Diagnostic guide
What Size Standby Generator Do You Need for Central AC?
Last reviewed · Reviewed by the Electrician Pro technical team
Short answer
Most homes with a 4-ton or 5-ton central air conditioner require an 18kW to 24kW standby generator equipped with a soft-starter module, or a 14kW to 16kW unit for smaller single-zone cooling systems.
Understanding Starting Amps vs. Running Amps
Air conditioning compressors require substantial locked-rotor starting current (LRA) to start up—frequently three to five times higher than their normal running wattage. Sizing a generator requires calculating these startup spikes.
- A typical 4-ton AC draws approximately 3,500 running watts but up to 15,000 starting watts
- Soft-start kits reduce compressor startup current spikes by up to 65%, allowing smaller generators
- Refrigerators, well pumps, and lighting circuits must be added to total running load calculations
Whole-House vs. Essential Circuit Coverage
Homeowners can choose between whole-house backup power systems or essential-circuit setups based on their budget and lifestyle requirements.
- Whole-house 20kW–26kW systems power dual AC units, pools, ovens, and lighting without interruption
- Essential-circuit 12kW–16kW systems focus on refrigeration, one central AC unit, and critical medical devices
- Smart load management switches automatically shed non-critical circuits when AC compressors cycle on
Fuel Supply Considerations in Arizona
Standby generators connect directly to residential natural gas lines or on-site liquid propane (LP) tanks, eliminating the need to store gasoline during summer emergencies.
