WireWorks

Wattage Calculator

The most basic electrical formula — volts times amps equals watts.

Voltage (V)

Current (A)

Watts

1,200 W

P = V × I is the basic power formula for a purely resistive load — motors and other reactive loads need a power factor adjustment.

What you entered

Voltage (V): 120Current (A): 10

1
Apply P = V × I
120 × 10= 1200 W

Result

Watts: 1,200 W

120 V at 10 A draws 1200 watts of power.

Watts from volts and amps

Power is voltage times current. This is the calculation behind every circuit and appliance rating.

Voltage	Current	Watts	Typical use
12 V	10 A	120 W	Car or RV accessory circuit
120 V	5 A	600 W	Small appliance
120 V	10 A	1,200 W	Microwave or toaster
120 V	15 A	1,800 W	A full 15 A circuit
120 V	20 A	2,400 W	A full 20 A kitchen circuit
240 V	30 A	7,200 W	Electric dryer
240 V	50 A	12,000 W	Range or EV charger

Circuits should be loaded to at most 80% continuously, so a 15 A circuit carries 1,440 W of continuous load rather than 1,800 W. Electrical work is life-safety work and is subject to local code and inspection. Use these figures for planning and have a licensed electrician verify anything you intend to build.

This assumes a resistive load

P = V × I is exact for purely resistive loads (like a heater or incandescent bulb). Motors and other inductive loads have a 'power factor' below 1, meaning actual real power is somewhat less than volts × amps suggests.

Why this matters for circuit planning

Knowing an appliance's wattage lets you check it against a circuit's capacity (amps × voltage) or a generator's rated output — always leave headroom rather than loading a circuit to its exact rated maximum.

Frequently asked questions

My space heater draws 12.5 amps on a 120V outlet — how many watts is that?

1,500 watts (12.5 x 120 = 1,500W). That's the maximum for a standard 15-amp household circuit after accounting for the 80% continuous-load rule, which is why most space heaters are rated at exactly 1,500W.

How do I calculate watts from volts and amps?

Multiply volts by amps: W = V x A. A 120V circuit drawing 10 amps uses 1,200 watts. For 240V circuits (dryers, ovens), the same 10 amps delivers 2,400 watts — double the power from the same current.

Can I plug two 1,500W devices into the same circuit?

Not safely on a standard 15-amp circuit. 3,000W at 120V draws 25 amps, far exceeding the 15-amp breaker. Even a 20-amp circuit (2,400W max continuous) can't handle it. Use separate circuits for high-wattage devices.

How does this relate to the wire size and voltage drop calculators?

This tells you the power draw. The wire size calculator ensures the wire can carry that current safely. The voltage drop calculator checks whether the wire run length causes unacceptable voltage loss — all three work together for circuit planning.