1. Backup target
Is the proposal designed for critical loads only, normal whole-house comfort, or every connected load with little/no management?
Generator sizing
What size standby generator do you actually need?
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.
Research & editorial responsibility: Standby Generator Resource. Technical claims are checked against the primary sources that actually support the model, product family, location or rule being discussed. Updated September 3, 2026.
How we research →Direct answer
The shortest useful sizing method.
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.
“Am I being oversized?”
Make the installer explain the design target—not just the kW number.
Homeowners often ask this after receiving several quotes that range from “essentials” to a large whole-house generator. A larger unit is not automatically an upsell, and a smaller one is not automatically smarter. The useful question is what loads and starting events the contractor designed around.
2. Actual fuel
What is the generator's published output on natural gas or propane at this exact model—not just its model-name kW?
3. Motor start
Which HVAC, well or sump-pump starting event is driving the recommendation, and what source data was used?
4. Load management
Which large loads could be delayed or prioritized instead of automatically increasing generator capacity?
Ask for the load calculation, sizing worksheet or at least a written list of the assumed outage loads. If two contractors recommend different sizes, compare their assumptions before deciding one is wrong.
Sizing method
Seven checks before choosing a kW number.
01
Choose what must stay on
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.
02
Record real equipment data
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).
03
Add simultaneous running load
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.
04
Account for the largest motor start
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.
05
Check fuel-specific generator output
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.
06
Decide whether to manage loads
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.
07
Verify the result professionally
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
Why a generator can look big enough on paper and still struggle.
Running watts
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.
Starting surge
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
“2,000 square feet” is not a generator size.
Essentials-focused home
Typical load profile
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.
Comfort + central AC
Typical load profile
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.
All-electric / large home
Typical load profile
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
Check these before accepting a sizing recommendation.
Central AC / heat pump
Get the exact outdoor-unit data. Compressor starting demand can dominate the calculation.
Well & sump pumps
Pump horsepower and starting characteristics matter more than a generic 'pump' checkbox.
Electric resistance heat
Heat strips, electric furnaces and some auxiliary heat can be extremely large continuous loads.
Electric water heating
A conventional electric tank is a meaningful load; instantaneous electric water heating can be much larger.
Range, dryer & EV charging
These are often candidates for load management rather than automatically increasing generator size.
Medical / critical loads
Identify equipment that cannot be interrupted and discuss its power-quality and transfer requirements with the appropriate professional.
Common questions
What size generator do I need for a 2,000-square-foot house?
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.
Can a standby generator run central air conditioning?
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.
Is a bigger generator always better?
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.
Can load management let me buy a smaller generator?
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
Check manufacturer tools, then verify the actual home.
Cummins — home generator sizing calculator ↗Generac — home standby buyer's guide & wattage worksheet ↗
Online calculators use assumptions. Replace generic wattages with actual nameplate data whenever possible and have the final design checked before purchase.