A small-business generator is sized from the loads it must supply and how those loads start and operate. Square footage and the main-breaker rating cannot tell you whether a compressor will restart or a payment system will stay online.
Start by defining your outage operating mode. A shop that closes while protecting inventory has a different load schedule from one that keeps working. The schedule should name actual equipment, rather than broad labels such as “kitchen,” “office,” or “shop.”
Collect the information behind each load
Record quantity, voltage, phase, running kW or input current, power factor when relevant, and starting information. For motors, include the starter or drive arrangement. For a UPS, obtain input requirements with battery charging included. Note whether equipment restarts automatically after power returns.
Utility demand records can help describe historical operation, but they do not identify every short starting event or prove which loads will reconnect together after an outage. Nameplates, measurements, operating schedules, and manufacturer data serve different purposes.
An illustrative running-load worksheet
| Included load group | Assumed running input | Operating assumption |
|---|---|---|
| Lighting | 1.2kW | Selected work areas only. |
| Office, network, and payment equipment | 0.8kW | Measured input for the selected equipment. |
| Refrigeration | 2.5kW | Selected units running together. |
| Ventilation | 1.5kW | Needed for the chosen operating mode. |
| One HVAC system | 6.0kW | Other HVAC excluded. |
| Air compressor | 3.0kW | May cycle during operation. |
| Total | 15.0kW | Running demand only; starting and other checks remain. |
The arithmetic is useful, but it does not establish that a 15kW generator will work. This fictional schedule assumes all listed inputs have already been verified as electrically compatible. It deliberately leaves starting demand and power factor unresolved.
Check the event that creates the largest demand
Suppose the compressor is off while the other listed equipment draws 12kW. If verified starting data called for 9kW total input during its start, the simplified active-power event would be 12 + 9 = 21kW. Do not add 9kW to the full 15kW running total; that would count the compressor’s 3kW running input again.
The 9kW figure is invented for this calculation, not a motor multiplier. Real selection also needs starting kVA, event duration, alternator performance, and acceptable voltage and frequency dip. Caterpillar’s sizing tools evaluate running and starting demand, and its detailed tool calculates voltage and frequency dips.
Can sequencing reduce the requirement?
Possibly, if the installed controls actually enforce the assumed sequence. Write down which loads start first, what waits, and what happens when equipment cycles later. A plan that depends on the owner remembering to turn appliances off is a different operating arrangement from automatic load control.
Cummins’ load-step sequencing guidance notes that staged loading can reduce starting requirements. Your designer must also evaluate subsequent restarts, not only the first transfer.
Ask for the assumptions with the recommended model
Request a sizing report identifying the selected fuel and output rating, voltage and phase, included and excluded loads, starting sequence, site derating, and defined expansion allowance. “Add 20%” is not a substitute for documenting what that allowance is meant to cover.
Use the phase and voltage guide to check compatibility. Restaurants should also use the food-service operating plan so the load list includes the systems needed to serve safely.
Connected guidance
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