Generator sizing
Do not size a generator from square footage alone. The useful answer comes from the loads you want during an outage, their simultaneous running demand, the largest motor-starting surge, your fuel, and whether large loads can be managed.
Direct answer
For a first-pass homeowner estimate, list the loads you expect to run together, total their running watts, then account for the largest additional starting surge among motor-driven equipment. Cummins describes the same basic approach: total the running watts, then account for the highest starting requirement. Manufacturer sizing worksheets are planning tools, not substitutes for a site-specific load assessment.
Sizing method
Separate true outage essentials from convenience loads. Refrigerator/freezer, furnace blower, well or sump pump, selected lighting, internet and medical equipment create a very different load profile from central air, electric water heat, cooking, laundry, pool equipment and EV charging.
Use appliance and HVAC nameplates when possible. Running watts tell you continuous demand. Motors and compressors also have a much higher momentary starting demand, often shown as starting amps or locked-rotor amps (LRA).
Estimate what may actually operate at the same time during an outage. Do not simply add every breaker in the panel and assume every load is on at full demand simultaneously.
A common first-pass method is total simultaneous running watts plus the largest additional starting requirement. Central AC, heat pumps, well pumps and other motors can be the load that changes the generator class.
A dual-fuel generator may not produce the same rated output on natural gas and propane. Compare the manufacturer's rating on the fuel you will actually use, not just the largest number in the model name.
Load-management controls can temporarily delay lower-priority large loads so they do not all start together. That can reduce the generator size needed without giving up those appliances entirely.
Use the calculation as a planning range. Final selection should be checked against the exact generator's motor-starting capability, transfer-switch design, fuel supply, local electrical requirements and the actual equipment being backed up.
Running vs. starting power
The continuous power needed after equipment is operating. Resistive loads are relatively straightforward; homes with several electric heating/cooking loads can accumulate substantial continuous demand.
Motors and compressors can briefly demand much more power when starting. The largest motor event—often central AC or a pump—can determine whether the system starts cleanly or experiences a voltage/frequency dip.
Three very different homes
Refrigeration, gas/oil furnace blower, sump pump, lights, outlets, internet
Often substantially smaller than a true whole-home system; exact size depends heavily on pumps and motor starts.
Essentials plus central air conditioning and more general circuits
HVAC starting demand frequently becomes the sizing constraint. Load management or an approved soft-start strategy may change the required class.
Heat pump or resistance heat, electric water heater, range/dryer, multiple HVAC systems, well pump, EV charging
Can exceed typical air-cooled standby capacity unless large loads are intentionally managed; a larger liquid-cooled system may be considered.
Loads people underestimate
Get the exact outdoor-unit data. Compressor starting demand can dominate the calculation.
Pump horsepower and starting characteristics matter more than a generic 'pump' checkbox.
Heat strips, electric furnaces and some auxiliary heat can be extremely large continuous loads.
A conventional electric tank is a meaningful load; instantaneous electric water heating can be much larger.
These are often candidates for load management rather than automatically increasing generator size.
Identify equipment that cannot be interrupted and discuss its power-quality and transfer requirements with the appropriate professional.
Common questions
Square footage cannot answer this reliably. A 2,000-square-foot gas-heated home backing up essentials can have a dramatically smaller load than a similar-size all-electric home with central HVAC, electric water heating, cooking, a well pump and EV charging. Build the load list first.
Yes, when the system is sized for the AC's running and starting requirements and the generator manufacturer supports that motor-starting load. The HVAC nameplate and generator's published motor-starting capability matter more than a generic tonnage rule.
No. Buying unnecessary capacity increases equipment cost and can increase fuel demand. The goal is adequate continuous and starting capacity with appropriate reserve—not simply the largest unit that fits the budget.
Often, yes. A designed load-management system can prevent selected large loads from operating or starting simultaneously, reducing peak demand. The exact controls and compatible loads depend on the generator/transfer system.
Primary sizing resources
Online calculators use assumptions. Replace generic wattages with actual nameplate data whenever possible and have the final design checked before purchase.
Next decision