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What size generator do I need?
Pick what has to run at the same time. The size comes from the method generator makers publish: add up the running watts, then add the one biggest starting surge. You see every step, and real generators that cover it.
Generators that cover it
Every number comes from the maker's own page or spec sheet. If a maker doesn't publish a number, the card says so instead of guessing.
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Where the numbers come from
The method is the one on Champion Power Equipment's "Choosing a Generator" sheet:
“RUNNING WATTS NEEDED: Total the Running Watts of ALL ITEMS to be powered by the generator.”
“STARTING WATTS NEEDED: Add the HIGHEST SINGLE Starting Watt to Total Running Watts needed.”
Champion's own example: 3,880 running watts, plus the refrigerator's 2,200 starting watts, means a generator rated at least 3,880 running and 6,080 starting watts. Generac publishes the same formula: total running watts plus the highest single starting burst.
Sources: Champion, Choosing a Generator (PDF); Generac, What Size Generator Do I Need? Retrieved October 30, 2026.
The appliance watts
The starting numbers are Champion's Generator Wattage Chart (the table on this page). Champion gives a range for each appliance. The tool starts at the top of each range so the answer isn't short, and your appliance's nameplate replaces it. Items marked "—" need no extra starting watts, per Champion.
Source: Champion, Generator Wattage Chart: Common Appliances & Tools. Retrieved October 30, 2026.
Home standby
For a standby generator wired into the house, Champion's standby guide uses the same two steps, then: “Finally, add a 20% safety cushion to your total.” The tool does that in standby mode. Champion's worked example there subtracts the motor's running watts before adding its surge, which gives a smaller number than its own written rule. The tool follows the written rule.
The final size for a whole house is a dealer's job. In Champion's site survey, an authorized dealer checks your home's electrical connections and fuel needs and gives a recommendation. Generac says: “A professional site survey is the only way to ensure full accuracy.”
Source: Champion, What Size Home Standby Generator Do I Need? (published October 16, 2026). Retrieved October 30, 2026.
Safety
The safety lines are quoted from the CPSC and the CDC. The CPSC also says: “DO ask retailers for a portable generator with a safety feature to shut off automatically when high CO concentrations are present.” That's why each card shows the maker's CO shut-off, if it has one.
Sources: CPSC Safety Alert 5123; CPSC468; CDC, Avoiding Carbon Monoxide Poisoning. Retrieved October 30, 2026.
Questions
How do I figure out what size generator I need?
Add the running watts of everything that will run at the same time. Then add the single biggest starting watts among them, not all of them. In Champion's example, 3,880 running watts plus a refrigerator's 2,200 starting watts means at least 3,880 running and 6,080 starting watts.
What's the difference between running and starting watts?
Champion's definition of running watts: “The steady power a device needs to keep working after it’s started.” Starting watts: “The extra power some devices need to turn on due to their components.” Motors need them: refrigerators, pumps, air conditioners. Lights, TVs and microwaves don't, per Champion's chart.
What size generator do I need to run a house?
Champion's standby guide adds the running watts, adds the biggest single starting watts, then adds a 20% cushion. It says most homes need an 8.5 kW to 26 kW home standby generator. Champion and Generac both point to a dealer's site survey for the final size.
How far from the house does a generator have to be?
The CPSC says to run it outside only, at least 20 feet away from the house, with the exhaust pointed away from the home and other buildings. The CDC says at least 20 feet from doors, windows, and vents. Never run one inside a home, garage, crawlspace or shed, even with fans on or doors and windows open (CPSC).
Does a generator make less power on propane?
On most dual fuel models, yes, and the makers publish both numbers. FIRMAN rates its H08052 at 8,000 running watts on gasoline and 7,250 on propane. Pick your fuel above and the list uses that fuel's numbers.
Can I plug a generator into a wall outlet to power my house?
No. The CPSC calls it “backfeeding” and says it is extremely dangerous: it puts utility workers and neighbors on the same transformer at risk of electrocution. House circuits connect through a transfer switch. Generac, for one, says its GP8000E works with its HomeLink manual transfer switches “for safe home backup power.”
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