Watts to Amps Calculator (kW, kVA, HP)
kW, kVA, amps and horsepower for DC, single-phase and three-phase loads.
How it was calculated
The same load at other standard voltages
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About the Watts to Amps Calculator (kW, kVA, HP)
How many amps does a load draw? It depends on the voltage, on whether the supply is DC, single-phase or three-phase, and — for AC — on the power factor. This calculator handles all of them in one form: convert from real power (W, kW), apparent power (VA, kVA), current (A) or a motor’s rated output (hp or kW), and it returns the current, kW, kVA, kvar and horsepower together.
Every answer comes with its formula and your numbers in it, such as I = P ÷ (√3 × V × PF), so you can check it or copy it into a report. Three-phase voltage can be entered line to line (400 V) or line to neutral (230 V), and the standard voltages of IEC 60038 are one tap away.
How to use it
- Choose the supply — DC, Single-phase AC or Three-phase AC. For three-phase, say whether your voltage is line to line (400 V in a 230/400 V system) or line to neutral (230 V).
- In Convert from, pick what you know: real power, apparent power, current, or a motor’s rated output. Enter the value and choose its unit.
- Enter the voltage or tap a standard one. For AC, enter the power factor (1 for heaters; the nameplate value for motors). For motors, add the efficiency.
- Read the current and the other quantities. How it was calculated shows each formula with your numbers, and the table shows the same load at other standard voltages.
Examples
Single-phase, 2 kW, 230 V, PF 1
I = 2,000 W ÷ (230 V × 1) = 8.696 A
Single-phase, 1.5 kW, 120 V, PF 1
I = 1,500 W ÷ (120 V × 1) = 12.5 A
Three-phase, 10 kW, 400 V line to line, PF 0.85
I = 10,000 ÷ (1.732 × 400 × 0.85) = 16.98 A per line · S = 11.76 kVA · Q = 6.197 kvar
Three-phase, 100 kVA, 400 V line to line
I = 100,000 ÷ (1.732 × 400) = 144.3 A
Three-phase, 10 hp, 400 V, PF 0.85, efficiency 90%
P_in = 7,457 W ÷ 0.9 = 8,286 W · I = 8,286 ÷ (1.732 × 400 × 0.85) = 14.07 A
The formulas
- DC: I = P ÷ V and P = V × I
- Single-phase AC: I = P ÷ (V × PF), apparent power S = V × I, real power P = S × PF
- Three-phase AC (balanced): I = P ÷ (√3 × V_LL × PF) = P ÷ (3 × V_LN × PF), and S = √3 × V_LL × I
- Reactive power: Q = √(S² − P²)
- Motors: the rating is shaft output, so the electrical input is P = P_out ÷ η, where η is the efficiency
V_LL is the voltage between two lines and V_LN the voltage from a line to neutral; in a balanced system V_LL = √3 × V_LN, which is why √3 appears. PF is the power factor, cos φ.
Watts, volt-amperes and var
Real power (W) is what does work — heat, light, motion. Apparent power (VA) is simply voltage × current, and it is what cables, fuses, transformers, UPS units and generators must carry, so they are rated in VA or kVA. Reactive power (var, also written VAr or VAR) is the part that flows back and forth to build magnetic and electric fields in motors and capacitors. The power factor ties them together: PF = P ÷ S. A motor taking 10 kW from the supply at PF 0.85 draws 11.76 kVA — 18% more current than a 10 kW heater on the same supply.
These definitions follow IEEE Std 1459-2010 for sinusoidal (undistorted) voltage and current.
Horsepower
Three horsepower definitions are in use, and the calculator lets you choose:
- Mechanical horsepower (550 ft·lbf/s) = 745.699 872 W, from the exact foot (0.3048 m) and pound-force (4.448 221 615 260 5 N); NIST SP 811 rounds it to 745.6999 W. Motors rated in hp use it
- Electrical horsepower = 746 W exactly
- Metric horsepower (PS, CV, ch) = 75 kgf·m/s = 735.498 75 W
A motor’s hp or kW rating is the mechanical power at its shaft, not the electrical power it takes. With 90% efficiency, a 10 hp motor (7.457 kW at the shaft) takes 8.286 kW from the supply.
Standard voltages (IEC 60038)
IEC 60038 Table 1 lists the low-voltage systems. For 50 Hz: 230 V (three-wire), 230/400 V, 400/690 V and 1000 V; for 60 Hz: 120/208 V, 240 V, 230/400 V, 277/480 V, 480 V, 347/600 V and 600 V, plus 120/240 V single-phase three-wire. In a pair such as 230/400 V, the first value is line to neutral and the second line to line. 230/400 V replaced the older 220/380 V and 240/415 V systems; for 11 kV and other medium voltages see the standard’s Table 3.
Sources
- IEEE Std 1459-2010, Definitions for the Measurement of Electric Power Quantities Under Sinusoidal, Nonsinusoidal, Balanced, or Unbalanced Conditions
- IEC 60038:2009, IEC standard voltages
- NIST Special Publication 811 (2008), Guide for the Use of the International System of Units, Appendix B: horsepower conversion factors (550 ft·lbf/s = 745.6999 W, electric 746 W, metric 735.4988 W)
- IEC 60364-5-52 Annex G — uses PF 0.8 when the load power factor is not known
Limitations
- Assumes sinusoidal voltage and current. Variable-speed drives, LED drivers and other electronic loads draw distorted current; their true power factor is lower than cos φ, so measured current can be higher.
- Three-phase results assume a balanced load. For different currents in each phase, use the three-phase power calculator.
- Motor starting current is several times the running current and is not calculated here.
- For sizing motor conductors and protection in the US, NEC 430.6(A)(1) requires the full-load currents in Tables 430.247–430.250 rather than nameplate or calculated values.
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Frequently asked questions
How many amps is 1000 watts?
It depends on the voltage. At 230 V single-phase with a resistive load (PF 1), 1000 ÷ 230 = 4.35 A. At 120 V it is 8.33 A. On a 400 V three-phase supply it is 1000 ÷ (1.732 × 400) = 1.44 A per line.
How do I convert kVA to amps?
Single-phase: I = S ÷ V. Three-phase: I = S ÷ (√3 × V_LL). A 100 kVA transformer at 400 V gives 100,000 ÷ (1.732 × 400) = 144.3 A — no power factor is needed, because kVA already includes it.
What is the difference between kW and kVA?
kW is real power; kVA is apparent power (voltage × current). They are related by the power factor: kW = kVA × PF. A generator rated 100 kVA at PF 0.8 can supply 80 kW.
What power factor should I use?
Use 1 for resistive loads such as heaters, kettles and filament lamps. For motors and electronic equipment, use the nameplate or datasheet value. If it is unknown, 0.8 is a common assumption — it is the value IEC 60364-5-52 Annex G uses.
How many amps does a 1 hp motor draw?
Divide the shaft power by the efficiency, then by voltage × power factor. For 230 V single-phase with PF 0.8 and 75% efficiency: 745.7 W ÷ 0.75 = 994 W, and 994 ÷ (230 × 0.8) = 5.40 A. Check the nameplate for the real efficiency and power factor.
Why is there a √3 in the three-phase formula?
Total power is 3 × V_LN × I × PF (three phases). Since the line-to-line voltage is √3 times the line-to-neutral voltage, that equals √3 × V_LL × I × PF. Use the √3 form with 400 V, or the 3 × form with 230 V — the calculator does either.