Whole-house generator sizing guide

Critical loads → running vs starting → headroom → portable vs standby path — then size your list on the free calculator.

Start with critical loads

Build the board before you shop:

• Refrigerator and freezer — continuous food protection
• Well pump — if you have private water
• Sump pump — storm and basement risk
• Furnace / heat source fan and any medical loads
• Lights, phones, and a microwave or small cooking load if needed
• Central AC — only if you truly need cooling during the outage (it often drives the class up)

Use nameplate running and starting watts (or LRA × volts for locked-rotor motors). Do not invent measured demos — if you only have estimates, treat them as placeholders until you verify on the appliance.

Running vs starting, diversity, and headroom

Running watts are continuous draw. Starting (surge) watts are the brief spike when a motor or compressor starts. Read that spike on the nameplate. Method:

1. Sum running watts for everything that will be on at once.
2. Add only the extra surge of the single largest starter (diversity: not every motor starts together).
3. Apply the headroom you set on the calculator above that peak. It is a buffer for nameplate variance and a future load, not permission to overload.

Generators run poorly when chronically underloaded and trip or stall when overloaded. Right-size once.

Portable whole-house path vs standby path

A portable generator plus a manual transfer switch can cover critical circuits without a permanent pad. A standby unit (or a large 240 V portable class) is the usual answer when you want automatic whole-house coverage including central AC. Compare install, fuel, and use cases on portable vs standby — then come back here and size watts, not marketing tiers.

Worked example skeleton

NEEDS_HOUSE_PROOF: Replace placeholder rows with your nameplate or measured watts before you buy. Do not treat sample numbers as a purchase spec.

Enter the same list in the calculator — presets help, nameplates win.

Accessories: transfer switch, inlet, cords

Match amperage across generator outlet, cord, inlet box, and transfer switch (commonly 30 A or 50 A class). An interlock kit is an alternate code path in some jurisdictions. Never back-feed through a dryer outlet. Details: transfer switch sizing.

CTA and related guides

Size your exact list on the free generator size calculator, then deepen individual loads:

• Fridge & freezer
• Well pump watts
• Sump pump watts
• Best portable generators (class matching after sizing)
• Portable vs standby
• 10 kW generator for a home
• Which generator is right for me?

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FAQ

What critical loads should I size first for whole-house backup?

Start with fridge and freezer, well pump if you have one, sump pump, furnace or heat source fan, lights, and any medical loads. Add central AC only if you truly need it during an outage. It often drives the generator class up. Use nameplate running and starting watts, not guesses.

How do running watts, starting watts, and headroom work together?

Sum the running watts of everything that will run at once. Add only the extra surge of the single largest motor start (not every motor). Then apply the headroom you set on the calculator so nameplate variance and a future appliance do not push you into overload. Diversity helps: not every load starts at the same second.

Should I use a portable or a standby generator for whole-house backup?

A correctly sized portable plus a transfer switch can cover critical circuits. Standby (or a large portable class with 240 V) is the usual path when you want automatic whole-house coverage including central AC. Compare install and use cases on the portable vs standby guide, then size watts on the free calculator.

Do I need a transfer switch for whole-house generator sizing?

Yes to feed household circuits safely. Match switch amperage to generator output (commonly 30 A or 50 A class), and plan inlet box and cords with the same rating. Never back-feed through a dryer outlet. See transfer switch sizing.