Volts, Amps & Watts Calculator
Convert watts to amps, amps to watts, or find the voltage from the other two values. Choose DC or AC, enter the values from your equipment label, and see the calculation step by step.
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How this calculator works
Choose what to find, amps, watts or volts, and the electrical system, then enter the other two values. One formula does the work: watts equal volts times amps for DC, times the power factor for single-phase AC, and times √3 as well for balanced three-phase AC.
To find amps or volts, the calculator divides the watts by the other values instead of multiplying. Inputs must be greater than zero, and the power factor must be above 0 and no more than 1. The methodology page sets out the formulas, the input rules and the sources.
What is the difference between volts, amps and watts?
Volts are the electrical push, amps are the rate at which current flows, and watts are the rate at which energy is used. For DC, watts equal volts multiplied by amps; for AC, multiply by the power factor as well. One number is never enough: a conversion always needs two of the three values.
- Volts (V). Voltage is electrical potential difference: the push that can drive current through a circuit. Most US household appliances run on 120 V; larger ones, such as clothes dryers and electric cooktops, use 240 V.
- Amps (A). Current is the rate of electric charge flow. More current at the same voltage means more apparent power.
- Watts (W). Real power is the rate of energy transfer. A 1,000 W load uses 1 kWh of energy if it draws that power for one hour.
Power factor is not efficiency. It is the ratio of real power (watts) to apparent power (volt-amperes). Motors and electronic devices may have a power factor below 1; read their specifications or use appropriate measurements instead of assuming a value.
Formulas for DC, single-phase and three-phase power
DC power is volts times amps. Single-phase AC power is volts times amps times the power factor. Balanced three-phase AC power is √3 times the line-to-line voltage times the line current times the power factor. To find amps or volts, rearrange the same equation and divide the watts instead of multiplying.
| System | Find watts | Find amps | Find volts |
|---|---|---|---|
| DC | P = V × I | I = P ÷ V | V = P ÷ I |
| Single-phase AC | P = V × I × PF | I = P ÷ (V × PF) | V = P ÷ (I × PF) |
| Balanced three-phase AC | P = √3 × V × I × PF | I = P ÷ (√3 × V × PF) | V = P ÷ (√3 × I × PF) |
AC voltage and current are RMS values. Three-phase mode uses line-to-line voltage, line current and total three-phase real power; do not enter line-to-neutral voltage. The three-phase calculation assumes a balanced sinusoidal supply and load. Unbalanced or distorted systems require more detailed analysis.
Calculations use full precision internally, and results are shown rounded to at most two decimal places. This is a steady-state estimate; it does not predict motor starting current or compressor surge.
Practical appliance examples
A 1,500 W heater on 120 V draws 12.5 A at a power factor of 1, and the same 1,500 W at 240 V draws 6.25 A. A 12 V DC accessory drawing 5 A uses 60 W. A 120 V AC load drawing 5 A at a power factor of 0.8 uses 480 W of real power.
| Example | Inputs | Calculation | Result |
|---|---|---|---|
| Resistive heater | 1,500 W, 120 V AC, PF 1 | 1,500 ÷ (120 × 1) | 12.5 A |
| Same power at 240 V | 1,500 W, 240 V AC, PF 1 | 1,500 ÷ (240 × 1) | 6.25 A |
| DC accessory | 12 V DC, 5 A | 12 × 5 | 60 W |
| AC load with PF below 1 | 120 V AC, 5 A, PF 0.8 | 120 × 5 × 0.8 | 480 W (600 VA) |
| Balanced three-phase load | 480 V line to line, 20 A, PF 0.85 | √3 × 480 × 20 × 0.85 | 14,133.53 W |
The two heater examples compare equal power at different rated voltages; they are not instructions to connect a 120 V appliance to 240 V.
Use this tool to understand ratings, not to select wire sizes, circuit breakers or an installation design. A result below a breaker rating does not establish that an appliance is safe on that circuit. Follow the equipment instructions and have electrical work assessed by a qualified electrician.
Watts to amps chart at 120 V, 240 V and 12 V DC
At 120 V and a power factor of 1, amps equal watts divided by 120: 1,500 W is 12.5 A and 1,800 W is 15 A. At 240 V the same watts draw half the current. At 12 V DC, divide the watts by 12, so a 100 W load draws 8.33 A. The tables below use that rule.
| Watts | Amps at 120 V | Amps at 240 V |
|---|---|---|
| 100 W | 0.83 A | 0.42 A |
| 300 W | 2.5 A | 1.25 A |
| 500 W | 4.17 A | 2.08 A |
| 750 W | 6.25 A | 3.13 A |
| 1,000 W | 8.33 A | 4.17 A |
| 1,200 W | 10 A | 5 A |
| 1,500 W | 12.5 A | 6.25 A |
| 1,800 W | 15 A | 7.5 A |
| 2,000 W | 16.67 A | 8.33 A |
| 2,400 W | 20 A | 10 A |
| 3,000 W | 25 A | 12.5 A |
| 3,600 W | 30 A | 15 A |
| 5,000 W | 41.67 A | 20.83 A |
| 7,500 W | 62.5 A | 31.25 A |
| 10,000 W | 83.33 A | 41.67 A |
A motor, compressor or electronic power supply may have a power factor below 1, and then it draws more current than the chart shows for the same watts. For those, use the calculator with the power factor from the equipment specification.
Amps at 12 V DC
For the same power, a 12 V DC load draws ten times the current of a 120 V AC load at a power factor of 1.
| Watts | Amps at 12 V DC |
|---|---|
| 50 W | 4.17 A |
| 100 W | 8.33 A |
| 300 W | 25 A |
| 500 W | 41.67 A |
| 1,000 W | 83.33 A |
| 2,000 W | 166.67 A |
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Frequently asked questions
How many amps is 1,500 watts?
At 120 V and a power factor of 1, 1,500 W draws 12.5 A. At 240 V and a power factor of 1, it draws 6.25 A. The voltage and, for AC, the power factor are needed to calculate current.
How do I convert amps to watts?
Multiply the amps by the volts. For AC, multiply by the power factor as well: 10 A at 120 V and a power factor of 1 is 1,200 W. For balanced three-phase AC, use the line-to-line voltage and multiply by √3 too.
Is watts always volts times amps?
For steady DC, yes. For single-phase AC, volts times amps gives apparent power in VA; real power in watts is V × A × PF. For balanced three-phase AC using line-to-line voltage, include √3 as well.
Can I calculate watts from volts alone?
No. Voltage alone does not tell you current or power consumption. Enter the current too, and the power factor for AC. For a known resistance, a different formula, P = V² ÷ R, can be used for a resistive load.
What power factor should I enter?
Use the equipment specification or a suitable measurement. A power factor of 1 models a purely resistive AC load. Do not assume a power factor of 1 for motors, compressors or electronic power supplies.
Are these running amps or starting amps?
The results describe the values you enter under steady conditions. They do not estimate starting surge. Use the manufacturer's starting requirements when selecting a generator for a motor or compressor.
Does this calculate battery capacity or running cost?
No. Battery capacity needs energy units such as Wh or Ah plus voltage. Electricity cost needs power, running time and your electricity rate; the electricity cost calculator works that out.