Generator Sizing: How to Calculate Power Needs for Your Facility.

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.

Step 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, HVAC, security, IT equipment, and process machinery.

Read the nameplate rating (in watts or amps) for each device. If only amps are available: Watts = Volts × Amps. For three-phase equipment: Watts = Volts × Amps × 1.732 × Power Factor.

Step 2: Calculate Running vs. Starting Power

You need two numbers:

• Running kW: Sum of all loads’ running watts, divided by 1000.
• Starting kVA: 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. Example: 30kW running total + 40kW starting surge = 50kW (62.5kVA) minimum generator.

Step 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 / 3280ft
• 10% temperature derating for ambient above 40°C / 104°F

Final formula: Generator Size (kVA) = (Total Running kW × 1.25) ÷ 0.8

For a detailed walkthrough with downloadable calculator, see our Generator Sizing Guide.

Common Sizing Mistakes

• Ignoring motor starting current — 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 Generator Sizing
• Adding 100% redundancy unnecessarily — N+1 for critical, standalone for non-critical

FAQ">FAQ

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.

What is the 80% rule for generator sizing?

Generators 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?

Yes. Many manufacturers provide free sizing tools. However, always validate the results with a licensed electrical engineer for critical applications.

Related Resources

• Diesel Generator Fuel Consumption Guide
• Diesel Generator Price Guide
• Generator Sizing Guide
• kW vs kVA: What’s the Difference
• 1000kVA Generator
• 100kVA Generator Specs