30 Amps at 120 Volts to Watts: Power for a Resistive Load

Watts from any two electrical readings

A space heater pulling 12.5 A from a 120 V outlet turns 1.5 kW into heat, and two hours of that every day comes to 3 kWh. This calculator finds power from voltage and current, current and resistance, or voltage and resistance. For alternating current it also separates the watts the meter bills from the volt-amperes the wiring carries, on single-phase and three-phase supplies, and with hours of use and a price per kWh it adds the energy and the cost.

Five choices on the form

What you know - voltage and current (DC or a resistive load), voltage, current and power factor on single-phase AC, the same on three-phase AC, current and resistance, or voltage and resistance.
Voltage - in volts. A US outlet is 120 V, a dryer circuit 240 V, USB 5 V. On three-phase use the line-to-line figure, such as 208 V or 480 V.
Current or resistance - amps from a nameplate or a clamp meter, ohms from a multimeter across a heating element.
Power factor - AC modes only, a number from 0 to 1. Type 1 for heaters and incandescent bulbs, or the value printed on a motor nameplate.
Hours per day and price per kWh - both optional. Hours give kWh per day, month and year; the price adds dollars.

Three supplies compared

The same pair of readings means different things depending on the supply. On DC and on a purely resistive AC load, volts times amps is the answer. With motors and electronics the power factor tells you how much of the current does work.

Question	DC or resistive load	Single-phase AC	Three-phase AC
Real power	V × I	V × I × PF	√3 × V × I × PF
Apparent power	same as real power	V × I, in VA	√3 × V × I, in VA
Which voltage to enter	across the load	RMS, for example 120 V or 240 V	line to line, for example 208 V or 480 V
Pick this mode when	there is no motor or electronics, or the supply is a battery	the nameplate lists a power factor or a VA rating	the equipment is fed by three hot wires

The formulas, with three sets of numbers

Power is the rate at which energy moves: P = V × I. Ohm's law, V = I × R, turns it into P = I² × R and P = V² ÷ R, which is why any two of the three readings are enough. Energy is power multiplied by time, so a 1 kW load running for one hour uses 1 kWh.

1

Heater element, voltage and resistance.
An element measuring 9.6 Ω on a 120 V outlet: 120² ÷ 9.6 = 1,500 W, and the current is 120 ÷ 9.6 = 12.5 A. At 2 h a day that is 3 kWh daily, 1,095 kWh a year, and at $0.17 per kWh $186.15 a year.

2

A motor on single-phase AC.
120 V and 10 A give 1,200 VA of apparent power. With a power factor of 0.8 the real power is 960 W, and the reactive part is √(1,200² − 960²) = 720 var.

3

Three-phase equipment.
480 V line to line, 20 A per line, power factor 0.9: 1.7321 × 480 × 20 = 16.628 kVA, times 0.9 = 14.965 kW. Leaving out the √3 would report 8.64 kW, about 42% too little.

One wattage, two outlet voltages

For a resistive load the same wattage needs half the current on a 240 V circuit and four times the resistance.

Power	Current at 120 V	Resistance at 120 V	Current at 240 V	Resistance at 240 V
60 W	0.5 A	240 Ω	0.25 A	960 Ω
100 W	0.833 A	144 Ω	0.417 A	576 Ω
600 W	5 A	24 Ω	2.5 A	96 Ω
1,000 W	8.333 A	14.4 Ω	4.167 A	57.6 Ω
1,500 W	12.5 A	9.6 Ω	6.25 A	38.4 Ω
3,000 W	25 A	4.8 Ω	12.5 A	19.2 Ω
4,800 W	40 A	3 Ω	20 A	12 Ω

Where power estimates go wrong

Volts times amps on a motor. On AC with a power factor below 1, V × I is apparent power in VA, not watts. Multiply by the power factor for the figure the meter records.

Line-to-neutral voltage on a three-phase formula. The √3 version expects the line-to-line voltage. On a 208 V system the line-to-neutral reading is 120 V.

Watts treated as energy. The bill counts kWh, so hours matter as much as the rating.

A month of 30 days. Twelve such months add up to 360 days, so the monthly and yearly figures stop agreeing. The calculator uses 365 ÷ 12 days, so 91.25 kWh × 12 = 1,095 kWh.

Asked about watts, VA and kWh

How do I calculate watts from volts and amps?

Multiply them. 120 V × 12.5 A = 1,500 W. On AC with motors or electronics, also multiply by the power factor.

What is the difference between W and VA?

Watts are real power, the part that does work; volt-amperes are voltage times current. Their ratio is the power factor, so 960 W at 1,200 VA is a power factor of 0.8.

Why does resistance give power without a current?

Because Ohm's law fixes the current once voltage and resistance are known. 120 V across 9.6 Ω must drive 12.5 A, and 120² ÷ 9.6 skips that step to reach 1,500 W directly.

Where does the √3 in three-phase power come from?

A balanced three-phase load takes three times the power of one phase, and the line-to-line voltage is √3 times the phase voltage. Written with line-to-line voltage, 3 × V_phase becomes √3 × V_line, about 1.7321 × V.

Does a low power factor raise a home electricity bill?

A home meter records real energy in kWh, so the bill follows the watts. Utilities usually charge extra for a low power factor only on commercial and industrial accounts.

Can I use it for a battery or a USB charger?

Yes, with the voltage and current mode. A USB supply delivering 3 A at 5 V provides 15 W.

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How many watts is 30 amps at 120 volts?

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