Recommended size: 5,000–22,000W
For whole-house backup including central air conditioning, you need a 17,000 to 22,000W standby generator. For whole-house backup without central AC, a 12,000W large portable handles it. For essentials only (fridge, freezer, sump pump, well pump, furnace fan, lights, WiFi), a 5,000 to 7,500W portable is the right size. The exact number depends on your home's square footage, whether you have a well pump or electric water heater, and how much you want running at once.
Affordable dual fuel option with CO Alert safety shutoff and MX2 power boost technology. Runs on gas or propane for flexible fuel options during outages.
Backup generator sizing comes down to one question: do you want to keep the whole house running, or just the circuits you cannot live without during an outage. Most homeowners do not need a generator that powers everything. They need one that keeps food cold, water flowing, the furnace fan turning, and a few rooms lit until the grid comes back. That job is done by a 5,000 to 7,500W portable. Running the entire house, central AC included, is a different category that requires a permanently installed standby unit in the 17,000 to 22,000W range. The sizing tables and worked example below show exactly where your home lands, and the on-site calculator turns your specific appliance list into a precise wattage number.
Use square footage as a starting point, then adjust for your specific loads. A home under 1,500 sq ft with gas heat and gas water heating can run its essentials on a 5,000W generator. A 1,500 to 2,500 sq ft home with a well pump or electric furnace fan needs 7,500 to 10,000W for essentials. For whole-house backup excluding central AC, a 2,000 to 3,000 sq ft home needs a 12,000W large portable. To back up the entire home including central air conditioning, a typical 2,000 to 3,500 sq ft house needs a 17,000 to 20,000W standby unit, and a larger home with two AC systems needs 22,000W or more. These are starting points. An all-electric house (electric range, electric water heater, heat pump) lands one tier higher than a gas-equipped house of the same size.
Add the running watts of everything you want on at once, then add the single largest starting surge on top. Refrigerator: 150W running, 1,200W starting. Chest freezer: 100W running, 1,000W starting. 1/2 HP well pump: 1,000W running, 2,100W starting. Sump pump: 800W running, 2,000W starting. Furnace blower fan: 600W running, 1,200W starting. Lights throughout: 400W. WiFi, modem, phone chargers: 100W. Microwave: 1,000W. That essentials list totals roughly 3,150W running. Add the well pump's 2,100W surge and you peak near 5,250W, so a 7,500W generator covers it with headroom. Add a 3.5-ton central AC (5,800W running, 11,000W+ starting) and the peak jumps past 16,000W, which only a standby unit handles.
A standby generator is permanently installed beside the house, wired to your electrical panel through an automatic transfer switch, and fueled by your natural gas line or a large propane tank. It starts itself within seconds of an outage and runs unattended for days, which makes it the only practical choice for whole-house backup that includes central AC. A large portable (10,000 to 12,000W) is far cheaper, covers nearly every circuit except central AC, and is moved out of the garage and connected by hand through a manual transfer switch. The trade-off is involvement: a portable needs refueling every 8 to 12 hours and you start it yourself. For essentials-only backup, a 5,000 to 7,500W portable is the sensible, lower-cost answer for most households.
Consider a 2,200 sq ft house with a gas furnace, electric water heater, well pump, and a 3-ton central AC. During a summer outage the owner wants the fridge, freezer, well pump, furnace blower, lights, WiFi, and the central AC. Running load: fridge 150W, freezer 100W, well pump 1,000W, furnace blower 600W, lights 400W, WiFi 100W, central AC 4,800W. That is 7,150W running. The largest starting surge is the central AC at roughly 11,000W. Peak demand lands near 17,800W, so this home needs a 20,000 to 22,000W standby generator. Drop the central AC from the list and the peak falls to about 9,200W, which a 12,000W large portable covers, cutting the cost dramatically. This is why deciding whether central AC is on the list is the single biggest sizing decision.
A transfer switch connects the generator to your home's electrical panel safely and is required for any whole-house or multi-circuit backup. A manual transfer switch (used with portables) costs $200 to $400 for the hardware plus $300 to $500 for a licensed electrician, and lets you power chosen circuits by flipping breakers instead of running extension cords. An automatic transfer switch ships with standby generators and switches the whole panel over by itself the moment grid power drops. Without a transfer switch, connecting a generator to house wiring backfeeds the grid, which is dangerous to utility workers and illegal in most areas. Size the transfer switch to your panel amperage, and use our transfer switch calculator to confirm the right rating.
For a whole house including central AC, you need a 17,000 to 22,000W standby generator professionally installed. For whole-house backup without central AC, a 12,000W large portable covers nearly every circuit. For essentials only (fridge, freezer, well pump, sump pump, furnace fan, lights, WiFi), a 5,000 to 7,500W portable is the right size.
A 2,000 sq ft house running essentials (fridge, freezer, well or sump pump, furnace blower, lights, WiFi) needs a 7,500W portable generator. To back up the whole house excluding central AC, step up to a 12,000W large portable. To include central air conditioning, a 17,000 to 20,000W standby generator is required.
A typical whole house draws 5,000 to 7,500W for essential circuits, 9,000 to 12,000W for nearly all circuits except central AC, and 17,000 to 22,000W when central air conditioning is included. The exact figure depends on whether the home is all-electric, has a well pump, and how many large appliances run simultaneously.
A 12,000W generator runs nearly every circuit in an average home except central air conditioning. It handles the refrigerator, freezer, well pump, sump pump, furnace blower, water heater, microwave, lights, and electronics together. For central AC on top of those loads, you need a 17,000W or larger standby generator.
A portable in the 5,000 to 12,000W range is enough for essentials or whole-house backup excluding central AC, and costs far less. A standby generator is necessary only when you want the entire house including central AC to run automatically and unattended for multi-day outages. Most households are well served by a portable plus a manual transfer switch.
An all-electric house (electric range, electric water heater, heat pump or electric furnace) needs roughly one tier more capacity than a gas-equipped house of the same size. Plan on 10,000 to 12,000W for essentials and 22,000W or more for whole-house backup, because electric heating and water heating add large continuous loads.
DuroMax
Affordable dual fuel option with CO Alert safety shutoff and MX2 power boost technology. Runs on gas or propane for flexible fuel options during outages.
Westinghouse
Dual fuel flexibility with CO sensor safety shutoff. Transfer-switch ready with remote electric start and 6.6-gallon tank.
Generac
Powers most essential home circuits during an outage. Electric start and dual fuel flexibility from a trusted brand.
DuroMax
Enough power for most home backup scenarios. 50A outlet for direct panel hookup with a transfer switch.
Westinghouse
Transfer-switch ready with remote start. Excellent runtime from a large 6.6-gallon tank and reliable Westinghouse build quality.
Generac
Purpose-built for home backup with COsense carbon monoxide safety shutoff. Long 11-hour runtime at half load.
Generac
Market-leading whole-home standby generator. Includes 200A transfer switch. Powers everything including central AC automatically when the grid goes down.
Champion
Strong Generac alternative with 200A transfer switch included. Competitive pricing and quiet 67 dB operation for whole-home backup.
Select your specific appliances in our sizing calculator to get a precise wattage calculation and personalized generator match.
Use the Generator Sizing Calculator