To size a whole house generator, total the running wattage of every circuit and appliance that will be covered, then check the largest simultaneous motor starting surge against the generator surge capacity. For most American single-family homes with gas heat, central AC, a gas range, and standard household loads, a Generac 22kW generator covers the entire 200-amp panel. Homes with electric heating, heat pumps, or EV charging need to step up to 24kW or 26kW for the same whole-house coverage.
Step 1: List Every Load
Write down every circuit in your electrical panel that the generator will power. Include HVAC (central AC and furnace blower), kitchen appliances (refrigerator, dishwasher, microwave), water heating, laundry (washer and dryer), water pump if on a well, sump pump, all lighting circuits, and outlet circuits throughout the home. Also include any dedicated circuits for office equipment, garage tools, or other specialized equipment.
Step 2: Determine Running Watts for Each Load
Running watts is the continuous power each appliance draws during normal operation. Central AC (3-ton): 3,500 watts. Central AC (4-ton): 4,500 watts. Heat pump (3-ton, heating mode): 4,000 watts plus electric strip backup if equipped. Gas furnace blower: 500 watts. Electric furnace: 10,000 to 15,000 watts. Refrigerator: 200 watts. Electric water heater: 4,500 watts. Electric dryer: 5,500 watts. Well pump (0.5 HP): 1,000 watts. Sump pump (0.5 HP): 1,050 watts. Lighting throughout home: 400 watts. Computers and entertainment: 500 watts.
Step 3: Calculate Total Running Load
Add the running watts for all loads that may operate simultaneously during a peak demand period. In a gas-heat home with 3-ton AC and standard appliances, a realistic simultaneous peak is approximately 8,000 to 12,000 watts. In an all-electric home with heat pump and electric backup heat, the simultaneous peak can exceed 20,000 watts. The generator must have a rated output above this peak running load.
Step 4: Check Motor Starting Surge
Motors draw 2 to 3 times their running wattage for 1 to 3 seconds on startup. The worst-case surge scenario in a typical home is the AC compressor starting while the well pump and sump pump also cycle on. A 3-ton AC compressor starting surge is approximately 10,000 to 15,000 watts. The generator surge capacity must exceed this worst-case simultaneous surge. Generac Guardian models are rated with sufficient surge capacity for their output class, but this check confirms the sizing is appropriate for your specific load combination.
Step 5: Apply the Standard Sizing Result
Gas heat, up to 4-ton AC, gas range, standard appliances: 22kW Generac Guardian. Gas heat or heat pump, 4-ton to 5-ton AC, electric range or EV charger added: 24kW or 26kW Guardian. All-electric home with heat pump, electric backup heat, electric range, EV charger: 26kW or 28kW Guardian. When in doubt, size up. The incremental cost between adjacent generator sizes is typically a fraction of the total installation cost, while undersizing creates ongoing load management headaches for the life of the system.
Browse the full Generac home standby generator collection from 10kW to 28kW or call (800) 674-9019 for a free load calculation from our licensed master electrician team. Free Freight Insurance Included.