Size a commercial generator to the facility’s actual peak load, add the largest motor starting surge, then add 20–25% reserve capacity.
A generator that’s undersized trips the moment the building’s biggest motor kicks on. One that’s oversized burns fuel and budget on capacity you’ll never touch. Sizing a commercial generator correctly comes down to matching it to the facility’s actual peak electrical demand, then adding the largest starting surge and a reserve margin.
Skip that sequence and you get the two classic failures: a machine that can’t start the chiller, or one that idles through outages and burns money doing it. The method below follows the same workflow manufacturers use — measure the actual load, calculate the surge, add headroom, and verify compatibility before you commit to a purchase.
The Core Sizing Formula
The formula has four parts: verified running load, the largest motor starting surge, reserve capacity, and a final compatibility check against the generator’s rated output. The surge step is where most undersized selections go wrong.
Running load is the real, measured demand the building pulls at peak, gathered from nameplate data, utility-bill peak demand, or current measurements taken at the busiest hour. Starting surge is the extra draw a motor or chiller pulls in the first moments of starting — an across-the-line start can briefly demand several times its running watts. If the generator can’t cover the biggest single starting surge, the whole system trips on day one. That’s why the largest motor in the building often sets the minimum generator size instead of the total of everything running.
Step By Step: Calculating The Real Load
Gather documented load data first, and never size from square footage alone — building area only works as a rough screen before a real calculation. The standard workflow runs through six steps.
- Inventory every electrical load in the building, then split it into critical and non-critical lists. Critical keeps life safety and essential equipment running; non-critical can be shed in an outage.
- Record running watts and starting watts for each item from the nameplate data.
- Identify the largest motor start in the facility — that surge becomes part of the minimum generator size.
- Verify voltage and phase, and note whether the building needs single-phase or three-phase service.
- Add reserve capacity to the total (covered below).
- Cross-check runtime: confirm the fuel supply can cover the longest expected outage.
Caterpillar’s commercial sizing workflow starts the same way — define power needs, select application and fuel type, then run the selection through its sizing calculators. Generac’s Power Design Pro walks through the same load and motor-start math and flags problems before you commit to a model. The table below shows where each load figure comes from.
| Load Input | Where It Comes From | Why It Matters |
|---|---|---|
| Nameplate data | Rating plate on each motor and unit | Provides real running and starting watts |
| Utility-bill peak demand | Highest kW on the last 12 months of bills | Captures seasonal peaks a single reading misses |
| Current measurement | Clamp meter at the busiest hour | Confirms the bills against actual usage |
| Motor starting data | Chiller or motor spec sheets | Starting surge sets the minimum size |
| Critical vs. non-critical loads | Facility management review | Keeps emergency loads covered, sheds the rest |
| Voltage and phase | Service entrance specs | The generator must match building service |
| Fuel type and runtime | Site fuel supply review | Fuel must cover the longest expected outage |
How Much Headroom Should You Add?
Add 20 to 25% reserve capacity above the calculated peak. That buffer keeps the generator from running at full output for hours, which shortens service life and invites nuisance trips when loads shift.
Siemens’ commercial sizing guide recommends choosing a generator output approximately 20 to 25% higher than peak load, and a common commercial formula treats reserve capacity as full load kW × 0.25. Use the higher end of the range for motor-heavy buildings and the lower end for steady, predictable loads.
Before you buy, verify four things: the transfer switch amp rating must handle the generator’s full output; the voltage and phase must match the building’s service; the derated output must cover your site’s altitude and temperature, since both reduce a generator’s rating; and the fuel system must sustain the expected runtime. For emergency and standby systems, NEC Articles 700, 701, 702, and 708 govern how loads are classified and wired — walk through them with your electrical engineer before finalizing.
Run the final check in order: continuous rating ≥ peak load + largest surge + reserve; transfer switch amps ≥ generator output; voltage and phase matched; derated output confirmed for your altitude and temperature; fuel covering target runtime. When the numbers line up, the shortlist gets short fast — our tested roundup of commercial generators compares the leading units on rated output and value.
FAQs
Can I size a commercial generator by square footage?
Only as a rough screening step. Two buildings of identical square footage can have completely different loads — one runs a handful of lights and receptacles, the other drives chillers and compressors. Square footage narrows the search before a real calculation; the nameplate, utility-bill, and motor-start data described above is what actually sets the size.
What’s the difference between prime and standby generator ratings?
A standby rating covers backup use during outages, typically a limited number of hours per year at full load. A prime rating handles continuous, variable load as the main power source. Commercial buildings sized for emergency backup usually select on the standby rating, while facilities that rely on the generator daily for normal operation need the prime rating.
Why does one motor change the whole generator size?
Because a motor starting across the line can draw several times its running watts for the first few seconds. If that momentary surge exceeds what the generator can deliver, voltage sags and the protection trips. When the largest motor in the building is big relative to the rest of the load, that single starting surge — not the total running load — sets the minimum generator size.
References & Sources
- Siemens. “Commercial and Industrial Generator Sizing Guide.” Source of the 20–25% headroom recommendation used in this article.
- Caterpillar. “Commercial Generator Sizing.” Outlines the power-needs, application, and fuel-selection workflow referenced above.
- Generac. “Power Design Pro.” Load-entry and motor-start sizing tool for commercial projects.