The Basic Formula: Watts = Amps × Volts

To find watts from amps, multiply the amperage by the voltage. That's it. The formula is Watts = Amps × Volts. If a device draws 5 amps at 120 volts, it uses 600 watts. If it draws 10 amps at 240 volts, it uses 2,400 watts.

The reason you need both numbers is that amps alone don't tell you the full power picture. A 5-amp device at 120 volts uses far less power than a 5-amp device at 240 volts, even though the amperage is identical. Voltage is the electrical pressure pushing the current through, and watts measure the actual work being done.

Most household devices in North America run on either 120 volts (standard outlets) or 240 volts (larger appliances like dryers, ovens, and air conditioners). Once you know which voltage your device uses, the math takes seconds.

Key Takeaways

  • Multiply amps by volts to get watts: a 15-amp device on a 120-volt circuit uses 1,800 watts.
  • Household outlets are usually 120 volts; larger appliances use 240 volts, which doubles the wattage at the same amperage.
  • Check your device's nameplate or manual for the voltage, or look at your circuit breaker panel to see what voltage that outlet or circuit provides.
  • If you only have amps and no voltage information, you cannot calculate watts — you need both numbers.

Where to Find the Voltage Your Device Uses

The voltage is almost always printed on the device itself. Look for a label on the back or bottom that says "Input" or lists specifications. It will say something like "120V" or "240V" or "120/240V". If the label is worn or missing, check the manual or the manufacturer's website.

If you're working with a circuit in your home, the voltage depends on which breaker it's connected to. Standard outlets (the ones you plug lamps and phone chargers into) are 120 volts. Dedicated circuits for large appliances — the dryer, electric range, water heater, air conditioner — are usually 240 volts. If you're unsure, a multimeter set to voltage mode can measure the outlet directly, or you can call an electrician.

Real Examples: Common Household Devices

A typical kitchen microwave draws about 10 amps on a 120-volt circuit. Using the formula: 10 amps × 120 volts = 1,200 watts. A window air conditioner might draw 12 amps on 120 volts, which is 1,440 watts. A hair dryer often pulls 15 amps at 120 volts, reaching 1,800 watts — which is why it can trip a breaker if other devices are running on the same circuit.

Larger appliances use 240 volts. An electric clothes dryer typically draws 20 to 30 amps at 240 volts. At 25 amps, that's 25 × 240 = 6,000 watts. An electric water heater might draw 30 amps at 240 volts, which is 7,200 watts. A central air conditioner compressor can draw 40 to 60 amps at 240 volts, easily reaching 10,000 to 14,000 watts when running.

These numbers matter because they tell you whether a device will overload a circuit. A standard 15-amp household circuit is rated for 1,800 watts at 120 volts. If you plug in a 1,200-watt microwave and a 600-watt toaster on the same circuit, you've hit the limit. Add a third device and the breaker trips.

Three-Phase Power and AC Equipment

Most homes use single-phase power, so the straightforward formula works. But some commercial equipment and industrial machinery uses three-phase power, which requires a different calculation: Watts = Amps × Volts × Power Factor × √3. The √3 (square root of 3) equals about 1.73.

You'll encounter three-phase power mainly in commercial kitchens, workshops, or industrial settings. If you're working with a device that says "three-phase" on the nameplate, the formula above applies. For home use, ignore this section — you won't need it.

Why Amps Alone Aren't Enough

Amperage tells you how much current is flowing, but not how much work is being done. Think of it like water flowing through a pipe: amps are the volume of water, and volts are the pressure. A small volume at high pressure can do as much work as a large volume at low pressure. That's why you need both to know the total power.

This is also why a 240-volt circuit can power larger appliances than a 120-volt circuit. At the same amperage, doubling the voltage doubles the watts. A 20-amp circuit at 120 volts maxes out at 2,400 watts, but a 20-amp circuit at 240 volts can handle 4,800 watts. That's why your dryer and oven need their own 240-volt lines.

What to Do If You Only Have Amps

If a device lists only amperage and no voltage, you cannot calculate watts without measuring or finding the voltage elsewhere. Don't guess. Check the manual, the manufacturer's website, or the device label again — voltage is almost always listed somewhere.

If the device is already installed and you need to know the voltage, use a multimeter set to AC voltage mode and touch the probes to the outlet or terminals. If you're not comfortable doing that, ask an electrician or call the manufacturer's customer service line. They can tell you the voltage in minutes.

Frequently Asked Questions

Can I use this formula for DC power like batteries?

Yes. The formula Watts = Amps × Volts works for DC power too. A device drawing 5 amps from a 12-volt battery uses 60 watts. The math is identical; the only difference is that DC voltage is constant, while household AC voltage alternates 60 times per second.

What's the difference between watts and kilowatts?

A kilowatt is 1,000 watts. Your electric bill is usually measured in kilowatt-hours (kWh), which is how many kilowatts a device uses over one hour. A 1,200-watt microwave running for one hour uses 1.2 kilowatt-hours.

Do I need to know the power factor?

For most household devices, no. Power factor matters mainly for motors and fluorescent lights, where the actual power used is slightly less than amps × volts. For resistive loads like heaters, ovens, and incandescent bulbs, the formula is exact. If you're unsure, the device manual will list the wattage directly.

Why does my device label list watts but not amps?

Many devices list watts because that's what matters to the user — how much power it consumes. If you need amps, rearrange the formula: Amps = Watts ÷ Volts. A 1,200-watt microwave on 120 volts draws 10 amps.

Can I use this to figure out if a circuit will overload?

Yes. Add up the watts of all devices on the circuit and compare to the circuit's rating. A 15-amp, 120-volt circuit is rated for 1,800 watts. If your total devices exceed that, the breaker will trip. For 240-volt circuits, multiply amps by 240 instead.