Watts Calculator

Common Examples

Input	Result
120 volts, 15 amps	1,800 watts
240 volts, 30 amps	7,200 watts
1,500 watts, 120 volts	12.50 amps
100 watts, 0.83 amps	120.48 volts
12 volts, 5 amps	60 watts

How It Works

The Formula

The relationship between watts, volts, and amps is defined by Ohm’s Power Law:

P = V x I

Where:

P = power in watts (W)
V = voltage in volts (V)
I = current in amperes/amps (A)

This formula can be rearranged to solve for any of the three values:

Watts = Volts x Amps (when you know voltage and current)
Amps = Watts / Volts (when you know power and voltage)
Volts = Watts / Amps (when you know power and current)

Understanding Watts

A watt is the standard unit of electrical power. It represents the rate at which electrical energy is consumed or produced. Common household appliances range from about 10 watts (LED bulb) to 5,000 watts (electric water heater). The wattage rating on an appliance indicates how much power it draws during normal operation.

Common Household Circuits

In the United States, most household outlets provide 120 volts. A standard 15-amp circuit can handle up to 1,800 watts (120 x 15). A 20-amp circuit handles up to 2,400 watts. Large appliances like dryers and ovens typically run on 240-volt circuits, which allows higher wattage at lower amperage.

Practical Application

Knowing the relationship between watts, volts, and amps helps determine whether a circuit can handle a particular load. For example, plugging a 1,500-watt space heater into a 120-volt, 15-amp circuit draws 12.5 amps, leaving only 2.5 amps of capacity for other devices on that circuit. The National Electrical Code recommends loading circuits to no more than 80% of their rated capacity for continuous loads.

Worked Example

For a 120-volt circuit drawing 15 amps: Watts = 120 x 15 = 1,800 watts. To find amps for a 1,500-watt heater on 120 volts: Amps = 1,500 / 120 = 12.5 amps. To find the voltage needed to power a 100-watt device drawing 0.83 amps: Volts = 100 / 0.83 = 120.48 volts. For a 240-volt dryer circuit drawing 30 amps: Watts = 240 x 30 = 7,200 watts. This is why high-power appliances use 240-volt circuits; they can deliver more watts without requiring extremely high amperage.

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Frequently Asked Questions

What is the difference between watts, volts, and amps?

Volts measure electrical pressure (how strongly current is pushed through a wire). Amps measure the flow rate of electric current (how much charge passes per second). Watts measure power (the rate at which energy is used or produced). The relationship is Watts = Volts x Amps. A common analogy is water in a pipe: volts are the water pressure, amps are the flow rate, and watts are the total power delivered.

How many watts can a 15-amp circuit handle?

A 15-amp circuit at 120 volts can handle a maximum of 1,800 watts (120 x 15 = 1,800). However, the National Electrical Code recommends not exceeding 80% of the circuit's capacity for continuous loads, which is 1,440 watts. For 20-amp circuits, the maximum is 2,400 watts, with a recommended continuous load of 1,920 watts.

How do I calculate the amperage of an appliance?

Divide the appliance's wattage by the circuit voltage. For example, a 1,500-watt space heater on a 120-volt circuit draws 1,500 / 120 = 12.5 amps. The wattage is usually listed on the appliance's nameplate or in its specifications.

Does this formula work for AC and DC circuits?

This formula (P = V x I) works directly for DC circuits and for purely resistive AC loads like heaters and incandescent bulbs. For AC circuits with motors, transformers, or electronic power supplies, the actual power consumed may differ due to the power factor. In those cases, the formula becomes P = V x I x power factor, where the power factor is a value between 0 and 1.

Why do some appliances use 240 volts instead of 120 volts?

Higher voltage allows the same amount of power with lower amperage. A 7,200-watt dryer on 240 volts draws 30 amps, but on 120 volts it would draw 60 amps, requiring much thicker wiring and a larger breaker. Using 240 volts for high-power appliances is more practical and cost-effective.