Choosing the right generator size is the single most important decision in your purchase. Get it wrong, and you face either frequent overloads (and shutdowns) or wasted capital on capacity you will never use. This guide gives you the exact calculation method engineers use—not guesswork.
خطوة 1: Identify Your Critical Loads
List every device or system that must run during a power outage. These are your critical loads. For a typical commercial facility, that includes lighting, التدفئة والتهوية وتكييف الهواء, حماية, معدات تكنولوجيا المعلومات, and process machinery.
Read the nameplate rating (in watts or amps) for each device. If only amps are available: الواط = فولت × أمبير. For three-phase equipment: Watts = Volts × Amps × 1.732 × Power Factor.
| Device | تشغيل واتس | بداية واتس | ملحوظات |
|---|---|---|---|
| إضاءة LED (50 fixtures) | 500 | 500 | No surge |
| HVAC System (10-طن) | 12,000 | 36,000 | 6x surge for compressor |
| Server Rack | 3,000 | 3,000 | UPS handles ride-through |
| Security System | 500 | 500 | Always-on |
| Elevator (20 HP) | 15,000 | 75,000 | High starting current |
خطوة 2: Calculate Running vs. Starting Power
You need two numbers:
- Running kW: Sum of all loads’ تشغيل واتس, مقسمة على 1000.
- بدء كيلو فولت أمبير: The largest motor’s starting kVA (typically 4-6x its running kVA) plus the sum of all others’ running kVA.
The generator must satisfy both numbers. مثال: 30kW running total + 40kW starting surge = 50kW (62.5كيلو فولت أمبير) minimum generator.
خطوة 3: Apply Safety Margins
Never size exactly to your load. Industry best practice adds:
- 25% future expansion margin — you will add equipment later
- 10% altitude derating if above 1000m / 3280قدم
- 10% temperature derating for ambient above 40°C / 104درجة فهرنهايت
Final formula: حجم المولد (كيلو فولت أمبير) = (Total Running kW × 1.25) ÷ 0.8
For a detailed walkthrough with downloadable calculator, see our دليل تحجيم المولدات.
Common Sizing Mistakes
- تجاهل المحرك بدءا الحالي — motors draw 4-6x running current at startup
- Using nameplate kVA instead of kW — critical for engine loading calculations
- Forgetting power factor differences — UPS systems have harmonic loads that affect تحجيم المولدات
- Adding 100% redundancy unnecessarily — N+1 for critical, standalone for non-critical
التعليمات">التعليمات
How do I size a generator for motor loads?
Multiply the largest motor’s running kVA by 4-6 (depending on starting method) and add the sum of all other loads. For soft-start motors, use 2-3x. For VFD-driven motors, use 1.5x.
ما هو 80% rule for generator sizing?
مولدات should operate at 70-80% of rated capacity for optimal fuel efficiency. Sizing to 100% continuous load is not recommended for diesel engines as it accelerates carbon buildup.
Can I use a generator sizing calculator online?
نعم. Many manufacturers provide free sizing tools. لكن, always validate the results with a licensed electrical engineer for critical applications.
Related Resources
- Diesel Generator Fuel Consumption Guide
- Diesel Generator Price Guide
- دليل تحجيم المولدات
- kW vs kVA: What’s the Difference
- 1000مولد كيلو فولت أمبير
- 100kVA Generator Specs
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