Generator sizing starts with the electrical loads you need to run, not the generator’s headline number. List the equipment, starting demand, voltage, phase, fuel and environment before comparing commercial generators, standby generators or marine generators.

1. List running watts and starting watts

Record the continuous running watts for every load that may operate at the same time. Motors, compressors, pumps, welders and refrigeration equipment can require substantially more power during startup. Use the manufacturer’s nameplate or installation documentation rather than guessing.

2. Add the loads that operate together

Decide which equipment must start simultaneously and which can be sequenced. Add the running loads, then allow for the largest starting event. A qualified electrician should confirm the final calculation and whether the installation needs load management or staged starting.

3. Match voltage, phase and connections

Check single-phase or three-phase requirements, output voltage, frequency, receptacles, hardwire options and transfer-switch compatibility. A generator with enough kW can still be unsuitable if its voltage or phase does not match the equipment.

4. Choose the fuel and operating environment

Compare natural gas, propane, diesel or gasoline based on availability, runtime, storage and service. For a workshop or event, consider noise, weather protection, mobility and refueling. For a boat, account for marine-rated equipment, cooling, exhaust, ventilation, corrosion and installation clearances.

5. Verify transfer-switch and code requirements

Standby installations may require an automatic transfer switch and permits. Portable or event installations still require correct grounding, cable sizing, ventilation and safe fuel handling. Have a qualified installer verify the complete system and local code requirements before purchase.

Generator sizing checklist

  • Running and starting loads are documented.
  • Voltage, phase and frequency match the equipment.
  • Fuel, runtime, noise and environment are suitable.
  • Transfer-switch and connection requirements are confirmed.
  • Installation and local-code requirements are reviewed.

Review current listings in commercial generators, standby generators and marine generators. Product specifications and availability should be confirmed on the individual listing.