Why Generator Sizing Matters

An undersized generator risks critical equipment damage, while an oversized unit wastes fuel, reduces efficiency, and increases costs. Proper sizing balances performance, reliability, and operational economics.

• Reliability: Adequate capacity prevents brownouts and equipment failures during peak demand.
• Efficiency: Generators operate best at 70-80% of rated load, optimizing fuel consumption.
• Cost Control: Right-sized units reduce capital expenditure, fuel costs, and maintenance overhead.
• Compliance: Meets safety codes, load bank testing requirements, and warranty conditions.

Step-by-Step Sizing Process

Step 1: Identify Critical Loads

Create a comprehensive inventory of all equipment and systems that must operate during an outage:

• IT Infrastructure: Servers, network switches, UPS systems, cooling units.
• Safety & Security: Emergency lighting, fire alarms, CCTV, access control.
• HVAC: Critical ventilation, chillers, compressors, circulating pumps.
• Production: Manufacturing equipment, conveyor belts, packaging machinery.
• Support Systems: Communications, medical equipment, elevators.

Step 2: Calculate Running Load (kW)

For each piece of equipment, determine:

• Rated Power: Check nameplate specifications (kW or HP).
• Operating Load: Actual running load (often 60-80% of rated).
• Power Factor: Use 0.8-0.9 for motors, 1.0 for resistive loads, or actual measured values.

Formula: Running Load (kW) = Rated Power × Operating Load Factor

Step 3: Account for Starting Current (Surge Load)

Electric motors draw 3-7× their running current at startup. This is the most critical factor in generator sizing:

• Small Motors (<5 HP): 3-4× running load
• Medium Motors (5-50 HP): 4-6× running load
• Large Motors (>50 HP): 5-7× running load
• Soft Starters/VFDs: Reduce surge to 1.5-2× running load

Step 4: Calculate Total Peak Demand

Determine if equipment starts simultaneously or sequentially:

• All-at-Once Starting: Sum all surge loads (rare, most conservative).
• Staggered Starting: Add largest surge + running loads of others (more realistic).
• Load Sequencing: Use automatic controls to delay motor starts.

Step 5: Add Safety Margins

• Diversity Factor: 15-25% buffer for load variations and future growth.
• Derating for Altitude: Reduce capacity by 3.5% per 1,000 ft above sea level.
• Derating for Temperature: Reduce capacity by 1% per 10°F above 77°F (25°C).
• Non-Transferable Loads: Add 10-15% for hardwired or unmetered equipment.

Step 6: Calculate Required Generator Size

Final Sizing Formula:

Step 1: Running Load = Σ (Rated kW × Load Factor)

Step 2: Peak Load = Largest Surge Load + Sum of Other Running Loads

Step 3: Apply Safety Margins

Step 4: Correct for Environmental Conditions

Required Generator (kW) = Peak Load × (1 + Safety Margin) × Altitude Factor × Temp Factor

    

Practical Example

Consider a commercial office building with the following loads:

Sizing Calculation:

1. Running Load = 51 kW
2. Peak Load (staggered start) = 90 kW (largest surge) + 21 kW (other running loads) = 111 kW
3. Add 25% safety margin: 111 × 1.25 = 139 kW
4. Correct for altitude (2,000 ft): 139 × 0.93 = 129 kW

Recommended Generator Size: 150 kW (next standard size above 129 kW)

Operating Load Range: 34% (running) to 86% (peak), which is within the optimal 50-80% range.

Additional Considerations

Single-Phase vs. Three-Phase

• Residential and small commercial: typically single-phase.
• Industrial and large commercial: usually three-phase.
• Balanced loading across phases is critical for three-phase systems.

Generator Type Selection

• Prime Power: Continuous operation — size at running load only.
• Standby Power: Emergency use — must handle surge loads.
• Load Banks: Regular testing under full load ensures reliability.

Future Expansion Planning

• Add 20-30% capacity for planned facility expansion.
• Consider modular generators for scalable growth.
• Document load profile changes in maintenance logs.

Common Sizing Mistakes to Avoid

• ❌ Ignoring Starting Current: Motors need 3-7× their running power at startup.
• ❌ Using Nameplate Only: Actual load is often 60-80% of rated capacity.
• ❌ No Safety Margin: Under sizing leads to brownouts and failures.
• ❌ Forgetting Altitude/Temperature: High altitude or extreme heat reduces capacity.
• ❌ Oversizing Excessively: Operating below 30% load causes wet stacking, increased wear, and wasted fuel.
• ❌ Ignoring Power Factor: Reactive loads (motors) require higher kVA than kW.

Professional Sizing Tools

For complex installations, consider:

• Load Calculation Software: SKM PowerTools, ETAP, or vendor sizing tools.
• Professional Engineering: Licensed engineers for large or critical facilities.
• On-Site Load Surveys: Clamp-on meters to measure actual consumption.
• Vendor Consultation: Generator manufacturers provide free sizing assistance.

Quick Reference: Generator Size Guidelines