Amps To Watts Calculator

Examples & FAQs)

If you work with electrical loads—whether you’re sizing power supplies, planning solar systems, or checking equipment labels—you’ll often know current (amps) and need real power (watts). That’s exactly what the Amps to Watts Calculator does: it converts current to power using the correct formula for DC, single-phase AC, or three-phase AC, with voltage and (for AC) power factor.

This guide explains how the calculator works, which formula to use, how to enter values, and common pitfalls. You’ll also find worked examples, conversion tips, and 20 FAQs to cover edge cases.

Amps to Watts Calculator

What the Amps to Watts Calculator Does

  • Instantly converts current (A) to power (W/kW)
  • Handles DC, single-phase AC, and three-phase AC
  • Accepts voltage and (for AC) power factor (PF)
  • Outputs watts and kilowatts, with clear rounding

In short, enter I (amps), V (volts), choose phase type, and (for AC) set PF—the calculator does the rest.

The Plain-Text Formulas (No Symbols Required)

Use the correct equation for your circuit type:

DC circuits

  • Power (watts) = Voltage (volts) × Current (amps)
  • P = V × I

Single-phase AC

  • Power (watts) = Voltage (volts) × Current (amps) × Power Factor
  • P = V × I × PF

Three-phase AC (line-to-line voltage, balanced load)

  • Power (watts) = √3 × Voltage_LL (volts) × Current (amps) × Power Factor
  • P = 1.732 × V_LL × I × PF

Three-phase AC (line-to-neutral voltage, balanced load)

  • Power (watts) = 3 × Voltage_LN (volts) × Current (amps) × Power Factor
  • P = 3 × V_LN × I × PF

Convert watts to kilowatts

  • Power (kilowatts) = Power (watts) ÷ 1000
  • kW = W / 1000

Notes

  • PF (power factor) ranges from 0 to 1 and accounts for phase shift and waveform distortion.
  • When PF is unknown, a common assumption for general induction-motor loads is 0.8–0.9, but always use the nameplate if available.
  • For three-phase systems, ensure you know whether the given voltage is line-to-line (V_LL) or line-to-neutral (V_LN). Most utility three-phase ratings are expressed as line-to-line (e.g., 400 V, 415 V, 480 V).

How to Use the Amps to Watts Calculator

  1. Select circuit type: DC, Single-phase AC, or Three-phase AC.
  2. Enter voltage (V): Use the system or nameplate voltage.
  3. Enter current (I): In amps (A).
  4. If AC, set PF: Enter the known power factor (0–1).
  5. If three-phase, confirm voltage mode: Most calculators assume line-to-line; switch if your value is line-to-neutral.
  6. Calculate: The tool returns watts and kilowatts.
  7. Review rounding: Results may display to 1–3 decimals; you can round up if you’re sizing equipment.

Pro tips

  • For nameplate checks, read voltage, current, and PF right off the label.
  • For panels/feeders, ensure the current is per phase (not total) when using three-phase equations.
  • If your device lists apparent power (VA or kVA) instead of amps, you can use:
    • For single-phase: S (VA) = V × I
    • For three-phase: S (VA) = √3 × V_LL × I
      Then convert real power via P = S × PF.

Worked Examples (Step-by-Step)

Example 1: DC load (LED strip, 12 V, 5 A)

  • Given: V = 12 V, I = 5 A
  • Formula: P = V × I
  • Calculation: P = 12 × 5 = 60 W
  • In kW: 60 ÷ 1000 = 0.06 kW

Example 2: Single-phase AC (tool on 230 V, 8 A, PF 0.9)

  • Given: V = 230 V, I = 8 A, PF = 0.9
  • Formula: P = V × I × PF
  • Calculation: 230 × 8 = 1840; 1840 × 0.9 = 1656 W
  • In kW: 1656 ÷ 1000 = 1.656 kW

Example 3: Three-phase AC (400 V line-to-line, 10 A per phase, PF 0.85)

  • Given: V_LL = 400 V, I = 10 A, PF = 0.85
  • Formula: P = 1.732 × V_LL × I × PF
  • Steps: 400 × 10 = 4000; 4000 × 0.85 = 3400; 3400 × 1.732 ≈ 5888.8 W
  • Result: ≈ 5.89 kW

Example 4: Three-phase with line-to-neutral value (230 V LN, 15 A, PF 0.8)

  • Given: V_LN = 230 V, I = 15 A, PF = 0.8
  • Formula: P = 3 × V_LN × I × PF
  • Steps: 3 × 230 = 690; 690 × 15 = 10350; 10350 × 0.8 = 8280 W
  • Result: 8.28 kW

Choosing the Right Inputs (and Avoiding Common Mistakes)

  • Using AC formula for DC (or vice versa): DC does not use PF; AC does.
  • For three-phase, mixing LN and LL: If your voltage is LN but you use the LL formula (or the opposite), you’ll be ~73% off (because √3 ≈ 1.732).
  • Ignoring PF: You’ll overestimate power if you assume PF = 1 for an inductive load.
  • Using total current instead of per-phase: Three-phase formulas expect phase current, which is usually what clamp meters show on one conductor.

