What is power factor and when does it matter?

It is the ratio of real power to apparent power in an AC circuit. Resistive loads like heaters are close to 1.0, while motors and older fluorescent ballasts can be 0.7 to 0.9, meaning they draw more current than the wattage suggests.

Why does the same appliance draw fewer amps in Europe?

Because 230 volt supply carries the same power at roughly half the current of 120 volts. This is also why European kettles can be 3,000 watts on a normal socket.

Are volt-amps the same as watts?

Only in a purely resistive circuit. Volt-amps measure apparent power, which is what a UPS or generator must supply, while watts measure the real work done. UPS units are usually rated in VA for this reason.

Volts/Amps/Watts Calculator

Calculate volts, amps or watts from two knowns

Enter any 2 values to calculate the third (Ohm's Power Law: W = V × A)

Volts (V)

Amps (A)

Watts (W)

Quick Facts

•W = V × A
•US outlets: 120V, 15-20A
•EU outlets: 230V, 16A
•A 100W bulb at 120V draws 0.83A

Ohm's Power Law

The relationship between volts, amps, and watts is: Watts = Volts × Amps. This fundamental electrical equation helps you size circuits, choose breakers, and calculate energy consumption. A standard US outlet provides 120V at 15-20A (1,800-2,400W max).

Understanding the volts/amps/watts calculator

Volts, amps and watts describe three different things: pressure, flow and the work being done. Ohm's law and the power equation tie them together, so knowing any two gives you the third. This is the arithmetic behind sizing a breaker, checking whether a power strip is overloaded, and picking a power supply that will not run hot.

Worked examples

Amps drawn by a 1,500 watt heater on 120 volts

1Power equation: watts = volts × amps, so amps = watts ÷ volts.
21,500 ÷ 120 = 12.5 amps.
3A 15 amp circuit is rated for 12 amps continuous, which is 80 percent of capacity.
412.5 amps exceeds that, so the heater needs its own 20 amp circuit.

12.5 amps, which needs a dedicated 20 amp circuit for continuous use.

Watts on a three phase 400 V supply at 12 A

1Three phase power = √3 × volts × amps × power factor.
2√3 is 1.732. Assume a power factor of 0.9.
31.732 × 400 × 12 × 0.9 = 7,482 watts.

About 7.5 kW of real power.

Core formulas

Find	Formula
Watts	Volts × Amps
Amps	Watts ÷ Volts
Volts	Watts ÷ Amps
Ohms	Volts ÷ Amps
Watts from resistance	Amps² × Ohms
Three phase watts	1.732 × Volts × Amps × PF

Typical appliance loads

Appliance	Watts	Amps at 120 V	Amps at 230 V
LED bulb	9	0.08	0.04
Laptop	65	0.54	0.28
Refrigerator	150	1.25	0.65
Microwave	1,100	9.2	4.8
Space heater	1,500	12.5	6.5
Kettle	1,800	15.0	7.8
Clothes dryer	3,000	n/a	13.0
EV charger, level 2	7,200	n/a	31.3

Practical tips

•Size continuous loads at 80 percent of the breaker rating. A 15 amp breaker safely carries 12 amps for hours.
•Motors draw a starting surge of three to seven times their running current, which is why a compressor can trip a breaker that handles it fine once running.
•Never rely on a calculation alone for permanent wiring. Local electrical code sets conductor size and protection.

Frequently asked questions

More Electrical Tools

kW ↔ Horsepower

Convert between kilowatts and horsepower

Extension Cord Helper

Find the right gauge for your load

This calculator is for educational and reference purposes only. Always verify critical calculations independently.