Amps to kW calculator
How the formula works
The formulas depend on the supply type. Use the line to line voltage for three phase.
Amps to kW
Three phase: kW = √3 × V × I × pf / 1000
Single phase: kW = V × I × pf / 1000
DC: kW = V × I / 1000
kW to Amps
Three phase: I = kW × 1000 / (√3 × V × pf)
Single phase: I = kW × 1000 / (V × pf)
DC: I = kW × 1000 / V
Other values
kVA = kW / pf
kVAr = √(kVA² − kW²)
Motor input kW = Output kW / Efficiency
1 HP = 0.7457 kW
Where:
- V = voltage (line to line for three phase)
- I = current in amps
- pf = power factor (use 1 for DC)
Typical values
Power factor: heaters and lighting 0.95 to 1.0, induction motors 0.8 to 0.9, lightly loaded motors 0.7 or lower. Motor efficiency: 85 to 95 %. Voltages: 230 V single phase, 400 or 415 V three phase. These are typical values. The motor nameplate and the local code are final.
How to use the calculator: select the calculation and supply type, choose the voltage and power factor, enter the current or power, then click Calculate.
Important notes: this gives the electrical load only. For cable and breaker sizing, use the Cable Size Calculator.
Worked example
Example 1: Three phase feeder, amps to kW
415 V, 100 A, pf 0.85.
kVA = 1.732 × 415 × 100 / 1000 = 71.88 kVA. kW = 71.88 × 0.85 = 61.10 kW. kVAr = 37.87. Power = 81.93 HP.
Example 2: Single phase load, amps to kW
230 V, 20 A, pf 0.9.
kW = 230 × 20 × 0.9 / 1000 = 4.14 kW. kVA = 4.60. kVAr = 2.01.
Example 3: kW to amps, three phase
75 kW at 415 V, pf 0.9.
I = 75 × 1000 / (1.732 × 415 × 0.9) = 115.93 A. kVA = 83.33. kVAr = 36.32.
Example 4: 37 kW motor with efficiency
400 V three phase, 37 kW output, pf 0.88, efficiency 92 %.
Input power = 37 / 0.92 = 40.22 kW. I = 40.22 × 1000 / (1.732 × 400 × 0.88) = 65.96 A. kVA = 45.70. kVAr = 21.71.
Example 5: DC load
48 V, 50 A.
kW = 48 × 50 / 1000 = 2.40 kW, or 3.22 HP.
Example 6: Motor current, amps to shaft kW
440 V three phase, 60 A, pf 0.85, efficiency 90 %.
Input = 38.87 kW. Output (shaft) = 38.87 × 0.90 = 34.98 kW, or 46.91 HP.
Common mistakes
- Using the phase voltage instead of the line to line voltage for three phase.
- Forgetting the power factor and getting kVA instead of kW.
- Using the single phase formula for a three phase load.
- Mixing up kW and kVA when sizing a generator or transformer.
- Ignoring motor efficiency, so the input current is too low.
- Using the motor’s full load pf for a lightly loaded motor.
- Confusing HP (0.7457 kW) with metric HP (0.7355 kW).
- Using the calculated current directly as the breaker rating, with no margin.
- Applying the AC formula to a DC supply.
Frequently asked questions
How do I convert amps to kW?
For three phase, kW = √3 × V × I × pf / 1000. For single phase, kW = V × I × pf / 1000.
How do I convert kW to amps?
For three phase, I = kW × 1000 / (√3 × V × pf). For single phase, I = kW × 1000 / (V × pf).
How many amps is 1 kW at 230 V?
At pf 1.0 it is about 4.35 A. At pf 0.9 it is about 4.83 A.
How many amps is 1 kW at 415 V three phase?
At pf 0.9 it is about 1.55 A.
What is the difference between kW and kVA?
kW is real power. kVA is apparent power. kW = kVA × power factor.
Which voltage do I use for three phase?
Use the line to line voltage, for example 415 V or 400 V.
What power factor should I use?
Use the real value if known. Induction motors are typically 0.8 to 0.9. Heaters are close to 1.0.
Why add motor efficiency?
A motor draws more electrical power than its shaft output. The difference is lost as heat.
How many kW is 1 HP?
One HP is 0.7457 kW. For metric HP it is 0.7355 kW.
What is kVAr?
kVAr is reactive power. It is found from kVA and kW and is used for power factor correction.
Can I use this for DC?
Yes. Select DC and the calculator uses kW = V × I / 1000. Use the Custom voltage option for 12, 24 or 48 V.
Which cable size do I need for this current?
Use the Cable Size Calculator with the current from this page.