How to Size a Generator for a Commercial Building
Selecting the correct generator size for a commercial building is one of the most important decisions in backup power planning. An undersized generator can fail during peak demand, while an oversized generator increases capital costs, fuel consumption, and maintenance expenses.
Proper generator sizing requires technical evaluation, load analysis, and strategic planning. This guide explains the step-by-step process Facility Managers should follow when determining the appropriate generator capacity for a commercial building.
1) Understand Why Proper Sizing Matters
Incorrect generator sizing can lead to:
- Overloading and automatic shutdown
- Voltage instability
- Damage to connected equipment
- Excessive fuel consumption
- Reduced generator lifespan
The goal is to match generator output to actual operational needs — not to guess or oversimplify.
2) Identify Critical Loads
Not every electrical system needs to be powered during an outage. Start by identifying which loads are critical for safety and business continuity.
Typical critical loads include:
- Emergency lighting
- Fire protection systems
- Security systems
- IT infrastructure
- Elevators (at least one)
- HVAC for critical areas
Facilities must decide whether the generator will support only critical systems or the entire building.
3) Calculate Total Electrical Load
To size a generator properly, you must calculate total electrical demand.
Step 1: List All Required Equipment
Create a detailed inventory of equipment that must operate during a power outage.
Step 2: Determine Power Ratings
Each device has a power rating, typically expressed in kilowatts (kW) or kilovolt-amperes (kVA).
Step 3: Account for Starting Power
Motors and compressors require higher power during startup (inrush current). This is especially important for:
- HVAC systems
- Chillers
- Pumps
- Elevators
Starting load may be 2–6 times higher than running load.
Step 4: Apply Demand Factors
Not all equipment operates simultaneously. Use realistic demand factors when calculating load.
4) Convert kW to kVA
Generators are typically rated in kVA. To convert kW to kVA, use the formula:
kVA = kW ÷ Power Factor
Commercial buildings often assume a power factor of 0.8.
Example: If total load is 400 kW:
400 ÷ 0.8 = 500 kVA generator required
5) Consider Future Expansion
Facilities evolve over time. Plan for future load increases, such as:
- Additional tenants
- New equipment
- Technology upgrades
Adding a safety margin of 10–25% is common practice.
6) Evaluate Load Type (Resistive vs Inductive)
Load characteristics influence generator performance.
- Resistive loads: Lighting, heating
- Inductive loads: Motors, compressors
Inductive loads require careful startup sequencing.
7) Decide Between Standby, Prime, or Continuous Rating
Generator ratings differ:
- Standby: Emergency use only
- Prime: Variable load for extended periods
- Continuous: Constant load operation
Most commercial buildings use standby-rated generators.
8) Assess Fuel Type and Runtime
Common fuel options:
- Diesel
- Natural gas
- Propane
Runtime planning depends on fuel storage capacity and expected outage duration.
9) Evaluate Environmental and Space Constraints
Generator sizing also depends on:
- Available installation space
- Noise regulations
- Ventilation requirements
- Local emissions regulations
10) Work with Qualified Engineers
Generator sizing should involve electrical engineers and experienced contractors.
Professional load studies and simulations ensure accurate results.
11) Test the System Under Load
After installation, perform load bank testing to verify capacity and performance.
Conclusion: Accurate Sizing Protects Your Investment
Generator sizing is not guesswork. It requires detailed load analysis, understanding of equipment characteristics, and long-term planning.
An appropriately sized generator ensures reliability, protects equipment, and supports business continuity without unnecessary costs.
For Facility Managers, investing time in proper sizing pays off during the first major outage.
