Amps to Watts Calculator

Have watts and need amps? Use the watts to amps calculator.

Amps to watts in one step

An amps to watts calculator answers a simple question: how much real power does a given current deliver? Multiply the amps by the circuit voltage and by the load’s power factor. Watts = Amps × Volts × Power Factor. A 15 A load on a 120 V circuit with a power factor of 1 draws 1,800 W, or 1.8 kW.

Breakers, wire, and clamp meters all speak in amps. Appliance labels, generators, and your utility bill speak in watts. This tool translates between the two so you can compare a measured current with a rated wattage.

What each part of the formula does

• Amps are the rate of current flow. A clamp meter reading or a nameplate “FLA” value goes here.
• Volts set how much push each amp carries. In a North American home, most outlets are 120 V and large appliances such as dryers, ranges, and well pumps run at 240 V.
• Power factor (PF) is the share of the current that does real work. A heater turns all of it into heat, so PF is 1. A motor stores and returns some energy each cycle, so its real watts are lower than amps × volts. The difference shows up as volt-amps (VA), the figure UPS and transformer ratings use.

The calculator returns watts and kilowatts. Kilowatts are watts ÷ 1,000.

Worked examples

Example 1: Space heater on a kitchen circuit, 15 A, 120 V, PF 1

• Power: 15 × 120 × 1 = 1,800 W
• In kilowatts: 1,800 ÷ 1,000 = 1.80 kW

Example 2: Well pump measured with a clamp meter, 12 A, 240 V, PF 0.85

• Apparent power: 12 × 240 = 2,880 VA
• Real power: 2,880 × 0.85 = 2,448 W
• In kilowatts: 2.45 kW

The second example shows why power factor matters. Ignoring it would overstate the pump’s real draw by 432 W, which changes energy-cost estimates.

Amps to watts chart (power factor 1)

Amps	Watts at 120 V	Watts at 240 V
5 A	600 W	1,200 W
10 A	1,200 W	2,400 W
15 A	1,800 W	3,600 W
20 A	2,400 W	4,800 W
30 A	3,600 W	7,200 W
40 A	4,800 W	9,600 W
50 A	6,000 W	12,000 W

Circuit capacity at the 80% continuous-load rule

Loads that run for three hours or more are treated as continuous, and the common practice is to keep them at or below 80% of the breaker rating.

Breaker and voltage	Full rating	80% continuous limit
15 A, 120 V	1,800 W	1,440 W
20 A, 120 V	2,400 W	1,920 W
30 A, 240 V	7,200 W	5,760 W
50 A, 240 V	12,000 W	9,600 W

This is why a 1,500 W space heater is the practical maximum on a typical 15 A bedroom circuit.

How to get accurate inputs

1. Read the nameplate first. Look for amps, volts, and sometimes PF or “VA”. If both watts and VA are printed, PF = watts ÷ VA.
2. Measure the real current. A clamp meter around one hot conductor gives running amps. Start-up surge on motors can be several times higher for a moment, so read it once the motor is up to speed.
3. Confirm the voltage. Use the actual circuit voltage. Commercial buildings often have 208 V rather than 240 V between legs.
4. Pick the power factor. Use 1 for heating elements and incandescent lighting. Use the nameplate figure for motors and compressors.

Common mistakes

• Using 240 V for a 120 V device. That doubles the answer.
• Ignoring power factor on motors. Amps × volts gives VA, not watts. Size a UPS by VA and an energy bill by watts.
• Confusing running and starting current. A generator must cover the start-up surge even though the running watts are lower. The generator size calculator accounts for that.
• Applying the single-phase formula to three-phase loads. Multiply by 1.732 for balanced three-phase power.

Related calculators

Going the other way? The watts to amps calculator reverses this formula. The breaker size calculator turns a load into a breaker rating, and the electricity cost calculator prices the watts by the hour.

Electrical work is code-governed and safety-critical. Confirm circuit, breaker, and conductor sizing with a licensed electrician and your local code.