Amps to Watts Calculator
Convert electrical current in amps to power in watts and kilowatts for DC, single-phase AC, and three-phase AC circuits.
Calculator Inputs
Power Result
How to use this calculator
- Pick your circuit configuration. Select whether the power supply is DC, single-phase AC, or balanced three-phase AC.
- Input the current and voltage. Enter the measured or rated current in amps (A) and the supply voltage in volts (V).
- Specify the power factor for AC. For alternating current, enter the equipment power factor between 0.1 and 1.0 (enter 1.0 for purely resistive loads).
- Review the power output. The calculator computes the real power in both watts (W) and kilowatts (kW) instantly.
Check that your voltage reading corresponds to the circuit type: enter line-to-line voltage for three-phase circuits. If you need to convert in the opposite direction, use our watts to amps calculator, or see our kVA to kW calculator to evaluate apparent power.
Formulas
The formula for calculating electrical power depends on whether the circuit carries direct current or alternating current:
| System | Formula | Description |
|---|---|---|
| DC | P = V × I | Voltage multiplied by current |
| Single phase AC | P = V × I × PF | Voltage multiplied by current and power factor |
| Three phase AC | P = √3 × V × I × PF | √3 times line-to-line voltage times current and power factor |
Common mistakes
- Omitting the power factor on AC motors, which causes an overestimation of real power.
- Using line-to-neutral voltage instead of line-to-line voltage when calculating three-phase power.
- Confusing real power in watts with total apparent power in volt-amps.
- Assuming peak current instead of RMS current when working with AC measurements.
Worked Example
1. Single-phase branch circuit
A continuous load draws 10 A from a 230 V single-phase circuit at unity power factor (PF = 1.0).
- Formula: P = V × I × PF
- Calculation: P = 230 × 10 × 1.0 = 2,300 W (2.3 kW)
2. Three-phase industrial feeder
A commercial air-conditioning unit draws 20 A per phase at 400 V line-to-line with a power factor of 0.85.
- Formula: P = √3 × V × I × PF
- Calculation: P = 1.73205 × 400 × 20 × 0.85 ≈ 11,778 W (11.78 kW)
3. Small DC device
An auxiliary 12 V DC pump draws 5 A of continuous current.
- Formula: P = V × I
- Calculation: P = 12 × 5 = 60 W (0.06 kW)
After determining the power and current of your installation, check the cable size with our voltage drop calculator to make sure the run does not exceed recommended voltage drop limits.
Frequently Asked Questions
Why can I not convert amps to watts directly without voltage?
Amps measure electrical current flow, whereas watts measure the rate of energy consumption. Without the electrical potential (voltage) driving the current, the total power cannot be determined.
How do I calculate watts for a single-phase AC circuit?
Multiply current by voltage and the load's power factor: P = V × I × PF. For a purely resistive load where PF is 1.0, power is simply volts multiplied by amps.
What is the difference between watts (W) and kilowatts (kW)?
One kilowatt equals 1,000 watts. To convert calculated watts into kilowatts, divide the wattage by 1,000. For instance, 2,300 W equals 2.3 kW.
Where does the square root of 3 come from in three-phase power?
In a balanced three-phase system using line-to-line voltage, the phase conductors are 120 degrees apart. The factor √3 (approximately 1.732) accounts for the geometric sum of the line voltages.
Does direct current have a power factor?
No. In DC systems, current and voltage do not oscillate, so there is no phase angle. The power factor is effectively 1.0, and power is always P = V × I.
How does circuit power relate to generator or transformer sizing?
Generators and transformers are usually rated in apparent power (kVA). If you need to size a power supply for a known wattage, divide the real power in kW by the power factor to find the required kVA.
Disclaimer
This calculator is provided for educational and estimation purposes only. Always consult a licensed electrical professional and adhere to your local electrical codes and standards before making any decisions based on these calculations.
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