Standby Generator Guide Standby sizing basics

Decision tree · contractor-ready

What size generator?

One-tap presets, running vs. peak (largest motor start), fuel derate, panel amps and load management — mapped to common kW classes with ATS framing. Educational, not an engineered design.

Contractor mode off

Start from a preset (then edit)

Fuel (remembered)
Plan load management? (remembered)

Bring a photo of the main breaker for the site visit. Starting watts (motor surge) often drive the class more than running watts alone.

Loads on the generator

Decision tips

  • Prioritize life-safety and food before comfort loads
  • Motors (AC, well, sump) need start capacity beyond running watts
  • Essential-circuit ATS vs 200 A service-entrance ATS changes cost and size
  • Natural gas output is usually lower than the LP rating
  • Load-shed modules can shrink the generator class you need