Generator Sizing Calculator
This generator sizing calculator adds up the running and starting watts of the circuits you check below and maps the total to a recommended kW range for a St. Louis-area standby generator — no email required. It's a planning tool, not a substitute for a licensed electrician's on-site load calculation.
Generator Sizing Calculator: Check What You Want Covered
Estimated Load
0 W
Recommended Generator Size
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How the Recommendation Is Calculated
The calculator sums the running watts of every circuit you check, then adds the single largest starting-watt spike among your selections on top of that total — the worst-case moment when one motor kicks on while everything else is already running. That combined number is rounded up to the nearest common standby generator tier, since undersizing is the more expensive mistake.
Codes & Standards
Final load calculations for permit purposes follow NFPA 70 (National Electrical Code), and general outage-preparedness planning guidance is available from Ready.gov.
For the reasoning behind these wattage figures and sizing tiers, see the full generator sizing guide. Once you have a target kW, the cost calculator gives a budget range and the Generac vs. Kohler guide compares brands at that size.
Sizing Calculator FAQ
How accurate is this generator sizing calculator?
It gives a reliable planning-stage kW range based on standard appliance wattage figures, but your panel's actual amperage, wiring condition, and any unusual large loads still need a licensed electrician's on-site load calculation before you order equipment.
Why does the calculator ask about starting watts, not just running watts?
Motors like AC compressors and well pumps briefly draw 2-3x their running wattage the instant they start. A generator sized only for running watts trips the moment two of those motors start close together, so the worst-case starting spike has to be built into the total.
Do I need to enter my email to use this calculator?
No — the sizing worksheet runs entirely in your browser and doesn't require an email address or any personal information to see your recommended kW range.