Generator Power Calculator
Enter any two or three known electrical values and this calculator works out the rest: kilowatts, kVA, amps, volts, power factor, and horsepower. Choose single-phase, three-phase, or DC mode to match your generator or motor system. Results update instantly and the steps panel shows exactly how each value was derived.
What is generator power and why does it matter?
Generator power describes how much electrical energy a generator can deliver to a load. Three related quantities appear on every generator nameplate: real power in kilowatts (kW), apparent power in kilovolt-amperes (kVA), and power factor (PF). Real power is the actual work done by the circuit - running motors, lighting, and heating. Apparent power is the product of voltage and current, and it represents the total electrical load the generator windings must carry. Power factor is the ratio of real to apparent power, and for most AC loads it is less than 1.0 because motors and transformers store and release energy each half-cycle. A generator rated at 10 kVA and 0.8 PF can deliver 8 kW of real power - the kVA rating determines the required generator size, while kW determines what the load actually consumes.
Single-phase vs. three-phase vs. DC
Single-phase systems use two conductors (live and neutral) and are common in residential and light commercial applications at 120 V or 240 V. Power is P = V x I x PF. Three-phase systems use three conductors and a factor of sqrt(3) = 1.732 in every formula, which is why a 480 V three-phase motor draws far less current per phase than the equivalent single-phase device. Three-phase generators are standard for industrial and commercial buildings and are more efficient for motors above about 1 HP. Direct current (DC) systems - including solar inverters, battery banks, and many telecom sites - have no reactive power and no power factor; the formula is simply P = V x I. This calculator handles all three modes.
How to size a generator correctly
Start by listing every load you need to run simultaneously: lighting (W), HVAC compressors (starting kW is typically 2-3x running kW), power tools, pumps, refrigeration, and any UPS or battery chargers. Total the running watts, then identify which load has the highest starting surge and add that surge on top. The sum gives the peak apparent power in kVA. Add at least 20 % headroom so the generator runs at 80 % of rated load rather than 100 %, which reduces wear, fuel consumption per kWh, and heat. For example, a workshop with a 2 kW circular saw (surge 4 kW) plus 1.5 kW of lighting and a 1 kW dust collector needs about (2 + 1.5 + 1 + 2) x 1.2 = 7.8 kW, so an 8-10 kW generator is appropriate. If your loads are mostly resistive (heaters, incandescent lights), PF is close to 1.0 and kW and kVA are nearly equal. Inductive loads (motors, compressors) pull PF down to 0.7-0.85, so the kVA demand is higher than kW alone would suggest.
Power factor - the hidden cost of inductive loads
Power factor measures how efficiently a circuit converts apparent power into real work. A purely resistive load (electric heater, incandescent bulb) has PF = 1.0 and every volt-amp drawn from the generator becomes a watt of useful power. An inductive load (motor, transformer, fluorescent ballast) stores energy in its magnetic field each half-cycle and returns it slightly out of phase, so the current waveform lags behind the voltage. For a motor at PF = 0.8, the generator must supply 1.25 kVA for every 1 kW of real power delivered. Utilities sometimes penalise industrial customers with low PF because extra current flows through cables and transformer windings without doing work. The fix is power factor correction: capacitors connected in parallel with the inductive loads supply the reactive current locally so the generator only needs to supply real power. Most generators are rated at PF = 0.8, so a 10 kVA machine delivers 8 kW maximum at rated power factor.
Standard electrical power formulas
| Quantity | Single-phase | Three-phase | DC |
|---|---|---|---|
| kVA | V x I / 1000 | V x I x 1.732 / 1000 | V x I / 1000 |
| kW (real power) | V x I x PF / 1000 | V x I x 1.732 x PF / 1000 | V x I / 1000 |
| Amps from kW | kW x 1000 / (V x PF) | kW x 1000 / (V x 1.732 x PF) | kW x 1000 / V |
| Amps from kVA | kVA x 1000 / V | kVA x 1000 / (V x 1.732) | kVA x 1000 / V |
| Amps from HP | HP x 746 / (V x %EFF x PF) | HP x 746 / (V x 1.732 x %EFF x PF) | HP x 746 / (V x %EFF) |
| HP from watts | W x %EFF / 746 | W x %EFF / 746 | W x %EFF / 746 |
| Power factor | kW / kVA | kW / kVA | 1 (always) |
NEC / NEMA standard formulas for single-phase, three-phase, and DC systems. E = volts, I = amps, PF = power factor, %EFF = motor efficiency as a decimal.
Frequently asked questions
What is the difference between kW and kVA?
kW (kilowatts) is real power - the actual work the electricity does, such as running a motor or heating an element. kVA (kilovolt-amperes) is apparent power, the product of voltage and current regardless of phase. For DC and purely resistive AC loads they are equal. For inductive AC loads (motors, transformers), kW = kVA x power factor. Generator nameplates show kVA because that determines the maximum current the windings can carry, while kW tells you the maximum useful work output at a given power factor.
Why do generators use 0.8 power factor as a rating standard?
Most electrical standards bodies and generator manufacturers use 0.8 PF as the design power factor because it represents a typical industrial mixed load. A generator rated "10 kVA at 0.8 PF" is designed to deliver 8 kW of real power when its windings carry the full rated current at that phase angle. If your load is mostly resistive (PF closer to 1.0), the same generator can deliver more real kilowatts - up to 10 kW - before hitting the current limit, but the output voltage may sag. Running at higher PF than the design point stresses the voltage regulator.
How do I calculate amps from kW?
For single-phase AC: Amps = (kW x 1000) / (Volts x PF). For three-phase AC: Amps = (kW x 1000) / (Volts x 1.732 x PF). For DC: Amps = (kW x 1000) / Volts. For example, a 5 kW single-phase load at 240 V with PF 0.85 draws 5000 / (240 x 0.85) = 24.5 A. Use the calculator above and choose "Amps" in the Solve For dropdown.
What surge (starting) allowance should I add for motors?
Electric motors draw 2 to 6 times their running current at startup for 1-3 seconds - this is the inrush or starting surge. The exact multiple depends on motor type: direct-on-line induction motors are typically 5-7x, capacitor-start single-phase motors 2-3x, and soft-start or VFD-controlled motors can be limited to 1.5x. Add the starting kW of the largest motor to the total running load of all other devices when sizing the generator. The calculator shows running power; add the startup multiplier manually for the largest motor.
How do I convert horsepower to kilowatts or generator kVA?
One mechanical horsepower equals 746 W. To convert motor HP to required input kW, divide by the motor efficiency: Input kW = (HP x 0.746) / efficiency. To find the generator kVA needed, divide by power factor as well: kVA = (HP x 0.746) / (efficiency x PF). A 10 HP motor at 90 % efficiency and 0.85 PF needs 7.46 / 0.90 = 8.3 kW input, and 8.3 / 0.85 = 9.75 kVA from the generator. Set "Solve for: Horsepower" in the calculator and enter the other values to work this automatically.
What voltage should I use for a three-phase calculation?
Always use the line-to-line voltage (also called line voltage) in three-phase power formulas, not the phase voltage. Common three-phase voltages in North America are 208 V (commercial buildings), 480 V (industrial motors), and 600 V (heavy industry). In Europe, 400 V (line) = 230 V phase is standard. The sqrt(3) = 1.732 factor in the formulas accounts for the geometric relationship between the three phases, so you should not adjust the voltage to compensate.