Generator Sizing for Operations
Size a backup generator to cover motor starting surge, not just steady-state running load, which can leave a generator unable to start its largest motor.
Motor starting surge
Fuel cost
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Based on your results, these related decisions are available.
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How this works
The generator must cover all running load plus the extra surge the largest motor draws at startup, an electric motor can draw two to three times its running current for a brief moment. Its running contribution is replaced by its starting-surge contribution in the total.
Key terms
- Starting surge factor: the multiple of running draw a motor pulls at startup, the highest instantaneous demand the generator will ever see.
Assumptions
- Only the single largest motor's surge is modeled, not simultaneous multi-motor starts
Limitations
- Not a full electrical one-line or transient-response study
Sources
Standard backup-generator sizing practice (motor-starting surge accounted for separately from steady-state running load).
Worked example
Sizing to the 200 kW running load alone misses that the largest 50 kW motor draws 150 kW at startup (3x surge). Replacing its 50 kW running contribution with 150 kW pushes the true requirement to 300 kW, a 100 kW undersizing the running-load-only approach would miss.
Common use cases
- Sizing a backup generator for a facility with large motor loads
- Checking whether an existing generator can actually start critical equipment
- Budgeting fuel cost for planned outage events
Frequently asked
- Why does motor starting surge matter more than steady-state load?
- Motors draw several times their running current for a brief moment at startup; sizing to steady-state load alone can undersize a generator that then fails exactly when equipment tries to start.
- What is critical load versus total facility load?
- Critical load is the subset of equipment that must stay powered during an outage; sizing to total facility load instead is usually far more generator than actually needed.
- Does this include fuel cost for extended outages?
- Yes, runtime hours and fuel cost are both inputs, so the result includes the operating cost of running the generator for your expected outage duration, not just its sizing.
- Can this also size a generator used for demand-charge management, not just backup?
- The demand-charge-savings input lets this model that use case too, but the surge-sizing logic for critical load and motor starts still applies underneath either use.