Choosing a commercial generator starts with four checks: list the loads that must run, size for running load plus starting surge, confirm duty type and fuel, then verify transfer switch, voltage, and phase compatibility.
A grocery store that loses refrigeration for six hours writes off its entire deli case. That number — not a sales sheet — decides the generator. Commercial sizing runs on measured data, and the difference between a unit that carries your building and one that browns out on startup usually comes down to one motor you forgot to count.
The sections below walk the four checks in order, with the actual formulas current commercial guides use and the mistakes that show up again and again in undersized installs.
How Do You Build The Load List?
Split every circuit into critical loads that must stay energized and optional loads that can wait, then measure rather than estimate. Cat’s commercial sizing calculator page frames the job as defining your power needs first, and vendor sizing guides treat the load list as the foundation everything else rests on.
Two measurement routes work. A clamp-on ammeter at the service panel captures peak demand directly. If that’s impractical, pull the highest peak demand from the prior 12 months of utility bills — the demand charge line on your statements already tracked it for you.
Critical loads typically include refrigeration, life-safety systems, servers, and any process equipment that fails hard. Emergency and legally required systems fall under NEC Articles 700, 701, 702, and 708, and any generator supporting them must be sized for that duty separately from convenience loads. Optional loads — break-room outlets, decorative lighting, most general office circuits — come off the list first when budget or footprint tightens.
How Is Running Load Sized And Surge Added?
Size for measured running load, add a reserve, then handle the largest motor starting event as its own problem. Those are three separate steps, and skipping any one is how generators end up undersized.
Current commercial guides use a reserve margin of roughly 20–25% above calculated load. One sizing guide recommends computing full load kW and adding 25% on top. That margin absorbs load growth, equipment degradation over the unit’s life, and environmental derating.
Starting surge is the step that catches people. A motor draws far more current in its first seconds than at steady state, so the generator must carry running load plus the surge of the single biggest motor starting while everything else stays on. HVAC compressors, well pumps, and air compressors are the usual offenders. Size the surge around whichever motor is largest, not the sum of every motor.
For preliminary screening only, guides cite rough shortcuts: retail applications at 50 kW plus 10 W per square foot, other commercial applications at 50 kW plus 5 W per square foot. Treat these as a starting conversation with an engineer, never as final design.
Converting to kVA matters if you’re comparing units rated that way. Required kVA equals peak required kW divided by power factor, and 0.8 is the industry-standard power factor used in the cited guide.
| Sizing Factor | What Current Guides Say | Why It Matters |
|---|---|---|
| Reserve margin | 20–25% above calculated load | Covers growth, degradation, and derating |
| Power factor | 0.8 industry standard | Converts peak kW to required kVA |
| Prime/continuous duty | Size at 70–80% of max capacity | Prevents overload in sustained running |
| Standby duty | Sized for backup service | Different duty cycle than continuous |
| Retail screening | 50 kW + 10 W/sq. ft. | Rough first pass, not final design |
| Other commercial | 50 kW + 5 W/sq. ft. | Rough first pass, not final design |
| Altitude and heat | Reduces engine output | May require a physically larger unit |
| Load diversity | Apply diversity plus growth margin | Not every load peaks at once |
Standby Or Prime Duty, And Which Fuel?
Match the duty rating to how the generator will actually run, then pick fuel for the worst realistic outage. A current commercial sizing guide draws a hard line between standby/emergency duty and prime/continuous duty.
Standby units wait for outages and run intermittently. Prime/continuous units carry sustained load and should be sized to run around 70–80% of maximum capacity, which leaves headroom for the hours that stretch on. Buying a standby-rated unit for continuous service is one of the fastest ways to void a warranty and shorten engine life.
Fuel follows runtime math. Diesel and natural gas behave differently on tank size, refueling logistics, and autonomy during a wide-area outage. Work backward from your worst realistic scenario — how many hours, at what load — and size the tank or gas supply for that, not for a two-hour blip.
Emissions tier and county requirements belong in the decision set too. One current buyer guide lists them alongside sizing, and local air-quality rules can rule out a fuel type entirely before price enters the conversation.
What Must Match At The Transfer Switch?
The transfer switch amp rating must match both the generator output and the service panel rating, and the generator’s voltage and phase must match the facility’s electrical service. These are compatibility checks, not preferences.
Single-phase and three-phase are not interchangeable. Sizing three-phase equipment for a single-phase service — or the reverse — produces a generator that cannot be connected legally or safely. Confirm the service type before comparing any models.
If you’re working through which units actually fit these numbers, our tested commercial generator picks match real load profiles to specific models. Then, before you commit to a brand, run your load figures through the Cat genset sizing tool, which starts by asking for application type and fuel type.
Those are reseller estimates, not manufacturer list prices — treat them as a planning range and get a firm quote against your actual load list.
Common Sizing Mistakes
- Sizing to nameplate rating instead of measured running load and true peak demand.
- Ignoring motor starting surge on HVAC, pumps, and compressors.
- Skipping reserve margin for growth, degradation, and environmental derating.
- Never separating critical loads from optional ones.
- Using square-foot formulas as final engineering design instead of a first screening pass.
Get the load list right, measure instead of guessing, and add the surge and reserve on top. That sequence produces a generator that starts the building on the worst day, which is the only day that counts.
FAQs
Can I size a generator from my square footage alone?
No. Square-foot formulas — 50 kW plus 10 W/sq. ft. for retail, 50 kW plus 5 W/sq. ft. for other commercial — are screening shortcuts from current guides, not design methods. They ignore motor surge, power factor, and which loads you actually need. Use them only for a rough first pass before measuring real demand at the panel.
What happens if I skip the motor starting surge?
The generator may run fine under steady load and still trip on startup. HVAC compressors, pumps, and air compressors draw several times their running current in the first seconds. Size the surge around the largest single motor starting while other loads stay on, then confirm the unit handles both together.
Does altitude change my generator sizing?
Yes. Current commercial guides warn that high altitude and extreme temperatures reduce engine performance, which can push you into a physically larger generator to deliver the same output. Derating factors apply on top of your reserve margin, so a unit that fits at sea level may come up short at elevation.
Why does power factor matter when comparing kVA ratings?
Required kVA equals peak required kW divided by power factor, and 0.8 is the industry-standard figure cited in current guides. If you compare a kW-rated unit against a kVA-rated one without converting, you can end up with a generator that looks adequate on paper and fails under load.
References & Sources
- Cat. “Genset Sizing.” Frames commercial sizing as defining power needs by application and fuel type.
- Norwall Power Systems. “Sizing Guide for Commercial Generators.” Source for load measurement, reserve margin, and duty-type guidance.
- NMC CAT. “Business Generator Sizing.” Source for commercial load categories, diversity factors, and screening formulas.
- Gardening Beyond. “Best Commercial Generator.” Same-site tested roundup matching load profiles to specific models.