Generator sizing
Generator sizing is the process of matching a generator’s electrical capacity and transient performance to the loads it must supply. A useful assessment includes steady running demand, motor starting events, the order in which loads connect, and the rating available on the chosen fuel. [1]
In this topic
Generator sizing fundamentals
The main inputs for sizing a generator: simultaneous loads, starting events, kW, kVA, fuel ratings, and site conditions.
Generator sizingCalculating running load
Build a running-load inventory using electrical input, realistic simultaneous operation, and clearly labeled estimates.
Generator sizingCalculating motor starting load
Understand starting watts, locked-rotor current, starting kVA, surge duration, and the effect of load sequencing.
Generator sizingLoad management
How sequencing, load shedding, and priorities help a generator serve selected loads within its capacity.
Where to start
Begin with a list of the equipment that matters during an outage. Record its electrical input, voltage, operating pattern, and any starting requirements. A building’s floor area or monthly energy use does not describe the instantaneous load a generator must accept.
The fundamentals article explains the overall decision. Calculating running load covers the steady total; calculating motor starting load covers the temporary demand. Load management explains how selected equipment can be delayed or withheld when capacity is limited.
From estimate to selection
A wattage total is a planning input. It does not establish that a particular generator can start a compressor, hold an acceptable voltage, or deliver the same output on every fuel. For a building system, the installer or designer checks the load assessment against manufacturer performance data and the proposed transfer and fuel equipment. Keep estimated values clearly identified until they can be replaced with equipment-specific information. [1] [2]
Sources and further reading
- 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.