What size generator to run a whole house with central AC?

One appliance changes the answer from a $500 inverter to a $1,500 workhorse: the compressor on your central AC. Here's the math, tier by tier.

The honest answer for most homes with central air is 10,000–12,000 watts. But the number isn't a guess — it falls out of the same three numbers our generator size calculator uses: running watts, the single biggest starting surge, and 20% headroom. Walk through it once and you'll know whether your house is the exception.

The 2.5-ton central AC math

A 2.5-ton unit (the most common size for a 2,000–2,500 sq ft home, roughly 30,000 BTU) is the pivot of the whole calculation:

Figures from our wattage presets, drawn from manufacturer published specs and DOE/consumer-testing ranges. Compressors start at up to 4× running watts — the reason the AC and not the fridge dominates the surge budget.

The compressor's startup is the trap. While the AC is already running, it's just 3,500 W of load. The moment it kicks on — which happens repeatedly, because thermostats cycle — it can spike toward 5,000 W, and the unit's nameplate LRA (locked-rotor amps) can exceed that worst case. Your generator must absorb that spike while everything else in the house is still running. That's why sizing to "the AC's running watts" fails in practice.

Tier 1: essentials only (no AC)

The board most people actually need during an outage:

• Refrigerator 150 W + chest freezer 200 W (600 W surge)
• Well pump ½ HP 1,000 W running / 2,100 W starting
• Sump pump ½ HP 800 W running / 1,300 W starting
• Furnace fan 700 W running / 1,400 W starting
• Lights whole home 80 W, microwave 1,000 W, phone charging

Total: about 3,900 W running, with the well pump's 2,100 W spike as the biggest surge. Peak demand (all running + extra surge of the largest starter) lands around 5,300 W, and 20% headroom takes it to roughly 6,400 W → the 5,000–7,500 W portable class. This is the tier the calculator's presets default to, and where a dual-fuel unit from the best portable generators for home backup list shines.

Tier 2: whole home + central AC

Add the AC to the essentials board:

Two things push this up or down. An electric water heater (4,500 W, resistive — no surge) adds a flat block if you run it during the outage; most people simply don't, or heat water in short bursts. A well pump on the larger side competes with the AC for the surge budget. And a soft-start kit on the AC compressor cuts its startup draw roughly in half — the cheapest way to make a 9,000–10,000 W generator comfortable instead of marginal.

At this tier you also need the wiring to match: a 50 A transfer switch (12,000 W capacity) and a generator with a 240 V / 50 A-class outlet. The transfer switch sizing guide covers the pairing.

Why not just buy bigger?

Generators run worst when lightly loaded. At low load they burn proportionally more fuel, foul up with carbon, and deliver no extra capability. A 12,000 W unit idling through an essentials-only overnight stretch costs fuel for nothing. The 20% headroom rule exists precisely to buy you the right size once, not the biggest size — sustained overload still trips the breaker or stalls the engine no matter what you bought.

Fuel: what 10 kW actually drinks

At 50% load, portables burn roughly 1 gallon of gasoline per 7,000 W of rated output per hour — so a 10,000 W generator at a 5,000 W load goes about 1.4 hours per gallon, around 6 gallons per 8-hour day. Propane trades ~25% of that runtime for indefinite storage. Two-day outage on AC duty: plan roughly 12 gallons of gas or the propane equivalent, and stagger loads (AC on, water heater off) to keep the burn down.

FAQ

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

For a typical 2,000–2,500 sq ft home with a 2.5-ton central AC: about 10,000–12,000 watts. The AC itself is 3,500 W running / 5,000 W starting, the rest of the house adds roughly 3,000–5,000 W running plus the biggest other motor's surge, and the 20% headroom rule lands you in the 10–12 kW class. Essentials only — no AC — drops the requirement to 5,000–7,500 watts.

How many watts does central air conditioning use on a generator?

A 2.5-ton central AC draws about 3,500 watts running and about 5,000 watts at compressor startup — a 4×-class surge because it's a compressor, not a plain motor. The AC's outdoor-unit nameplate lists RLA (running) and LRA (locked-rotor); LRA × 240 V is the true worst-case startup. Every ½ ton of capacity adds roughly 700 W running / 1,000 W starting.

Can a portable generator run central air?

Yes, if it clears two bars: a 240 V output (NEMA 14-50 or L14-30) to power the AC's 240 V condenser, and enough surge headroom — roughly 9,000+ rated watts once the house essentials are running too. A soft-start kit on the compressor cuts startup draw roughly in half and makes a 9,000 W generator comfortable instead of marginal.

Why not just buy a bigger generator than I need?

Because generators run worst at low load: they burn proportionally more fuel, build carbon, and run louder relative to output. A 12,000 W generator loafing through an essentials-only night wastes fuel without buying anything. That's why the sizing method is loads + 20% headroom — not 'buy the biggest'.

How much fuel does whole-house running burn?

At 50% load, most portables burn roughly 1 gallon of gasoline per 7,000 watts of rated output per hour — a 10,000 W unit goes about 1.4 hours per gallon at 5,000 W load. Two full days of alternating AC-and-essentials duty at 8 hours/day means planning on roughly 6 gallons per day, or the propane equivalent (about 25% less runtime per gallon-equivalent).

What if my AC is bigger or smaller than 2.5 tons?

Scale by half-tons: roughly 700 W running / 1,000 W starting per ½ ton. A 3-ton unit is about 4,200 W running / 6,000 W starting — which pushes the whole-house class toward 12,000+ W or makes a soft-start kit near-mandatory. A 1.5–2 ton unit can squeeze into a 9,000 W generator when essentials are modest.