How the number is made
How to use this calculator
Type the volts and the amps. 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 is the product of voltage and current. Use it when both of those are on a nameplate or a meter, and you need watts of real power. It is the same arithmetic as amps to watts. The form leads with volts because that is how many people read a label: volts first, then amps. The next step still goes to watts to amps so you can change the voltage and get a new current.
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. Enter a documented factor; do not use 1 to relabel apparent power as real watts.
The formulas
DC: P = V × I
Single-phase AC: P = V × I × PF
Three-phase AC: P = √3 × V × I × 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 wye or delta loads, DOE Volume 3 gives P = √3 × VLL × Iline × PF. The three-phase result requires line-to-line RMS voltage and line current; unbalanced loads are not modeled.
DC
Direct current has no power factor. Watts are volts times amps. A 12 V circuit at 10 amps is 120 watts. A 48 V circuit at the same 10 amps is 480 watts. The current did not change. The voltage did. The DC choice ignores whatever power factor is typed in the form, and the steps say so.
Battery math still needs hours and amp-hours on other pages. This tool stops at watts. Multiplying watts by hours to get watt-hours is a later step, and it is not on this form.
Single-phase AC
At a power factor of 1, single-phase watts match the DC formula for the same volts and amps. At 0.8, real power is 80% of the voltage-current product. The voltage-current product alone is apparent power in VA. Neither result establishes an allowable circuit load or equipment rating.
If a nameplate lists volts and amps and also lists watts, run this page as a check. Type the volts and amps. If the result is far from the listed watts, the listed figure may be volt-amperes, or the power factor is not 1, or one of the numbers is an output rating rather than an input rating.
Three-phase
Three-phase watts are higher than single-phase watts for the same volts, amps, and power factor, because the square root of 3 is in the product. The volts have to be line to line. If you type 120 V on a 208 V system, the watts come out low. This page does not convert 120 V to 208 V for you.
Type the current of one phase conductor. Do not add the three clamp readings. The formula already uses the three-phase factor.
Worked example
Type 240 volts, 20 amps, single-phase, power factor 1. The engine returns 4800 watts. The step is 240 times 20. Nothing else is added. Change the power factor to 0.8 and the watts fall. 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 10 amps and 20 amps across common voltages. It is not a list of appliance ratings.
What this page does not do
It does not select installation equipment or apply motor starting current, installation rules or efficiency. Use amps to watts if you prefer to enter current first, or watts to amps for the inverse calculation. Shareable calculations include your values 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 10 A and 20 A
Single-phase, power factor 1. Every cell is computed by the same function as the result card.
| Volts | Watts at 10 A | Watts at 20 A |
|---|---|---|
| 12 | 120 | 240 |
| 24 | 240 | 480 |
| 48 | 480 | 960 |
| 120 | 1200 | 2400 |
| 208 | 2080 | 4160 |
| 240 | 2400 | 4800 |
| 277 | 2770 | 5540 |
| 480 | 4800 | 9600 |
Common questions
How do I convert volts to watts?
Multiply volts by amps for DC, or for single-phase AC at a power factor of 1. If the power factor is lower, multiply by that factor as well. Three-phase also multiplies by the square root of 3, and the volts must be line to line.
Is this the same as amps to watts?
Yes, as arithmetic. Both pages multiply volts, amps, and power factor. This page leads with volts because that is how a nameplate is often read. The steps still name the circuit you chose.
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 volts to watts for three-phase?
Multiply the square root of 3 by the line-to-line volts, then by the amps, then by the power factor. Do not use the single-phase formula on a three-phase load.
Where does the next current come from?
From watts to amps . The next-step button carries the watts, the volts, the circuit, and the power factor. Change the volts there if the new circuit is a different system.
How to read a shared result
A shared address preserves volts, amps, 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, 240 volts at 20 amps gives 4800 watts. Balanced three-phase uses a different formula. The table uses 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 current, 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.
Nameplate traps
Keep input and output ratings separate: conversion losses mean they need not describe the same power. Use voltage and current from the same side and operating condition, with the matching circuit model and, for AC, the power factor. Input values calculate input real power; output values calculate output real power. Combining values from different sides is not a valid efficiency calculation.
A breaker rating is not a load current. Typing 20 amps because the breaker is 20 A overstates the watts for whatever is actually plugged in. Type the measured current or the equipment current, not the device that protects the circuit.
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.