To size a generator correctly, you need to understand the distinction between real power (kW) and apparent power (kVA). The relationship between them is defined by power factor — and getting it wrong is the most common cause of undersized generators.
The portion of electricity that performs useful work — turning motors, powering heating elements, driving pumps. This is what your equipment actually consumes.
The total power the generator must supply, including both real power (kW) and reactive power (kVAR) that inductive loads need to build magnetic fields.
The ratio of real power to apparent power: PF = kW ÷ kVA. For industrial loads, 0.8 is the standard generator rating convention.
The conversion is straightforward once you know the power factor. kVA = kW ÷ Power Factor. At the standard 0.8 power factor, this becomes kVA = kW ÷ 0.8 — or equivalently, kVA = kW × 1.25. For example, a 100 kW load at 0.8 PF requires 125 kVA. Conversely, kW = kVA × Power Factor.
The copper and iron losses in the alternator — and therefore its heating — are driven by total current, which corresponds to apparent power (kVA).
Industry practice standardizes generator ratings at 0.8 lagging, dating back to motor-heavy industrial loads which historically averaged around 0.8 PF.
The engine produces mechanical power (kW), while the alternator converts it to electrical power (kVA). Both limits matter for correct sizing.
Enter your real power (kW) and power factor to get the equivalent apparent power (kVA) and full-load current. Use the preset buttons for common generator sizes, or enter your own values.
Convert kW to kVA using your actual power factor, or use the industry-standard 0.8.
This table shows the most common generator sizes with their equivalent real power (kW) at the standard 0.8 power factor. Use it as a quick check before confirming your final load calculation.
| Generator (kVA) | Real Power (kW) at 0.8 PF | Typical Application |
|---|---|---|
| 10 kVA | 8 kW | Small backup, home standby |
| 20 kVA | 16 kW | Small retail, telecom |
| 50 kVA | 40 kW | Workshops, small factories |
| 100 kVA | 80 kW | Commercial buildings |
| 200 kVA | 160 kW | Mid-size industrial |
| 500 kVA | 400 kW | Large factories, hospitals |
| 1000 kVA | 800 kW | Data centers, mining |
| 2500 kVA | 2000 kW | Heavy industry, utilities |
Converting kW to kVA is step one. Selecting a machine that will start and run your load reliably is the real goal. Follow this process to avoid under-sizing — the most common and costly generator mistake.
List every load the generator must support, then sum the real power (kW) of each. For motor-driven equipment, use the nameplate kW. If you only have horsepower, convert it: 1 HP ≈ 0.746 kW.
Divide your total kW by your measured (or estimated) power factor. If your facility has significant motor load, use 0.8. If it is predominantly IT or lighting, use 0.9 or higher.
Motors draw a surge of current on startup that can exceed running load several times. Add a margin of around 25% to your calculated kVA to accommodate starting current and future growth.
Generator ratings are not all equal. Confirm whether the figure you are comparing is standby, prime, or continuous. A standby-rated unit cannot be run as a prime power source without derating.
Round up to the nearest standard rating. For example, a load calculated at 376 kVA with a 25% margin becomes roughly 470 kVA — round up to a standard 500 kVA unit.
An 80 kW connected load at 0.8 PF corresponds to 100 kVA. Applying a 25% sizing allowance gives 100 × 1.25 = 125 kVA. The practical selection is therefore a 125 kVA generator, equivalent to approximately 100 kW at 0.8 PF.
A connected-load calculation is only the starting point. Engine output, alternator performance and transient capability can all change with the operating environment and the type of electrical load.
Engine output decreases as air density falls. A planning estimate of roughly 1% loss per 100 m above 1,000 m means a site at 3,000 m may have substantially less available engine output than the sea-level rating.
High ambient temperature reduces cooling effectiveness and usable output. Above approximately 40 °C, additional derating may be required for hot climates and poorly ventilated installations.
Direct-on-line motors can draw around 6–7× full-load current during startup. Star-delta starters, soft starters and VFDs reduce the transient demand and can sometimes allow a smaller set.
Standby generators operate intermittently during utility failures, while prime-rated sets are designed for extended operation under varying loads. The required rating should reflect the actual duty cycle.
UPS systems, VFDs and electronic loads can introduce harmonics. If non-linear loads form a large percentage of the system kVA, alternator sizing and transient performance require additional consideration.
Planned equipment additions should be considered before ordering the set. Allowing approximately 10–25% reserve can reduce the risk of replacing an otherwise suitable generator as demand grows.
Most sizing problems come from a small number of recurring assumptions. Avoiding these errors helps prevent both undersized equipment and unnecessary excess capacity.
kW is real usable power, while kVA is apparent power. The relationship depends on power factor. Failing to convert between the two can directly result in an undersized generator.
A generator may carry the continuous load normally and still fail when a compressor, pump or HVAC motor starts. The largest transient demand must be included in sizing.
Diesel generators operating for long periods at very low load can experience incomplete combustion and wet stacking. Excess capacity also increases capital cost and reduces operating efficiency.
A generator rated under standard conditions may not produce the same usable output at high altitude or in extreme heat. Apply the necessary derating before choosing the final capacity.
Facilities frequently add machines, HVAC capacity, pumps or production equipment after commissioning. Leaving a realistic expansion margin can avoid a costly generator replacement later.
Final generator selection should be checked against the complete load schedule, motor-starting sequence, power factor, site conditions, duty rating and selected engine-alternator combination before equipment is ordered.
A kW to kVA calculator gets the number right. Getting the machine right takes an experienced manufacturer. Shandong Huali Electromechanical Co., Ltd. has delivered diesel generator sets for more than 25 years, with ISO-certified production lines and strict testing before every unit ships.
A: Divide your kW by the power factor: kVA = kW ÷ PF. At the standard 0.8 power factor, divide by 0.8 — so 100 kW becomes 125 kVA.
A: At 0.8 power factor, 100 kW equals 125 kVA. The exact figure depends on your power factor; a higher PF produces a lower kVA for the same kW.
A: For any real-world inductive load, yes. Because power factor is less than 1, dividing kW by the power factor always yields a larger kVA. At a perfect 1.0 PF (purely resistive load), kW equals kVA.
A: Use 0.8 for general industrial and motor-heavy loads — the standard generator rating. Use 0.9 or higher for predominantly IT, lighting, or resistive loads. When in doubt, use 0.8 to build in a safety margin.
A: Yes. For three-phase systems: Amps = (kVA × 1000) ÷ (Volts × 1.732), or Amps = (kW × 1000) ÷ (1.732 × Volts × PF). Our calculator includes the amps output when you enter a voltage.
A: Because the alternator’s electrical capacity — and its heating limit — is determined by total current (apparent power, kVA), while the engine’s output is measured in real power (kW). Rating in kVA communicates the machine’s full electrical capacity without assuming a specific power factor.
