Generator Size Calculator
Estimate generator running and starting capacity requirements.
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What this tool does
Estimate generator running and starting capacity requirements. Enter surge additional to running load, not total starting watts. Simultaneous motor starts may require more power.
How to use the Generator Size Calculator
- Enter or select total running watts.
- Enter or select largest extra starting surge watts.
- Enter or select reserve allowance (%).
- Read the calculated result; change any measurement to compare alternatives.
Formula
running capacity=running×(1+reserve/100); peak capacity=(running+extra starting surge)×(1+reserve/100)
- running
- Total running watts
- surge
- Largest extra starting surge watts
- reserve
- Reserve allowance (%)
Enter surge additional to running load, not total starting watts. Simultaneous motor starts may require more power.
Worked example
For generator size calculator, the following measurements illustrate the exact method: Total running watts: 4000; Largest extra starting surge watts: 2000; Reserve allowance (%): 20.
Inputs
- Total running watts4000
- Largest extra starting surge watts2000
- Reserve allowance (%)20
Result
- Peak capacity required (watts)7,200
- Running capacity (watts)4,800
Results explained
- Peak capacity required (watts)
- Peak capacity required (watts) from the formula above. Enter surge additional to running load, not total starting watts. Simultaneous motor starts may require more power.
- Running capacity (watts)
- Running capacity (watts) from the formula above. Enter surge additional to running load, not total starting watts. Simultaneous motor starts may require more power.
Frequently asked questions
running capacity=running×(1+reserve/100); peak capacity=(running+extra starting surge)×(1+reserve/100)
Enter surge additional to running load, not total starting watts. Simultaneous motor starts may require more power.
No. Electrical and HVAC results are estimates. Installation must comply with local codes and be checked by a qualified licensed professional.
Nameplate values, conductor temperature, power factor, efficiency, weather and actual installation conditions may differ from your entered assumptions.
Confirm units, manufacturer data and the formula's assumptions, then verify sizing with the equipment instructions and applicable local standards.