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Electrical Power Calculator

Convert between watts, amps, volts and kVA for DC, single-phase AC and three-phase AC circuits.

Level: Grades 11–12

Printable worksheet

Electrical Power Calculator

Know the answer you need? Pick an input to work backward.

Inputs
Circuit type

For three-phase, the line-to-line voltage (for example 208, 400 or 480 V).

Between 0 and 1. Use 1 for resistive loads such as heaters; motors are often 0.8 to 0.9.

Result

Current

12.5 A

Current is 12.5 A (single-phase AC, power factor 1).

Real power
1,500 W
Real power in kilowatts
1.5 kW
Apparent power
1.5 kVA
Voltage
120 V
Reactive power
0 kVAR

See the step-by-step solution ↓

Step-by-step solution

  1. Turn P = V × I × PF around to get the current: divide the watts by V × PF

    I=1,500120×1=12.5 A

  2. Apparent power, the total load the supply carries: multiply volts × amps, then divide by 1000 for kVA

    S=120×12.51,000=1.5 kVA

Test yourself

20 quick questions built from this calculator's formula: work out results, predict what happens when an input changes, and work backward from an answer. Every answer comes with the working.

What this calculator does

Switch between watts, amps, volts and kVA for any electrical load. These are four views of the same thing. Watts measure the real power that does useful work. Amps measure the current in the wires, and volts the electrical push behind it. kVA (thousands of volt-amps) is the apparent power, the total load the supply and cables really have to carry. The calculator works for direct current (DC), ordinary single-phase AC, and balanced three-phase AC.

You might use it to find how many amps an appliance or motor draws from its wattage. Or you might check how much load a generator or transformer rated in kVA can handle. You can also work back from a measured current to the power being used. For AC circuits, the power factor is included. That's because it changes how much current a given number of watts needs.

Where it's used

Why this math matters outside the classroom:

  • Circuit breakers. Electricians check that a 1,800 W heater on a 120 V circuit draws 15 A, the full rating of a 15 A breaker.
  • Backup generators. Homeowners add up the watts of the appliances they need in an outage to pick a generator big enough.
  • Three-phase motors. Plant engineers find a 400 V three-phase motor drawing 20 A at power factor 0.85 uses about √3 × 400 × 20 × 0.85 ≈ 11.8 kW.
  • Transformer sizing. Facility managers convert a load of 80 kW at power factor 0.8 into 100 kVA when sizing a transformer.

How to use it

  1. Choose the circuit type: DC, single-phase AC, or three-phase AC.
  2. Choose what to work out: amps from watts, watts from amps, amps from kVA, or volts from watts.
  3. Enter the values you know. For three-phase, use the voltage between two lines (line-to-line) and the current in one line.
  4. For AC, enter the power factor. Use 1 for heaters and old-style bulbs; motors and many electronic loads are lower. Then read the answer and the related figures below it.

What the inputs mean

Real power
Real power in watts or kilowatts, the figure on most appliance labels. For motors, the label usually shows the power the motor puts out. The electricity it takes in is higher, because some is lost as heat.
Apparent power
Apparent power in kVA, as printed on generators, transformers and UPS units (battery backups). It already includes the effect of power factor, so amps follow straight from it.
Voltage
For three-phase systems, enter the voltage between any two lines, such as 208, 400 or 480 V. Don't use the line-to-neutral voltage. Common single-phase supplies are 120 or 240 V in North America and 230 V in much of the world.
Current
For three-phase, the current in each line.
Power factor
How much of the apparent power does real work, as a number from 0 to 1. It's 1 for loads that just turn electricity into heat, like heaters. If you don't know it, check the nameplate. 0.8 is a common planning figure for motors and generators.

Formula and method

DC:            P = V × I
Single-phase:  P = V × I × PF,        S = V × I
Three-phase:   P = √3 × V × I × PF,   S = √3 × V × I
Amps from watts:  I = P ÷ (k × V × PF)
Amps from kVA:    I = 1000 × S ÷ (k × V)
k = 1 for DC and single-phase, √3 ≈ 1.732 for three-phase
Reactive power:   Q = √(S² − P²)

In the formulas, P is real power in watts, V is voltage and I is current in amps. PF is the power factor, and S is apparent power. In DC circuits, power is just voltage times current. AC circuits with motors, transformers or electronic power supplies are different. There, the current runs a little out of step with the voltage, or gets distorted. So only part of V × I does useful work. That share is the power factor. V × I itself is the apparent power S, measured in volt-amperes (VA) or kVA.

A balanced three-phase system sends power down three lines. Their voltages peak at different times, 120° apart in the cycle. Using the line-to-line voltage, total apparent power is √3 × V × I, where I is the current in each line. The calculator rearranges whichever formula it needs and reports the other quantities. That includes the reactive power Q, power that flows back and forth without doing any work.

Worked example

A three-phase motor load draws 10 kW of real power at 480 V line-to-line, with a power factor of 0.85. How much current flows in each line?

  1. Convert to watts: 10 kW × 1000 = 10,000 W.
  2. Current: I = 10,000 ÷ (√3 × 480 × 0.85) = 14.1507 A.
  3. Apparent power: S = √3 × 480 × 14.1507 ÷ 1000 = 11.7647 kVA.
  4. Reactive power: Q = √(11.7647² − 10²) ≈ 6.1974 kVAR.

Result: Each line carries about 14.15 A, and the supply sees 11.76 kVA of apparent power.

Understanding the result

Use the current to choose fuses, breakers and wire sizes. But follow your local electrical code and the equipment's nameplate. Codes often require extra margin for motors and for loads that run for hours, which this calculator doesn't add.

The kVA figure is what matters when sizing a generator, transformer or UPS. They're limited by current and heat, not by real watts alone. A low power factor means more kVA, and more current, for the same useful power.

On three-phase, the same power needs far less current per wire than on single-phase at the same voltage. That's why large loads use it.

Assumptions and limitations

  • Voltages and currents are RMS values (the usual meter readings) of steady, smooth-wave AC, or steady DC.
  • Three-phase loads are balanced, with equal current in each line.
  • The power factor you enter applies to the whole load.
  • Unbalanced three-phase loads need each phase worked out separately.
  • A motor can draw several times its running current while starting. That isn't modeled.
  • These are estimates for planning and learning. They are not a substitute for your local electrical code or a licensed electrician.

Frequently asked questions

How many amps is 1500 watts?

At 120 V with a power factor of 1, such as a space heater, 1500 ÷ 120 = 12.5 A. At 230 V it's about 6.52 A. A lower power factor raises the current for the same wattage.

What is the difference between kW and kVA?

kW is real power, the part that becomes heat, light or motion. kVA is apparent power, voltage times current. They're equal only when the power factor is 1. Otherwise kW = kVA × PF.

Why does three-phase use √3?

In a three-phase system, the voltage between two lines is √3 times the voltage from each line to neutral. That's because the phases are 120° apart. When you write the total power of all three phases using line voltage and line current, a factor of √3 ≈ 1.732 is left over.

Can I convert watts to amps without knowing the voltage?

No. Power is voltage times current (times power factor for AC). So the same wattage means very different currents at 12 V, 120 V or 480 V. You need the voltage to get amps.

How this calculator is checked

Method: Standard power relations: P = VI (DC), P = VI·PF (single-phase AC), P = √3·V_LL·I·PF (balanced three-phase AC), S = P ÷ PF. The calculation is covered by automated tests with known reference values and edge cases. Last updated 2026-10-18. Found a problem? Tell us.

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