Start with the load, not the generator label
Generator sizing has two jobs: cover the watts that run continuously and survive the brief surge when the largest motor or compressor starts. The safest planning number comes from the actual nameplates/manuals for your equipment. Typical tables are useful for screening, but they are not a substitute for the equipment data in your house.
| Load | Typical running range | Possible starting surge | What changes it |
|---|---|---|---|
| 3-ton central AC compressor/fan | Often ~3,000–4,000W | Can be 2x or more without soft start | SEER, compressor, LRA/RLA, blower |
| Gas furnace/air-handler blower | 400–1,000W | Motor surge | ECM vs PSC motor |
| Refrigerator | 150–400W | Up to ~1,200W | Compressor |
| Other essentials | 500–2,000W | Varies | Pumps, lights, outlets |
Use the site's Wattage Lab to build the simultaneous load, then leave headroom rather than planning to run a generator continuously at its ceiling.
Sizing scenarios
Read the HVAC nameplate
Look for voltage, minimum circuit ampacity, compressor rated/load amps and locked-rotor amps where shown. The generator has to survive startup, not merely the steady-state cooling draw.
Soft start changes the conversation
A properly selected and installed AC soft starter can reduce compressor starting current substantially. It does not reduce the normal running load to zero, and it must be compatible with the HVAC equipment.
240V is mandatory for most central AC
A large 120V inverter generator is not a substitute for a 120/240V generator if the condenser requires 240V.
Think in load priorities
During an outage, you may choose to run central AC while avoiding electric water heating, range, dryer, EV charging and other large loads. A transfer/load-management plan can make a portable generator practical without pretending it is utility service.
What to shop for
8,000–10,000W 120/240V class
A starting shopping category when a soft-started 3-ton AC and modest critical loads are in the plan.
10,000–12,000W+ tri-fuel class
More headroom for central AC plus pumps, refrigeration and household circuits; size from the intended fuel's running watts.
Generator safety is not optional
Portable generators are combustion engines. The U.S. Consumer Product Safety Commission says to operate them outside only, at least 20 feet from the home, with exhaust pointed away from the home and other occupied buildings. A garage, shed, carport, porch, basement, crawlspace, or partly enclosed area is not made safe by opening doors or windows. Install and maintain carbon-monoxide alarms on every level of the home and outside sleeping areas.
For home circuits, do not improvise a connection and never backfeed a house through a receptacle. A transfer switch or properly approved interlock/inlet arrangement should be selected and installed in accordance with the generator manual, electrical code, utility requirements, and qualified professional guidance. For rain or snow, follow the manufacturer's weather instructions rather than moving a running unit into an enclosure.
Output ratings also change by fuel. Propane and natural gas commonly produce less power than gasoline, so size the system around the fuel you actually expect to use. Verify the appliance nameplate, motor starting behavior, voltage, receptacle type, and generator manual before purchase.
FAQ
Will a 7500-watt generator run a 3-ton AC?
Sometimes with a soft starter and limited other loads, but it can also be inadequate. The compressor startup data and generator surge capability decide it.
Does a soft starter mean I can buy a tiny generator?
No. It reduces startup current; the generator still has to supply the AC's running load plus everything else.
Can I power central AC through extension cords?
Central HVAC is normally part of the home's electrical system and should be supplied through properly designed transfer/inlet equipment, not improvised cords.
Primary spec & safety sources
- U.S. CPSC — carbon-monoxide and portable-generator safety
- Westinghouse — WGen11500TFc
- DuroMax — XP13000HXT specifications
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