Outage scenario

Whole-House Generator Sizing With Central AC

Data reviewed 2026-07-05 ·how we calculate

This is the build people mean when they say "I want to run the whole house", and it is where this site’s math delivers its most expensive honest answer. Adding 3-ton central air to the essentials package drives the peak requirement to 13,050W — past every portable class — and headroom lands it at 16,313W: home standby territory, professionally installed, with a transfer switch. The numbers below show exactly why, and the FAQ maps the two ways to change the answer: soft-start hardware, or cooling one room instead of the envelope.

The load math, spelled out

Running watts: central AC (3-ton) 4,200W + refrigerator 700W + chest freezer 500W + electronics 250W = 5,650W. Largest single starting delta — and it is not close — is the AC compressor: 11,600W − 4,200W = 7,400W. Peak = 5,650W + 7,400W = 13,050W, already past the 12,000W portable ceiling before headroom. With 25%: 13,050W × 1.25 = 16,313W → the 14–22kW home standby class, 240V, installed with a transfer switch by professionals. This page’s calculator will show the same conclusion however you rearrange the smaller loads: the AC compressor IS the requirement.

Generator safety — read every time

Never run a generator indoors, in a garage, or near windows, doors or vents.Carbon monoxide kills quickly and without warning — place the generator 20+ feet from the house with the exhaust pointed away, and use battery CO detectors inside.
Never backfeed your house through a wall outlet or dryer outlet. Backfeeding can electrocute utility line workers and destroy equipment when power returns.
Connecting a generator to home circuits requires a transfer switch or interlock installed by a licensed electrician. There is no safe DIY shortcut, and this site does not provide wiring instructions.
Use heavy, outdoor-rated extension cords (12-gauge or thicker), fully uncoiled, sized to the load and the distance.

Running this build, step by step

Accept the structure of the problem first: the AC’s 7,400W starting delta is 57% of the entire peak. Every path to a smaller answer goes through that one number.
Get a standby quote as the baseline: a 14–22kW unit with an automatic transfer switch, sized by the installer against your panel and nameplate data (they will run this same math with your actual LRA figures).
Price the soft-start alternative: an HVAC tech installs a soft-start kit (~$300–500 plus labor), the AC delta drops to roughly 2,200–3,000W, and the build lands near 9,900–11,000W with headroom — a 9,000–12,000W 240V portable with an interlock becomes arguable. Quieter wallet, more manual lifestyle.
Or reshape the goal: the window-AC-plus-essentials strategy (see the window AC page) delivers a cool bedroom, cold food and connectivity from a 4,500W-class portable — a quarter of the standby’s cost for most of the livability.

Pro tips

Check your nameplates first — every figure on this page is a planning estimate built from typical values; your appliances' labels beat any chart.
Whatever path you choose, do it before you need it: standby installs run 4–12 weeks lead time, and even the soft-start-plus-interlock route needs two trade visits that don’t happen during a heat emergency.
If you get standby quotes, ask specifically how the AC’s locked-rotor amps were handled — installers who size with soft-start or load-shedding modules can quote a smaller, cheaper unit honestly.

The data behind this scenario

The whole-house-with-AC build, itemized
Load	Figures	Source
Central AC, 3-ton (240V)	4,200W running / 11,600W starting (delta +7,400W)	Generator sizing charts; condenser LRA convention
Refrigerator	700W running / 2,200W starting	Generac & Honda sizing charts
Chest freezer	500W running / 1,500W starting	Honda & Champion wattage worksheets
TV + Wi-Fi + lights bundle	250W running, no surge	Itemized on the bundle page
Soft-start kit (the math-changer)	AC starting delta cut ~60–70%	Micro-Air EasyStart / Hyper Engineering data

Each appliance in this build has its own page with full ranges and sources:central air conditioner, refrigerator, chest freezer, tv, wi-fi & lights bundle.

Whole house with central AC questions, answered

What size generator do I need to run my whole house including central AC?

For a 3-ton system with the basic essentials alongside: 5,650W running, but a 13,050W peak — the AC compressor’s 7,400W starting delta dominates everything — and 16,313W once 25% headroom is applied. That is the 14–22kW home standby class, permanently installed with an automatic transfer switch by a licensed electrician. No standard portable reaches it; the calculator on this page exists mostly to show that conclusion transparently rather than sell it on vibes.

Could a 12,000W portable with a soft-start kit really do it?

It moves from impossible to arguable. A soft-start kit cutting the AC’s delta ~65% turns the peak into roughly 5,650W + 2,600W ≈ 8,250W — about 10,300W with headroom, inside a 9,000–12,000W 240V portable’s envelope with the essentials still aboard. The honest trade-offs: manual operation (wheel it out, connect, manage loads), a portable running near capacity through every hot afternoon, fuel logistics of 5+ gallons a day, and no automatic restart when you’re away. Households that are home during outages and allergic to standby pricing genuinely choose this; know which household you are.

Why is standby-generator sizing best left to the installer if this math is public?

Because the planning figures on this page become YOUR nameplate figures at purchase time: the installer reads the actual LRA off your condenser, counts the real circuits the transfer switch will carry (well pump? second AC? electric range?), applies local code (NEC 702 load calculation), and warranties the result. This page’s value is walking in knowing WHY they quote 18kW instead of nodding at a number. Treat mismatches between their math and this page’s as questions to ask, not errors to allege.

Is whole-house backup with AC worth it compared to the essentials approach?

It is a climate-and-health decision wearing a financial costume. Gulf Coast summers with week-long hurricane outages, or household members whom heat genuinely endangers: the $12,000–$18,000 installed standby earns its keep as infrastructure. Elsewhere, the honest comparison is stark — the essentials-plus-window-AC strategy delivers cold food, one cool room, connectivity and the furnace in winter for $1,000–$1,500 all-in. This site’s job is the arithmetic; the standby industry’s job is selling past it. Both numbers are now yours.

See alloutage scenarios or build your own combination in themulti-appliance builder.

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