 V AC three-phase (line-to-line, PF 0.9): 1200 / (1.732 × 480 × 0.9) ≈ 1.6 A per line.

Higher voltage means lower current for the same power, which is why power is distributed at high voltage and stepped down near the point of use.

Watts to amps conversion table (all voltages)

This matrix converts common wattages to amps across the voltages you are most likely to meet. DC and the 120 V / 240 V columns assume a resistive load (power factor 1.0); the three-phase columns use the line-to-line voltage at a 0.9 power factor.

Reading across any row shows the core rule at a glance: the higher the voltage, the lower the current for the same wattage. The tiny per-line currents in the 480 V column are why heavy equipment runs on high-voltage three-phase.

Factors that change the amps

If you adjust the inputs and watch the current move, a few things dominate the result:

• Voltage: the biggest lever - doubling the voltage halves the current for the same watts.
• Power factor: on AC, a lower PF raises the current. Dropping PF from 1.0 to 0.8 raises the current by 25% for the same wattage.
• Number of phases: three-phase spreads the load across three lines, so each line carries far less current than a single-phase equivalent.
• Whether you used line-to-line or line-to-neutral voltage: mixing these up is the most common three-phase error and changes the answer by a factor of √3.

Practical tips

• Read the nameplate. Most devices list watts and volts, and many list amps directly - if the nameplate gives amps, trust it over a converted value.
• Use the rated power factor for motors. A guessed PF can throw the current off by 20% or more; the motor nameplate lists full-load amps and PF.
• Add a margin for continuous loads. NEC requires sizing continuous loads (running 3+ hours) at 125% of the current.
• Don't forget inrush. Motors and compressors can draw several times their running current for a fraction of a second at startup.
• Match the voltage to your supply. Using 120 V when the device is on 240 V doubles your calculated amps - always confirm the actual circuit voltage.

Converting amps back to watts

If you know the current and want the power, simply reverse the formulas. For DC, watts = volts × amps. For single-phase AC, watts = volts × amps × PF. For three-phase AC (line-to-line), watts = √3 × volts × amps × PF. So a motor drawing 10 A at 240 V with a 0.85 power factor uses 240 × 10 × 0.85 = 2,040 W of real power, while its apparent power is 240 × 10 = 2,400 VA. The gap between watts and volt-amps is the reactive power the utility still has to deliver.

Limitations and assumptions

This is a planning and learning tool, not a code-compliant design. Keep these limits in mind:

• It returns the steady-state running current for the values you enter - not motor inrush, surge, or fault current.
• It does not size conductors, breakers, or fuses; that requires NEC ampacity tables, temperature and bundling derating, and the 125% continuous-load rule.
• It assumes a balanced three-phase load; unbalanced loads draw different currents on each line.
• It uses the power factor you enter; harmonics and distorted waveforms in real electronics can make the true RMS current higher.
• It assumes the nominal voltage you type; real supply voltage varies within utility tolerance.

Related conversions and calculators

This page answers "how many amps does this wattage draw?" For related questions, a sister tool fits better:

• To work through Ohm's power law in any direction, use the Scientific Calculator for the raw arithmetic.
• To convert between units of length, weight, and volume for a project, use the Unit Converter.
• To express very large or very small electrical numbers cleanly, try the Scientific Notation Calculator.

This is an electrical conversion, not general arithmetic - if you actually need everyday percentage math, a different tool fits. The Percentage Calculator works out "what is X% of Y", the Percentage Increase Calculator and Percentage Change Calculator compare two figures, and the Discount Calculator finds a sale price - none of them need a voltage, unlike this watts-to-amps tool.

About this formula

The watts-to-amps conversion is a deterministic application of the electrical power law (P = V × I) and the standard three-phase relationship using the √3 factor. These are universal physics and engineering identities, not regional figures, so they need no external source - the math is the same everywhere. The only judgment calls are the power factor you assign to an AC load and which voltage reference you use for three-phase, both of which you control in the inputs above. For circuit design rather than a bare current figure, defer to the National Electrical Code and a licensed electrician.