1. Two Key Power Ratings (Critical)

1. Prime Power (PRP)

• Continuous 24-hour operation; 10% overload allowed for 1 hour within any 12-hour period

• For long-term, continuous use: construction sites, farms, factories, etc. → Select based on PRP

2. Standby Power (ESP)

• Only for emergency backup when mains power fails; total annual running time ≤ 500 hours

• Not suitable for long-term full-load operation

• Never size the generator by standby power

Industry rule: Standby Power ≈ Prime Power × 1.1

2. Load Classification & Power Calculation

2.1 Resistive Load (Smooth startup, no inrush current)

• Applications: lighting, heaters, electric cookers, computers

• Calculation: Sum actual power directly

• Margin factor: 1.2–1.3×

2.2 Inductive Load (High inrush current)

• Applications: motors, water pumps, air compressors, fans, refrigeration compressors

• Feature: starting current = 3–7× rated current

• Calculation:

• Small motors (≤15kW): 2–2.5× rated power

• Large motors (>15kW): 2.5–3× rated power

• Multiple simultaneous startup: (largest motor ×3) + total power of other loads

2.3 Capacitive / Rectified Load (Switching power supplies)

• Applications: UPS, VFDs, servers, LED screens, welding machines

• Feature: harmonics, low power factor

• Margin factor: 1.3–1.5×

2.4 Welding Equipment (Extreme inrush)

• MIG/MMA welders: 1.5–2.0× rated capacity

• Multiple welders (not simultaneous): apply a diversity factor of 0.6–0.8

3. Professional Sizing Formulas

3.1 Pure resistive load (lighting, heating)

3.2 Mixed load (motors, pumps, compressors)

3.3 Non-simultaneous operation

4. Power Factor & kW/kVA Conversion

• Standard generator power factor: 0.8

  → 100 kVA generator ≈ 80 kW active power

• For many inductive loads: further upsizing by 10–20%

5. Recommended Margin Factors by Application

6. Key Selection Pitfalls

1. Do not size to full rated power

   Long near-full load causes overheating, smoking, rapid wear, and shortened lifespan.

2. Always size by PRIME power

   Many suppliers overstate using standby power.

3. Altitude & temperature derating

• Altitude >1000m: power decreases 3–5% per 1000m

• Ambient temp >40°C: power decreases 5–10%

4. Long cable runs require larger generators

   Voltage drop may prevent equipment starting.

5. Soft start / VFD reduces sizing factor

   With soft starter or VFD, motor multiplier reduces to 1.5–2×.

7. Quick Examples

Example 1: Home / Shop

• Load: 3HP AC (2.2kW) + fridge + lighting + computer ≈ 8kW

• Recommend: 10kW prime power generator

Example 2: Construction Water Pump

• 7.5kW pump + 3kW small tools

• Calculation: 7.5×3 + 3 = 25.5kW

• Recommend: ≥30kW prime power generator

Example 3: Livestock Farm

• Total load 30kW (many small motors)

• Recommend: 45kW prime power generator

8. Summary

1. Resistive load: ×1.2–1.3

2. Inductive load (motor/pump): ×2–3

3. Mixed load: largest motor ×3 + other loads

4. Use prime power only, ignore standby power

5. Upsize one more rating for high altitude, high temperature, or long cables