Practical Reference Values

  • Common single-phase mains: 120 V, 230–240 V
  • Common three-phase mains (LL): 208 V, 400–415 V, 480 V
  • Typical PF ranges:
    • Incandescent/Resistive heaters: ~1.00
    • Power supplies with PFC: ~0.95–1.00
    • Induction motors (light load): ~0.6–0.8
    • Induction motors (rated load): ~0.8–0.9

When You Only Know Amps (No PF Listed)

If your device is purely resistive (heaters, incandescent lamps), you can assume PF ≈ 1. For motors or compressors, use the nameplate PF if possible; otherwise, estimate 0.8–0.9 for a ballpark.

Beyond Watts: VA and kVA

  • Apparent power (VA or kVA) tells you how much current your wiring/transformer must carry:
    • Single-phase: S = V × I
    • Three-phase: S = 1.732 × V_LL × I
  • Real power (W or kW) is what you pay for (P = S × PF).
  • Reactive power (var or kvar) handles the energy stored and released each cycle by inductors/capacitors.

Knowing the difference helps you pick properly sized breakers, generators, and UPS systems.

Extra Tips for Accurate Conversions

  • Temperature and wiring length affect voltage drop—measure voltage at the load if precision matters.
  • Motor inrush is higher than running current; sizing must consider starting conditions (for protective devices).
  • Rounding: For planning, round up to ensure headroom. For audits, keep at least two decimals in kW.

20 Frequently Asked Questions (FAQs)

  1. What is the basic amps to watts formula?
    For DC: P = V × I. For AC single-phase: P = V × I × PF. For three-phase (LL): P = 1.732 × V × I × PF.
  2. Do I need power factor for DC?
    No. PF applies to AC only.
  3. What if I don’t know the power factor?
    Use the device nameplate. If unknown, estimate 0.8–0.9 for typical motor loads, 1.0 for resistive loads.
  4. How do I convert watts to kilowatts?
    kW = W ÷ 1000.
  5. Can the calculator handle three-phase systems?
    Yes—choose three-phase and enter line-to-line voltage, current per phase, and PF.
  6. What’s the difference between line-to-line and line-to-neutral?
    LL is measured between any two phases; LN is one phase to neutral. Use the matching three-phase formula.
  7. Is apparent power the same as real power?
    No. Apparent power (VA) = V × I (× 1.732 for three-phase). Real power (W) = Apparent × PF.
  8. Why does PF matter so much?
    A lower PF means more current for the same real power, which increases conductor and transformer loading.
  9. Can I assume PF = 1 for electronics?
    Only if the device has active power-factor correction; otherwise PF may be lower.
  10. How accurate is the conversion?
    It’s exact if your inputs (V, I, PF) reflect real operating conditions.
  11. What if my supply voltage sags under load?
    Use the actual measured voltage during operation for best accuracy.
  12. Does frequency (50/60 Hz) change the formula?
    The formulas are the same; frequency can affect PF and device behavior but not the conversion equation.
  13. How do I convert amps to watts for a UPS or generator?
    Use the correct formula with the manufacturer-specified PF and voltage rating.
  14. Why is my measured wattage lower than V × I?
    Because V × I is apparent power. Real power is V × I × PF (for AC).
  15. Can this help size breakers?
    It informs load power; breaker sizing must follow current, duty cycle, inrush, and code requirements.
  16. What about split-phase systems (e.g., 120/240 V)?
    Treat each branch as single-phase at its voltage; for 240 V loads across both legs, use single-phase at 240 V.
  17. How do I handle unbalanced three-phase loads?
    Formulas assume balance. For unbalanced systems, compute each phase separately and sum real power.
  18. What units should I enter?
    Voltage in volts, current in amps, PF as a unitless decimal (e.g., 0.85).
  19. How do I get from kW to horsepower?
    Mechanical hp ≈ kW × 1.341; electrical hp often uses kW ÷ 0.746.
  20. Does cable length matter for the calculation?
    Not directly. It affects voltage drop; measure voltage at the load for accurate power.

Final Thoughts

The Amps to Watts Calculator saves time and eliminates guesswork by applying the right equation for your circuit type. Provide voltage, current, and (for AC) power factor, and you’ll get accurate watts and kilowatts—ready for equipment sizing, energy estimates, or quick checks.