Most users make mistakes when purchasing diesel generators by estimating power based on experience, calculating only steady-state load, ignoring motor starting inrush current, confusing prime power and standby power, and neglecting altitude and temperature derating correction. These errors result in insufficient generator capacity, startup tripping, voltage drop, or excessive generator size that operates under long-term low load, causing carbon deposition and high failure rates.
This article serves as a professional engineering-level generator sizing guide. It elaborates on step-by-step power calculation methods with universal formulas and practical cases, which can be directly applied to project quotation, equipment procurement, and construction acceptance.
1. Core Concepts: Where 90% of Sizing Mistakes Occur
It is critical to distinguish between two generator power ratings. Mixing them up is the biggest industry pitfall.
1.1 Prime Power (PR / Continuous Power)
Rated for 24-hour continuous operation with a permissible 10% overload for 1 hour within every 12-hour cycle. Suitable for long-term power supply at construction sites, factory self-provided power systems, and continuous operation scenarios. It is the only standard benchmark for engineering selection.
1.2 Standby Power (PRP / Emergency Power)
Only used for emergency backup during mains power failure. Long-term full-load continuous operation is prohibited with limited annual operating hours. Industry standard conversion: Standby Power = Prime Power × 1.1.
Important Reminder: Most suppliers quote standby power by default, while users mistakenly take it as prime power, leading to undersized generators and field operation failures. Always select generators based on prime power.
2. Five Core Steps for Accurate Generator Sizing (Standard Engineering Procedure)
Step 1: Count All Electrical Equipment and Classify Load Types
Register all generator-powered equipment with rated power and load type. Two load categories require completely different calculation logic.
1) Resistive Load (No Starting Inrush)
Including lighting, electric heating tubes, water heaters, computers, office equipment, and UPS systems. Features: stable starting current without inrush; power values can be directly summed up.
2) Inductive Load (Motor Load, Critical for Sizing)
Including water pumps, fans, air compressors, refrigeration compressors, machine tools, and mixers. Features: extremely high instantaneous starting current (multiple times the rated current). Generator size is determined by the maximum motor starting load rather than total steady-state load.
Essential Unit Conversion Formula:
Standard motor power factor: cosφ = 0.8
Step 2: Calculate Total Steady-State Load (Simultaneity Factor)
Not all equipment operates simultaneously. Multiply the total power by the simultaneity factor (K) to obtain the actual operating load.
General Simultaneity Factor Reference
- Ordinary office and commercial backup power: K = 0.5~0.7
- Factory workshops and general construction sites: K = 0.6~0.8
- Fire emergency systems (fire pumps, exhaust fans): K = 1.0 (100% full-load operation mandatory)
- Precision computer room UPS backup: K = 0.8~0.9
Step 3: Verify Motor Starting Inrush (Most Critical Step)
This is the leading cause of sizing failure. Many generators work well under steady-state conditions but trip or experience voltage collapse when motors start. Verify the minimum generator capacity according to the motor starting method.
Motor Starting Multiplier (Based on Prime Power)
- Direct-on-line starting (DOL): Generator Prime Power ≥ Motor Power × 2.5~3.0
- Star-Delta reduced voltage starting: Generator Prime Power ≥ Motor Power × 1.5~2.0
- Soft starter: Generator Prime Power ≥ Motor Power × 1.2~1.5
- Variable frequency drive (VFD): No obvious inrush, no extra capacity amplification required
Core Rule: For multiple motors, verify capacity based on the single largest motor starting load, then superimpose the steady-state power of other equipment.
Step 4: Altitude & Temperature Derating Correction (Mandatory for Plateau & High-Temp Areas)
Generator nameplate power is tested under standard conditions (altitude ≤ 1000m, ambient temperature 25°C, standard atmospheric pressure). Harsh on-site conditions cause power attenuation and must be corrected.
- Altitude Derating: For altitude above 1000m, power decreases by 3%~5% per 1000m elevation increase
- Temperature Derating: For ambient temperature above 40°C, power decreases by 2%~3% per 5°C temperature increase
Superimpose derating coefficients for high-altitude and high-temperature environments and select a larger generator to compensate for power loss.
Step 5: Reserve Safety Margin & Optimize Load Range
Reserve sufficient power margin to avoid full-load operation, accommodate future capacity expansion, and prevent carbon deposition caused by low-load operation.
- Emergency backup power: Reserve 10%~20% power margin
- Long-term continuous main power: Maintain load at 70%~80% of generator prime power (optimal operating range with minimum failure rate)
- Prolonged operation below 30% load or at 100% full load is strictly prohibited
3. Complete Engineering Calculation Case
Project Background: General construction site backup power, altitude 800m, normal temperature (no derating required)
Equipment List:
- Lighting & office resistive loads: 40kW
- Main water pump (DOL starting): 30kW
- Other small equipment total load: 20kW
- Simultaneity factor K = 0.7
Calculation Process:
1. Total steady-state load = (40+30+20) × 0.7 = 63kW
2. Maximum motor starting load verification (30kW DOL): 30×2.5 = 75kW
3. Starting load dominates over steady-state load, take 75kW as the benchmark
4. Add 15% safety margin: 75×1.15 = 86.25kW
5. Standard model selection: 100kW Prime Power Diesel Generator
4. Eight Common Generator Sizing Mistakes (Must Read)
- Only checking kVA instead of kW: Convert generator apparent power (kVA) to active power (kW) by ×0.8; electrical equipment is rated in kW.
- Confusing prime power and standby power: Using emergency standby power for long-term continuous operation causes generator overload and damage.
- Calculating only steady-state load and ignoring starting inrush: Generators sufficient for steady-state load trip during motor startup.
- Ignoring altitude and temperature derating: Nameplate power meets standards, but actual output attenuation causes insufficient load capacity.
- Oversized generator selection: Long-term low-load operation leads to engine carbon deposition, oil leakage, and high failure rates.
- Underrating fire-fighting loads: All fire equipment must operate at full load simultaneously; simultaneity factor shall not be less than 1.0.
- No reserved capacity for future expansion: Marginal sizing results in insufficient power after new equipment addition.
- Uniform motor starting multiplier: Failing to distinguish starting modes causes oversized or undersized generator selection.
5. Quick Self-Selection Checklist
Provide the following 6 sets of information for accurate generator model matching:
- Complete equipment list (power rating & load type)
- Motor starting method (DOL / Star-Delta / Soft Starter / VFD)
- Generator application type (fire emergency / site backup / long-term main power)
- Maximum simultaneous operating quantity of equipment
- On-site altitude & maximum ambient temperature in summer
- Reserved expansion capacity (if required)
6. Conclusion
Diesel generator sizing is not a simple power summation, but a systematic engineering process including steady-state load calculation, motor starting verification, environmental condition correction, and safety margin reservation. 90% of on-site generator failures stem from inaccurate sizing, parameter confusion, and neglected environmental power attenuation.
Following this standardized selection procedure completely solves common problems such as insufficient load capacity, startup tripping, frequent failures, and budget waste, applicable to all civil, industrial, and fire emergency scenarios.
Post time: Aug-18-2026
