Why Generator Sizing Matters#
An undersized generator risks overload, voltage sag, and premature failure. An oversized generator wastes fuel, increases maintenance costs, and can suffer from wet-stacking (incomplete combustion at low loads). Proper sizing hits the sweet spot: enough capacity to handle peak demand with room to grow, without paying for power you'll never use.
Step 1: Inventory Your Electrical Loads#
Start by walking your facility with an electrician and listing every piece of equipment that needs backup power. For each item, record:
- Equipment name and location
- Nameplate wattage (or voltage × amperage)
- Priority: critical (immediate power needed) or deferrable (can wait 30-60 seconds)
Common loads by facility type:
| Facility | Typical Critical Loads | Typical Range |
|---|---|---|
| Medical office | HVAC, lighting, medical equipment, IT | 30–100 kW |
| Retail store | POS systems, refrigeration, lighting, security | 20–75 kW |
| Small warehouse | Loading dock, lighting, office area | 50–150 kW |
| Data center | Server racks, cooling, UPS | 500 kW–3 MW |
| Manufacturing | Production lines, compressors, lighting | 200 kW–2 MW |
Step 2: Calculate Total and Starting Loads#
Running load is the sum of all equipment wattages during normal operation. But generators must also handle starting surge — electric motors draw 3–6x their rated wattage for the first few seconds of operation.
Example calculation:
| Equipment | Running (kW) | Starting (kW) |
|---|---|---|
| HVAC compressor | 15 | 45 |
| Elevator | 20 | 60 |
| Lighting | 10 | 10 |
| IT servers | 25 | 25 |
| Total | 70 | 140 |
Your generator must handle at least 140 kW to start the highest-surge loads. Using sequenced starting (staggering motor startups via the ATS) can reduce the peak starting requirement.
Step 3: Choose Your Fuel Type#
| Fuel | Pros | Cons | Best For |
|---|---|---|---|
| Diesel | Highest energy density, long shelf life with treatment, reliable | Emissions permits required, fuel degradation over time, CARB restrictions in CA | Facilities needing high power output, remote locations |
| Natural gas | Clean burning, unlimited runtime via utility line, lower emissions | Lower energy density, utility disruption risk | Urban commercial, healthcare, facilities with gas service |
| Propane (LP) | Long storage life, clean burning, no permits in most cases | Tank storage space required, delivery logistics | Rural locations, smaller facilities, backup to natural gas |
| Bi-fuel | Diesel starting reliability + natural gas runtime | Higher upfront cost, more complex maintenance | Large facilities wanting extended runtime |
Step 4: Apply a Safety Margin#
Never size a generator to exactly match your calculated load. Add a 20–25% safety margin for:
- Future growth — new equipment, facility expansion
- Calculation errors — real loads often exceed nameplate ratings
- Efficient operation — generators run most efficiently at 50–80% of rated capacity
Formula: Recommended generator size = Peak starting load × 1.25
From our example: 140 kW × 1.25 = 175 kW minimum rated capacity
Step 5: Select and Plan Installation#
With your sizing calculation in hand, work with a licensed generator dealer to:
- Select the right unit — match capacity, fuel type, and voltage configuration
- Plan ATS placement — automatic transfer switch location affects cable runs and transfer times
- Design fuel storage — per NFPA 110 requirements, some facilities need 96+ hours of on-site fuel
- Handle permitting — building permits, electrical permits, AQMD permits (for diesel), and HCAI approval (for healthcare)
- Address noise — sound-attenuated enclosures may be required by local ordinance
- Schedule installation — minimize downtime with planned cutover
Need Help Sizing Your Generator?#
Every facility is different. Our team has sized and installed generators for hospitals, data centers, retail chains, and government facilities across California and Nevada. Get a free sizing consultation — we'll review your electrical loads and recommend the right solution for your budget and requirements.