The method in one line
Running load → worst-case motor start → × 1.25 reserve → site derate → next nameplate size up. Every credible sizing worksheet — and our own selector wizard — runs this same chain. This guide walks each step with real numbers so you can sanity-check any quote you receive.
Step 1 — establish the running load
You have three ways in, from most to least accurate:
- Measure it. A clamp ammeter on the main feeder during a busy period, or twelve months of utility bills showing peak demand kW, beats every formula.
- Convert your panel amps. kW ≈ amps × volts × 1.732 × 0.8 ÷ 1,000 on three-phase (drop the 1.732 on single-phase). A 200 A, 208 V three-phase service fully loaded is about 58 kW — but real buildings rarely load a service fully, which is why measuring wins. Our kW ↔ amps converter does this math at any voltage and phase.
- Estimate from square footage. The standard planning formulas:
Food service, grocery, convenience, gas station, retail: 50 kW + 10 W per sq ft Offices, clinics, warehouses and most other buildings: 50 kW + 5 W per sq ft
The higher factor covers buildings dense with refrigeration and cooking. If you know nothing else yet, these running-load anchors are typical planning midpoints by building type:
| Building type | Typical running-load anchor |
|---|---|
| Small office | 30 kW |
| Gas station | 40 kW |
| Small data / server room | 40 kW |
| Restaurant | 55 kW |
| Retail store | 60 kW |
| Warehouse / light industrial | 60 kW |
| Grocery / convenience store | 75 kW |
| Nursing home | 100 kW |
| Medical clinic | 150 kW |
These are starting points for a conversation, not answers — a 2,000 sq ft clinic and a 12,000 sq ft clinic are different machines.
Step 2 — add the motor-starting surge
This is where undersized generators come from. Motors — HVAC compressors, walk-in refrigeration, elevators, pumps, air compressors — briefly draw about 2.5× their running watts while spinning up (2–3× is the accepted range). Resistive loads such as lighting, ovens and IT gear have no surge.
Typical running kW for the common commercial motors:
| Motor load | Typical running kW | Surge it adds (× 1.5) |
|---|---|---|
| Central HVAC / rooftop unit | 15 | +22.5 kW |
| Elevator | 15 | +22.5 kW |
| Walk-in refrigeration | 10 | +15 kW |
| Water / well pump | 7.5 | +11.25 kW |
| Air compressor | 7.5 | +11.25 kW |
The rule: for every motor that can start at the same moment, add 1.5 × its running kW on top of your running load (the motor's running watts are already in the base; the surge is the extra 1.5×). If your controls stagger the starts — refrigeration recovers before HVAC is allowed to kick on — only the single largest surge counts. Staggering is the cheapest kilowatts you will ever buy.
Step 3 — add the 25% reserve
Multiply the governing figure by 1.25. The reserve covers loads you will add later, voltage stability when motors start, and engine health — a generator cruising at 60–80% load runs cleaner and lasts longer than one pinned at its nameplate. It also keeps monthly NFPA 110 test loads meaningful without oversizing into wet-stacking territory.
Step 4 — correct for the site
Thin air and hot air both cost capacity. Planning derates, applied by dividing your reserved figure by (1 − derate):
| Site condition | Derate |
|---|---|
| Below 1,000 ft elevation | 0% |
| 1,000–3,000 ft | 5% |
| 3,000–6,000 ft | 10% |
| Above 6,000 ft | 15% |
| Hot climate (add to the above) | +5% |
A 128 kW requirement in a hot Gulf Coast climate becomes 128 ÷ 0.95 ≈ 135 kW of needed nameplate. Manufacturers publish exact derate curves per model; use these bands for planning.
Three worked examples
Restaurant, 3,000 sq ft, gas cooking. Running load: 50 + (3,000 × 10 ÷ 1,000) = 80 kW. Motors: central HVAC (15 kW) and walk-in refrigeration (10 kW), staggered starts — only the largest surge counts: +22.5 kW → governing load 102.5 kW. Reserve: × 1.25 = 128.1 kW. No derate. Needed nameplate ≈ 129 kW → a 150 kW-class unit for whole-building backup. Backing up only critical loads typically cuts this 30–50% — see the full restaurant walkthrough.
Convenience store, 2,500 sq ft. Running load: 50 + 25 = 75 kW. One motor cluster — walk-in refrigeration: +15 kW → 90 kW. Reserve: × 1.25 = 112.5 kW. Needed nameplate ≈ 113 kW → a 125 kW-class unit.
Medical clinic, 6,000 sq ft, hot climate. Running load: 50 + (6,000 × 5 ÷ 1,000) = 80 kW. Central HVAC surge, staggered: +22.5 kW → 102.5 kW. Reserve: × 1.25 = 128.1 kW. Hot-climate derate: ÷ 0.95 ≈ 135 kW → a 150 kW-class unit. Clinics with life-safety loads also carry NFPA 110 obligations — fuel and testing choices change, size math does not.
Single-phase or three-phase?
The generator must match the building's electrical service — a hard constraint:
- 120/240 V single-phase — small storefronts and offices. In the 20–200 kW commercial class, single-phase offerings generally top out around 100 kW; above that, the lineup is three-phase only.
- 120/208 V three-phase — the standard service in most commercial buildings and strip malls.
- 277/480 V three-phase — larger buildings and heavier machinery.
Your main panel label or electrician confirms this in minutes. Sizing doesn't change; the available equipment does.
From a number to a machine
Nameplates come in steps (20, 30, 45, 60, 80, 100, 125, 150, 175, 200 kW in the Cat Olympian line), so you buy the next size at or above your requirement — a 113 kW answer becomes a 125 kW unit. Then the automatic transfer switch gets sized to your electrical service, and fuel gets chosen on logistics and code, not price alone.
These are planning calculations. Before any purchase, have a licensed electrician or the supplier's engineer run a formal load calculation on your actual panels — site visits routinely move the number.
Frequently asked questions
What size generator do I need for a small business?
Most small commercial buildings land between 30 and 150 kW for whole-building backup. Start with 50 kW + 5–10 W per square foot depending on building type, add motor-starting surge and 25% reserve, and confirm with a measured load.
How many kW is a 200-amp commercial service?
Fully loaded: about 48 kW at 120/240 V single-phase, or about 58 kW at 120/208 V three-phase (at 0.8 power factor). Size to your measured demand plus reserve, not to the service rating — services are rarely fully loaded.
Is it better to oversize a generator?
A 25% reserve is engineering; 100% oversize is a problem. A lightly loaded diesel wet-stacks and fails emissions and NFPA 110 loading expectations. Target running at 60–80% of nameplate.
Do I size the generator to my breaker panel or my load?
To your load, always. The panel tells you the ceiling the utility service can deliver; your load calculation tells you what the building actually draws. The transfer switch — not the generator — gets sized to the service.
Get a model, not a math lesson
Our selector wizard runs this exact method — square footage or kW, motor loads, staggering, service voltage, fuel, derates — and returns a specific commercial standby model in about two minutes. Convert panel amps first with the kW ↔ amps tool, and when you're ready for real numbers from real suppliers, post your project and let dealers bid on a pre-scoped spec.