How to Size a Whole-House Standby Generator

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Start with the loads you must run

Walk through your home and write down what has to keep working during an outage. Think heating or cooling, the fridge and freezer, a well pump or sump pump, lights, the router, a range or water heater if they are electric.

For each item, find its own number. Look at the nameplate or label on the appliance, the power brick, or the maker's spec page. For plug-in items, a plug-in watt meter shows what they draw in your home. Do not guess, because draws vary widely by model.

• Running watts: what the item draws while it runs
• Starting watts or locked-rotor amps: the brief surge some motors need to start
• Voltage: whether the item runs on 120 or 240 volts
• Whether it cycles on and off or runs steadily

Running watts vs starting watts

Use the generator’s rated continuous output for loads that stay on. Have your installer check motor-starting capability and the effect of loads switching on together; do not assume every standby model has a separate advertised peak-watt rating.

For example, if your loads add up to 9,000 running watts and the largest motor on your list needs 6,000 extra watts for a moment to start, you need a unit that can deliver 15,000 watts during that start and at least 9,000 watts continuously. Then size up from there so the generator is not working at its limit.

Central air conditioners and well pumps are often the loads that decide the size. Check their nameplates for locked-rotor amps, and ask your electrician whether a soft-start device makes sense for yours.

Whole house or essential circuits

A whole-house unit is sized to run nearly everything at once. An essential-circuit setup covers only chosen circuits, so it can be smaller.

Many standby systems also offer load management, which briefly cuts power to a big load like an air conditioner or electric dryer so the rest of the house stays on. That can let a smaller generator cover a larger home. Ask your electrician which approach fits your panel and your list.

• Whole house: largest unit, least effort during an outage
• Essential circuits: smaller unit, you choose what runs
• Managed loads: big loads take turns so the unit is not overloaded

Fuel type changes the rating

Standby generators usually run on natural gas or propane. Many list a different output for each fuel, so read the spec sheet for the number on the fuel you will actually use.

Your gas meter or propane tank must be able to supply the generator at full load along with your other gas appliances. Your installer or gas utility can check the meter and line size.

Installation and safety

A standby generator connects to your home through a transfer switch installed by a licensed electrician. The electrician should also run a load calculation, which is the best check on your sizing.

Placement matters. Generators must be outdoors, away from windows, doors, and vents, because exhaust contains carbon monoxide. Follow the maker's clearance rules, and ask your electrician and local building department about permits.

Compare a standby option with your load plan

The linked Champion aXis home standby system is a permanently installed option to compare with your load calculation, not a preset size for every home. Have your installer check its continuous output on your chosen fuel and its motor-starting capability against your own loads. The worked example above is arithmetic, not a test or a capacity claim for this model.

Check seller delivery eligibility for your location before ordering. Confirm the fuel supply, transfer-switch configuration, installation requirements, and any additional load-management modules with your installer. A whole-house transfer switch does not mean the generator can run every appliance at once.

Sources

Recorded AI check: Oct 9, 2026. The sources below may include later additions. AI checks compare factual numbers and checkable claims with web sources; they do not verify every statement.

• [PDF] Standby Generator Sizing Guide
• Standby Generator Sizing Calculator: How to Choose the Right kW
• Home Standby Generator Sizing: Find Your Ideal Generator Size - Champion Power Equipment
• What Size Generator Do I Need? | Constellation Home
• What size generator do I need for my home? | KOHLER Generators

Best inverter generator for a home outage compares units from the makers' numbers. Every guide in this class is in Portable generators.

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