How to Calculate Gas Generator Set Capacity? Sizing Guide & Formulas

Choosing a generator that is oversized leads to wasted initial investment and "wet stacking" (carbon buildup) due to low-load operation. Conversely, an undersized unit will fail to meet demand, leading to frequent trips or potential equipment damage. Accurately calculating power requirements is the critical first step in procurement.

This guide provides a practical framework for power sizing.

1. Distinguish Between Power Ratings

Understanding the ISO ratings is essential for long-term reliability:

Pro Tip: For projects requiring continuous operation, Prime Rated Power (PRP) should be your primary selection benchmark.

2. Calculate Total Load

• Step 1: Inventory All Equipment: List everything, including lighting, HVAC, pumps/motors, production machinery, and IT hardware.

• Step 2: Classify Load Types:

• Resistive Loads: (e.g., lighting, electric heaters) Power factor ≈1.0.

• Inductive Loads: (e.g., motors, compressors, pumps) These create high inrush currents during startup, typically 3 to 7 times their rated current.

3. Account for Startup Current (Critical!)

Startup inrush is the leading cause of generator tripping.

• Single Large Motor: Generator Power ≥ Motor Rated Power × Startup Factor (3–7x).

• Multiple Motors: Use the "Peak Startup Method":

  (Startup power of the largest motor) + (Running power of all other equipment).

Simplified Formula:

Required Power = Total Running Load × 1.1) + (Largest Motor Start-up Inrush)

4. Apply the Diversity Factor (Simultaneity)

Not all equipment runs at the same time. Apply a Diversity Factor based on your specific scenario:

5. Environmental Derating Factors

Generator performance decreases in harsh environments. You must "derate" the unit based on site conditions.

• High Temperature Derating: Standard range is -30℃ to 50℃ . When ambient temperature exceeds 40℃, the rated power typically drops by 3% for every 5℃ increase.

• High Altitude Derating: Standard for altitudes below $500\text{m}$. Above 500m, the power drops by approximately 5% for every additional 500m.

Calculation:

Actual Available Power=Rated Power×(1−Temp Derating Rate)×(1−Altitude Derating Rate)

6. Future Expansion

Always allow a buffer for growth. We recommend adding a 10–20% margin to the final calculated power to accommodate future equipment additions.

7. Selection Example

Scenario: A farm in the Middle East requiring 24/7 continuous power.

• Lighting: 20kW

• Ventilation Fan: 30kW(Largest motor, startup factor of 5)

• Water Pump: 15kW

• Feed Processing: 10kW

• Domestic Use: 5kW

Calculation Steps:

1. Total Running Load: 20 + 30 + 15 + 10 + 5 = 80kW

2. Peak Motor Inrush: 30kW× 5 = 150kW

3. Applying Diversity Factor (0.85): 80kW× 0.85 = 68kW

4. Max Demand Power: 68kW + (150kW× 0.3  impact coefficient})≈ 113kW

5. Temperature Derating (15% for 45°C): 113kW÷ 0.85 ≈ 133kW

6. Future Expansion (15%): 133kW × 1.15 ≈ 153kW

Recommendation: Select a gas generator set with a Prime Power (PRP) of 150–160kW.

8. Professional Consultation

Power sizing is nuanced. Before finalizing your purchase, please provide the following to our specialists:

1. Full Load List: (Name, rated power, quantity, and load type).

2. Starting Methods: (Direct-on-line, Star-Delta, or VFD/Soft Starter).

3. Site Conditions: Ambient temperature and altitude.

4. Future Plans: Any anticipated equipment upgrades or expansions.