Diesel Generator Set — FAQ with Professional Answers Sizing & Load Calculation

Q1. How do I calculate the right generator size (kW/kVA) for my home or business?

List every device you intend to run, sum their rated running watts, then add the highest single starting/surge watt draw on top (usually the largest motor-driven appliance). Multiply the total by a 20-25% safety margin to allow for voltage dips and future load growth. For mixed inductive loads (motors, compressors, pumps), a qualified load audit is recommended rather than a rough estimate.

Q2. What's the difference between kW and kVA?

kW (kilowatts) is real, usable power that actually does work; kVA (kilovolt-amperes) is apparent power, which includes both real and reactive power. They are related by the power factor (kW = kVA × PF), typically 0.8 for industrial gensets. When sizing, always convert to the same unit before comparing generators.

Q3. What is prime power vs standby power rating?

Prime power rating is the maximum output a generator can deliver continuously for unlimited hours with a variable load (typical average load 70-80% of rating), used where there is no utility grid. Standby rating is a higher, short-term output (usually limited to 200-500 hours/year) intended only to cover utility outages. Standby-rated units should never be run as a primary, full-time power source.

Q4. How many appliances can a 5kW/10kW/20kW generator run at the same time?

A 5kW unit typically covers lighting, a refrigerator, a few small appliances and a window AC unit. A 10kW unit can add a well pump, several room air conditioners or small workshop tools. A 20kW unit can support a small home's central HVAC, multiple circuits, or light commercial equipment. Exact capacity depends on each appliance's starting current, not just its running wattage.

Q5. What size diesel generator do I need to run central air conditioning?

Central AC units draw 3-5x their running wattage at start-up due to the compressor motor. A typical 3-5 ton residential central AC needs roughly 15-20kW of generator capacity when combined with other household loads, and larger commercial rooftop units require detailed load calculations or soft-start devices to reduce inrush current.

Q6. How do I calculate starting (surge) load vs running load?

Running load is the steady-state wattage a device consumes once operating; starting/surge load is the momentary spike (often 2-7x running load) drawn by motors, compressors and pumps at startup. Generator sizing must be based on the sum of all running loads plus the single largest starting load, not the sum of all starting loads, since motors rarely start simultaneously.

Q7. Single phase or three phase — which one do I need?

Single-phase power (110V/220V) suits residential use and small equipment; three-phase power (380V/400V) is required for larger motors, industrial machinery and commercial buildings because it delivers power more efficiently and starts large motors more smoothly. If your existing electrical panel or equipment is three-phase, the generator must match; mismatched phase configurations will not work.

Q8. What voltage and frequency options are available (110V/220V/380V, 50Hz/60Hz)?

Most manufacturers offer reconnectable alternators supporting 220V/380V (50Hz) for regions following the IEC standard, and 110V/220V/240V (60Hz) for regions following the NEMA/ANSI standard (e.g., North America). Frequency is tied to engine RPM (1500rpm for 50Hz, 1800rpm for 60Hz), so confirm your local grid standard before ordering, as after-sale conversion is costly.

Q9. Can one generator power an entire house?

Yes, provided the generator's continuous rating exceeds your home's total connected load including the largest appliance's starting surge, and it is connected through a properly installed automatic or manual transfer switch. Whole-house standby units in the 15-30kW range typically cover an average North American home; larger homes with electric heating or well pumps may need more.

Q10. How do I size a generator for a construction site or workshop?

Add the running wattage of all tools/equipment expected to run simultaneously, then add the highest starting wattage among motor-driven tools (welders, air compressors, saws). Construction sites should also factor in a duty cycle margin (typically sizing to 80% of rated capacity) since prime-rated units running near 100% continuously will wear faster and consume more fuel per kWh.