How do I convert watts to amps at 120V?

Divide watts by 120. A 1,200W toaster draws 10A. A 60W light bulb draws 0.5A. This works perfectly for resistive loads like heaters and lights. For motor-driven appliances (fridges, A/C units, power tools), multiply the amps result by about 1.15-1.25 to account for power factor. Or enter the power factor in the calculator for an exact result.

Why does a 240V appliance draw fewer amps than a 120V appliance of the same wattage?

Power equals voltage times current. Double the voltage means half the current for the same power. A 5,000W dryer at 240V draws 20.8A, but the same power at 120V would need 41.7A — which is impractical for household wiring. This is why high-power appliances (dryers, ovens, EV chargers) use 240V circuits. It keeps the current manageable and allows smaller wire gauges.

What is the 80% rule for circuit loading?

NEC requires that continuous loads (running 3+ hours) use no more than 80% of the circuit breaker rating. A 15A breaker should carry no more than 12A continuous. A 20A breaker tops out at 16A continuous. This margin protects the wiring from heat buildup during extended use. Intermittent loads like a vacuum cleaner can briefly draw more, but anything running for hours — space heaters, grow lights, servers — must stay under 80%.

Amps Draw Calculator — Free | VoltCalcs

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Find out how many amps a device draws from its wattage and voltage. This is the most basic electrical calculation you will ever need — and the one that prevents tripped breakers, blown fuses, and overloaded circuits.

The Relationship Between Watts, Volts, and Amps

Three quantities define every electrical circuit: voltage (V), current (A), and power (W). They are linked by a simple formula:

Amps = Watts / Volts

A 1,800W hair dryer at 120V draws 15A. That same hair dryer on a 240V circuit (common in Europe and Australia) would draw only 7.5A. The device does the same amount of work either way — higher voltage simply means less current is needed to deliver the same power.

This formula assumes a power factor of 1.0, which is true for purely resistive loads: heaters, incandescent lights, toasters, and hair dryers. These devices convert all their electrical energy directly to heat or light.

Motors, fluorescent ballasts, and power supplies have a power factor below 1.0, typically between 0.75 and 0.95. They draw more current than the simple formula predicts because some of that current oscillates back and forth without doing useful work. For a 1,000W motor with a 0.80 power factor at 120V, the actual current is 1,000 / (120 x 0.80) = 10.4A instead of the 8.3A you would expect from watts and volts alone.

For three-phase systems, the formula becomes: Amps = Watts / (Volts x 1.732 x Power Factor). The 1.732 factor accounts for the three-phase voltage relationship. Three-phase loads draw less current per conductor than single-phase loads of the same power — one reason commercial and industrial buildings use three-phase power.

Amp Draw of Common Household Devices

Device	Watts	Voltage	Amps
Hair dryer (high)	1,800	120	15.0
Microwave	1,500	120	12.5
Space heater	1,500	120	12.5
Toaster	1,200	120	10.0
Vacuum cleaner	1,000	120	8.3
Coffee maker	900	120	7.5
Window A/C (8k BTU)	800	120	6.7
Refrigerator (running)	150	120	1.3
LED TV (55")	80	120	0.7
LED bulb	10	120	0.08
EV charger (Level 2)	7,200	240	30.0
Electric dryer	5,000	240	20.8
Electric oven	4,000	240	16.7

Devices over 1,800W at 120V need a dedicated circuit. A standard 15A kitchen outlet already maxes out with a single hair dryer. Plug in a microwave at the same time and the breaker trips instantly — that is not a faulty breaker, it is your wiring doing its job.

Example: How Many Devices Can Share One Circuit?

A standard US bedroom circuit is 15A at 120V. NEC says you should load it to no more than 80% for continuous use — that is 12A or 1,440W.

Say you plug in a desktop PC (300W), a monitor (50W), a desk lamp (10W LED), and a phone charger (20W). That totals 380W, or about 3.2A. You are at 21% of the circuit's safe capacity, with plenty of room for an additional device like a laser printer (600W at peak).

Now try a different room. A space heater (1,500W / 12.5A) on a 15A circuit uses 83% of the safe capacity by itself. Add a table lamp and phone charger and you are fine. Add a vacuum cleaner and the breaker trips. This is why kitchens and bathrooms have 20A circuits — high-draw appliances need the extra headroom.

If you are adding up multiple loads for a whole panel, our electrical load calculator handles demand factors and gives you the total service size needed.

Worked Examples

Can a 15A Kitchen Circuit Handle a Microwave and Toaster?

Context

Your kitchen has a single 15A, 120V circuit. You want to run a 1,200W microwave and an 800W toaster at the same time.

Calculation

Microwave: 1,200 / 120 = 10.0 A

Toaster: 800 / 120 = 6.7 A

Combined: 10.0 + 6.7 = 16.7 A

Interpretation

16.7A exceeds the 15A breaker rating. Running both simultaneously will trip the breaker. NEC also limits continuous loading to 80% of breaker capacity (12A), so even the microwave alone pushes the limit.

Takeaway

If your kitchen trips breakers regularly, you likely need a dedicated 20A circuit. Use our electrical load calculator to add up all the appliances on that circuit and see the total draw.

Three-Phase Current for an Industrial Heater Bank

Context

A factory has a 15 kW resistance heater bank on a 480V three-phase circuit with a power factor of 1.0 (purely resistive load).

Calculation

I = 15,000 / (480 x 1.732 x 1.0)

= 15,000 / 831.4

= 18.0 A per phase

Interpretation

At 18A, this load fits on a 30A three-phase breaker with room to spare. The unity power factor makes this straightforward — motors with lower power factor would draw more current for the same kilowattage.

Takeaway

For motor loads on this same panel, the power factor drops below 1.0 and current increases. Use our motor FLA calculator to get accurate current draw for each motor.

Frequently Asked Questions

Glossary

Current Draw

The amount of electric current (in amps) a device pulls from the circuit. Higher wattage devices draw more current at the same voltage. Exceeding a circuit's current capacity trips breakers or blows fuses.

Power Factor

For resistive loads like heaters and incandescent bulbs, power factor is 1.0. For motor and electronic loads, it drops below 1.0, meaning the device draws more current than a simple wattage-to-amps conversion would suggest.

Circuit Capacity

The maximum continuous current a circuit can safely carry, determined by the wire gauge and breaker rating. NEC limits continuous loads to 80% of the breaker rating to prevent overheating.

Running these devices from a battery system? Our battery runtime calculator shows how long a given battery bank lasts under any load. Try it now →

Knowing the amp draw of your devices is the foundation of safe electrical practice. Before plugging in a new high-draw appliance, check the circuit breaker rating and add up what else is on that circuit. Overloaded circuits are the most common cause of tripped breakers and one of the leading causes of residential electrical fires. The values from this calculator are theoretical — always check the device nameplate for the manufacturer's rated current draw.

Last updated: March 23, 2026

Written and maintained by Dan Dadovic, Developer & Off-Grid Energy Enthusiast. On the energy side, Dan has hands-on experience with residential solar panel installation, DIY battery bank construction, off-grid power systems, and wind power — all from building and maintaining his own systems..

Disclaimer: Calculator results are estimates based on theoretical formulas. Actual performance varies with temperature, battery age, load patterns, and equipment condition. For critical electrical work, consult a licensed electrician.