How generator sizing works
- Running watts. Add the power every appliance uses while running at the same time.
- Starting surge. Motors in refrigerators, pumps, furnace blowers and air conditioners draw two to three times their running watts for a second or two when they start. A generator must cover the running load plus the largest single surge, because motors rarely start at the exact same moment.
- Headroom. Generators should not run above about 80% of their rated output for long periods. The calculator sizes for whichever is larger: running watts ÷ 0.8, or running watts plus the largest surge.
Appliance wattage chart
Typical figures. Check the nameplate on your appliance for exact values, especially for well pumps and air conditioners.
| Appliance | Running W | Starting W |
|---|
Generator size by home
| What you want to run | Running W | Recommended |
|---|---|---|
| Essentials: fridge, sump pump, furnace blower, lights, TV and internet, microwave | 4,100 W | 7.5 kW portable |
| Essentials plus 3-ton central AC with soft start | 7,600 W | 9.5 kW portable |
| Whole house: AC, electric water heater, range, dryer, well pump and the rest | 24,875 W | 26 kW+ standby |
Common sizes: portable generators of 3.5, 5, 7.5, 9.5 and 12 kW, and home standby generators of 14, 18, 22 and 26 kW.
Portable or standby?
- Portable generators (3.5–12 kW) cost less and run on gasoline or dual fuel. You wheel them out, start them by hand and connect through a transfer switch or interlock kit. They suit essentials during occasional outages.
- Standby generators (14–26 kW for most homes) are permanently installed, run on natural gas or propane, and start automatically within seconds of an outage. They can run central AC and most of a house, but cost several times more installed.
Safety and connection
- Never run a generator indoors, in a garage or near windows. Carbon monoxide from generators kills people every year. Keep it at least 20 feet from the house with the exhaust pointing away.
- Never plug a generator into a wall outlet ("backfeeding"). It can electrocute utility workers. Use a transfer switch or an interlock kit installed by an electrician.
- Install battery carbon monoxide alarms on every floor.
Frequently asked questions
What size generator do I need to run a house?
Essentials such as the fridge, sump pump, furnace blower, lights and internet need about 7.5 kW portable. Adding central air pushes most homes to 9.5 kW portable, and running the whole house with electric appliances usually needs more than 26 kW of standby capacity, or a 22–26 kW standby unit with load management.
Will a 7,500 watt generator run central air?
Often a 2- to 3-ton unit, if few other loads are running and the AC has a soft-start kit. Without soft start, the AC's starting surge alone can exceed 9,000 watts.
What size generator do I need for a refrigerator?
A refrigerator runs on about 700 watts but needs about 2,200 watts to start. A 3,500 watt generator runs it with room for lights and a phone charger.
Is it bad to oversize a generator?
Not dangerous, but it costs more and burns more fuel. Running a generator at very light load for long periods can also cause carbon build-up in some engines.
What is the difference between running and starting watts?
Running watts are the steady power an appliance uses. Starting watts are the brief surge a motor needs to get going, usually two to three times higher.
These results are estimates. Have a licensed electrician size the transfer switch and confirm loads from appliance nameplates.