How the number is made
How to use this calculator
Type the amps and the volts. Choose DC, single-phase AC, or three-phase AC. For AC, set the power factor. The watts appear in the result card, with the steps beside it. Submit the form if JavaScript is off. The same formulas run on the server, so the first number is already on the page.
This page starts from load current, not power. Use compatible electrical input values from equipment documentation or a suitable measurement at one operating point. A breaker trip rating is not the load current. The result is real input power in watts; apparent power in VA omits the power-factor multiplier.
For AC, use RMS voltage and RMS current from the same operating condition. Three-phase results require a balanced load and line-to-line voltage. Power factor is real power divided by apparent power, not efficiency. Leaving an unknown factor at 1 can overstate real power; obtain it from equipment documentation or a suitable measurement.
The formulas
DC: P = I × V
Single-phase AC: P = I × V × PF
Three-phase AC: P = I × √3 × V × PF
P is watts, V is volts, I is amps, and PF is power factor. The DOE Fundamentals Handbook gives P = V × I for DC and P = VRMS × IRMS × PF for single-phase AC. For balanced three-phase loads, DOE Volume 3 gives P = √3 × VLL × Iline × PF for wye or delta connections. This result uses line-to-line RMS voltage and line current; it does not model unbalanced loads.
DC
Direct current has no power factor. The watts are amps times volts, and that is the whole working. A 12 V battery bank and a 48 V bank at the same amps do not produce the same watts. Higher voltage means more power for the same current. The DC choice ignores whatever power factor is typed in the form, and the steps say so.
That ignore step matters when you copy a filled address. Someone may have left a motor power factor in the box, then switched the circuit to DC. The card still prints "Not used on DC" so the printed page does not look like the factor was applied.
Single-phase AC
At a power factor of 1, single-phase watts match the DC formula for the same voltage and current. At 0.8, real power is 80% of the voltage-current product. A nameplate rating and a measured operating value need not describe the same condition. Keep all inputs consistent and do not infer installation capacity from this calculation.
If the label lists watts and amps, this page is a check, not a discovery. Type the amps and volts, then compare the result to the listed watts. A large gap often means the label is in volt-amperes, or the power factor is not 1, or the volts on the label are not the volts on your circuit.
Three-phase
Three-phase watts are higher than single-phase watts for the same amps, volts, and power factor, because the square root of 3 is in the product. The volts have to be the line-to-line value. If you type the phase-to-neutral voltage by mistake, the watts come out low. This page does not convert 120 V to 208 V for you. Type the voltage you measured or the voltage the equipment is listed for.
A three-phase motor often lists both horsepower and full-load amps. Horsepower is not on this form. Convert horsepower to watts on a later tool if you need that path. Here the inputs are amps, line-to-line volts, and power factor.
Worked example
Type 15 amps, 120 volts, single-phase, power factor 1. The engine returns 1800 watts. The step is 15 times 120. Nothing else is added. Change the power factor to 0.8 and the watts fall, because the formula multiplies by 0.8 as well. Change the circuit to three-phase and type the line-to-line voltage if that is the system you have. The reference table below is the same engine at power factor 1, single-phase, for a set of round amp values. It is not a list of appliance ratings.
What this page does not do
It does not select wiring, protective devices or equipment, or apply installation rules, motor starting current or efficiency. Use watts to amps when you know real input watts and need current. Shareable calculations include your inputs in the page address, which may appear in browser history and server logs. Do not enter confidential information. Search engines should use the clean calculator address.
Watts at 120 V and 240 V
Single-phase, power factor 1. Every cell is computed by the same function as the result card.
| Amps | Watts at 120 V | Watts at 240 V |
|---|---|---|
| 1 | 120 | 240 |
| 5 | 600 | 1200 |
| 10 | 1200 | 2400 |
| 12 | 1440 | 2880 |
| 15 | 1800 | 3600 |
| 16 | 1920 | 3840 |
| 20 | 2400 | 4800 |
| 24 | 2880 | 5760 |
| 30 | 3600 | 7200 |
| 40 | 4800 | 9600 |
| 50 | 6000 | 12000 |
Common questions
How do I convert amps to watts at 120 volts?
Multiply amps by 120 for DC, or for single-phase AC at a power factor of 1. If the power factor is lower, multiply by 120 and then by that power factor. The table on this page already did the power-factor-1 case.
Does power factor change the watts?
Yes. Single-phase AC uses P = V × I × PF. Balanced three-phase AC uses P = √3 × V_LL × I × PF. For fixed voltage, current and circuit type, a power factor of 1 gives the largest real power within that model. DC does not use power factor.
How do I convert amps to watts for three-phase?
Multiply amps by the square root of 3, then by the line-to-line volts, then by the power factor. Do not use the single-phase formula on a three-phase load.
Is the DC result the same as single-phase AC at power factor 1?
Yes, for the same amps and voltage. Both use amps times volts. Balanced three-phase AC also multiplies by √3 and uses line-to-line voltage, so it is not the same result. The steps retain the selected circuit assumption.
Why is the next step watts to amps?
You can send the calculated real power to watts to amps to check another arithmetic scenario. Keep the circuit type and power factor consistent. These conversions do not select wiring or establish that equipment can use a different supply.
How to read a shared result
A shared address preserves amps, volts, circuit and power factor. The server calculates those inputs even without JavaScript; enhanced calculations update the same result in place. The clean canonical address identifies the calculator and opens its default example, not your particular scenario.
Results are rounded to four decimal places. For DC or single-phase AC at power factor 1, 15 amps at 120 volts gives 1800 watts. Balanced three-phase uses a different formula. The card and reference table use the same rounding. This is an explanatory calculation, not a precision measurement or an installation-design method.
Keep the assumptions with the number. Watts without the voltage, the circuit, and the power factor cannot be checked later. The card lists those four items above the steps so a printed page still shows them. The print button hides the menu and the form. The steps stay.
Watts and volt-amperes
Watts measure real power; VA and kVA measure apparent power. Do not enter a VA rating as watts or assume an unknown power factor is 1. Use values and a circuit model that describe the same operating condition. A current measurement alone does not establish real power.
For sinusoidal single-phase AC, RMS volts times RMS amps gives apparent power. Balanced three-phase also includes √3 and uses line-to-line RMS voltage. Real power additionally requires the power factor. If it is unknown, do not invent a value or use this result to size equipment.
Sources
- DOE Electrical Science Volume 4: DC power identity (formula-only use). Accessed 2026-10-26.
- DOE Electrical Science Volume 3: sinusoidal RMS AC power (formula-only use). Accessed 2026-10-26.
- DOE Electrical Science Volume 3: balanced three-phase line-value power (formula-only use). Accessed 2026-10-26.