Generator sizing fundamentals
A generator must support the intended running load and the changes that occur as equipment starts, stops, or reconnects. Sizing therefore combines a load inventory with checks of the generator’s electrical ratings, short-term response, and installed operating conditions.
Define the service
First identify which loads must operate and which may wait. Essential-circuit backup, a managed whole-building system, and unrestricted operation of every appliance are different requirements. Include recurring automatic loads, such as refrigeration and pumps, rather than assuming they will remain off during an outage.
Make an equipment inventory using model-specific data. Note voltage, phase, running input, starting information, and operating priorities. Generic wattage charts help identify missing loads, but the figures remain estimates until checked against the actual equipment. [3]
Check steady and starting loads
Sum the loads expected to run together, then examine each important starting event with the loads already operating. A motor’s total starting demand includes its own running demand; adding both in full can double-count that part of the load. Multiple starts may overlap unless controls reliably separate them.
Power factor connects real and apparent power: kW = kVA × power factor. Low power factor can require more current than a simple watt total suggests. Nonlinear loads and an uneven distribution of loads also affect the electrical assessment. A rated kW figure alone does not describe every alternator limitation. [1]
Match the generator to the site
Compare the assessment with the relevant running rating, motor-starting capability, and permitted voltage and frequency deviations. The duration of a starting event matters as well as its peak. A short advertised surge rating is not a promise that a difficult motor will accelerate successfully. [2]
Use the rating for the actual fuel and the manufacturer’s corrections for the site. Document planned future loads and any allowance for uncertain inputs instead of applying a universal percentage margin. Final building-system selection should also confirm fuel delivery, transfer equipment, and the behavior of managed loads. [1]
Sources and further reading
- Cummins — Application manual T-030 ↗powersuite.cummins.com
- Cummins — Understanding transient performance ↗www.cummins.com
- Honda — Determining generator power requirements ↗powerequipment.honda.com
Use the manual and nameplate for the exact model. Local requirements and site conditions take precedence over general examples.