A generator sizing chart connects three values: kVA (apparent power), kW (real power) and amperage (current). Knowing how they relate helps you read the chart correctly and avoid undersizing.
The real electrical power your equipment consumes while operating. kW = kVA × power factor.
The total apparent power the generator must deliver. Most industrial sets are rated in kVA at 0.8 power factor.
The current at your chosen voltage. Amperage determines cable size, breaker rating and distribution panel capacity.
kW = kVA × power factor · At 0.8 PF, a 100 kVA generator delivers 80 kW.
kVA = kW ÷ power factor · An 80 kW load at 0.8 PF needs a 100 kVA generator.
Amps (3‑phase) = (kVA × 1000) ÷ (V × 1.732) · Amps (1‑phase) = (kVA × 1000) ÷ V
| kVA | kW (0.8 PF) | Amps @ 400V (3‑ph) | Amps @ 480V (3‑ph) |
|---|---|---|---|
| 50 | 40 | 72 A | 60 A |
| 100 | 80 | 144 A | 120 A |
| 150 | 120 | 217 A | 181 A |
| 250 | 200 | 361 A | 301 A |
| 500 | 400 | 722 A | 601 A |
| 1000 | 800 | 1443 A | 1203 A |
Match your project type to a typical generator capacity range. kW values are shown at the standard 0.8 power factor. Use these as a quick check before finalising your load calculation.
| Application | Recommended kVA | Typical kW (0.8 PF) | Phase |
|---|---|---|---|
| Small apartment (essentials) | 5–10 kVA | 4–8 kW | Single |
| Small home / off‑grid backup | 7–18 kVA | 6–14 kW | Single |
| Large home / villa with central AC | 18–60 kVA | 14–48 kW | Single / 3‑ph |
| Small office / retail store | 10–50 kVA | 8–40 kW | 3‑phase |
| Restaurant / small shop | 20–50 kVA | 16–40 kW | 3‑phase |
| Farm / irrigation (pumps) | 20–50 kVA | 16–40 kW | 3‑phase |
| Construction site (tools, mixers) | 50–150 kVA | 40–120 kW | 3‑phase |
| Mechanical workshop / machine shop | 50–100 kVA | 40–80 kW | 3‑phase |
| Office building (5,000–10,000 sq ft) | 80–150 kVA | 64–120 kW | 3‑phase |
| Data center (servers + cooling) | 100–300 kVA | 80–240 kW | 3‑phase |
| Hospital (critical equipment + HVAC) | 200–500 kVA | 160–400 kW | 3‑phase |
| Factory / manufacturing plant | 100–1,000+ kVA | 80–800+ kW | 3‑phase |
| Large industrial / base‑load / mining | 1,000–4,000 kVA | 800–3,200 kW | 3‑phase |
The same physical generator carries different kW/kVA numbers depending on how it is rated. Confusing the rating type is a common cause of undersizing.
For use during utility outages. The generator supplies varying load for a limited number of hours per year (typically a few hundred) with no overload capacity. Suitable for hospitals and data centers.
For use when grid power is unavailable or unreliable. Carries varying load for unlimited hours with a small overload allowance. Prime rating is roughly 90% of standby. Use for construction, mining and off‑grid sites.
For a constant, unchanging load supplied around the clock — typically base‑load applications. The continuous rating is lower than both standby and prime, reflecting the sustained duty cycle.
Standard for residential and light commercial use. Practical up to roughly 50 kVA, after which three‑phase is more efficient. kW ≈ kVA because power factor is close to 1.0 for most single‑phase loads.
The norm for motors, pumps, HVAC systems and large equipment. Delivers more power with lower current and smaller conductors. Required for most commercial and industrial installations above a few horsepower.
Follow these four steps to move from your equipment list to a correctly sized generator set. The chart gives you a quick reference; these steps help you confirm the number.
Write down every appliance, motor and piece of equipment that may operate at the same time. Record the running watts from the nameplate or spec sheet.
Motor‑driven equipment — compressors, pumps and air conditioners — can draw 2 to 7 times running load at startup. Add only the single largest surge to your running total.
Divide total watts by 1,000 to get kW, then divide by the power factor (0.8 for inductive loads) to get kVA. Use the conversion formulas above.
Add 20–30% headroom for standby applications and up to 50% for prime‑power or future expansion. Then select the next standard generator size from the chart above.
Most sizing problems come from a few 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 directly leads to 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 or production equipment after commissioning. Leaving a realistic expansion margin avoids 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 the selected engine‑alternator combination before equipment is ordered.
Shandong Huali Electromechanical Co., Ltd. has manufactured diesel generator sets since 1999, with a range spanning 8 kVA to 4,000 kVA, single‑ and three‑phase, and 50 or 60 Hz. Every unit is 100% load‑tested in our national‑standard testing center before delivery, certified to ISO9001, CE and CCC, and exported to more than 20 countries.
A: Start by listing every load you must run, add the single largest starting surge, then divide by 1,000 to get kW and by 0.8 to get kVA. Add 20–30% headroom and select the next standard size from the application chart above. For a precise result, ask an engineer.
A: kW is the real power your equipment consumes to do work; kVA is the apparent power the generator delivers. They are related by the power factor: kW = kVA × power factor. At the standard 0.8 PF, a 100 kVA generator supplies 80 kW.
A: A 100 kVA three‑phase generator at 480 V draws about 120 amps. At 400 V the same unit draws about 144 amps, because amperage rises as voltage falls.
A: Standby is for limited‑hours backup use. Prime is for unlimited‑hours use as the primary source. Continuous is for a constant 24/7 load. Prime is roughly 90% of standby, so always match the rating type to how you will actually run the generator.
A: A 0.8 power factor means the generator delivers 0.8 kW of real power for every 1 kVA of apparent power. It is the standard for industrial generators because their inductive loads — motors, pumps and compressors — consume reactive power.
A: For a 200 amp, 240 V single‑phase service, a generator around 50 kW is typical. For a 200 amp three‑phase service at 480 V, the requirement rises to roughly 175 kW because of the higher voltage and phase configuration.
A: Engine output decreases as air density falls — roughly 1% loss per 100 m above 1,000 m. High ambient temperature (above 40 °C) also reduces cooling effectiveness and usable output. Apply the necessary derating before choosing the final capacity.
