What Size Generator Do You Need to Run a Central Air Conditioner?

Powering central air conditioning during an outage is the difference between an uncomfortable night and a normal one, but it demands far more generator capacity than any other household appliance. The main challenge is not running watts but the compressor’s starting surge. This guide shows how to size a generator for central AC, when a portable can work, when you need a standby unit, and how soft starters change the math.

Generator size by AC tonnage

These are starting-watt capacities. Essentials include a refrigerator, lights, electronics, a furnace/air handler blower and a sump or small well pump. Actual needs vary with your specific equipment.

Sizing method

1. Find compressor RLA and LRA on the outdoor unit. Running watts ≈ RLA × 240; starting surge ≈ roughly 60–100% of LRA × 240.
2. Add the air handler or furnace blower running watts (100–800W).
3. List all other loads that run simultaneously and total their running watts.
4. Required starting capacity = AC starting surge + all other running watts.
5. Required running capacity = AC running watts + all other running watts, plus 20% headroom.

Worked example

A 3-ton AC with 14 RLA and 73 LRA: running ≈ 3,360W; estimated starting surge ≈ 11,000W. Air handler 600W, fridge 200W, lights and electronics 600W, sump pump 800W. Starting requirement ≈ 11,000 + 2,200 = 13,200W. Running requirement ≈ 5,560W × 1.2 ≈ 6,700W. A 14kW+ standby unit handles it outright; with a soft starter reducing surge to about 5,000W, a 9,000–10,000W portable could manage it.

Soft starters: the game changer

A compressor soft starter uses electronics to ramp current gradually, cutting inrush by 50–70%. Installed on a 3-ton system, it can reduce required generator size by 4–6kW. Installation typically costs $300–700 with a technician and takes about an hour. It is often the cheapest way to make central air generator-compatible.

Portable versus standby for central AC

Central AC is a 240V load, so a portable must supply 240V through a 30A or 50A outlet (L14-30 or 14-50) into a transfer switch or interlock. Small 120V-only inverters cannot run it at all.

Fuel consumption

Running a 3-ton AC plus essentials averages roughly 4,000–6,000W over a hot afternoon. A 20kW natural-gas standby at that load might use about 200–250 cubic feet of gas per hour. A 12kW gasoline portable would burn roughly 0.8–1.1 gallons per hour — about 10 gallons for a 10-hour cooling day. Running the AC in intervals, such as keeping the house at 78°F, cuts fuel noticeably.

Load management strategies

• Use a load-shed module on standby systems to pause AC when the water heater or dryer runs.
• Cool one zone only if you have multiple systems.
• Pre-cool the house to 72°F before evening peaks, then let it drift to 78°F.
• Avoid running electric ovens, dryers and water heaters while the AC runs.

Altitude and heat derating

Generators lose about 3–4% of output per 1,000 feet above sea level, and some capacity above roughly 77–85°F ambient. A 12kW unit in Denver on a 95°F day may deliver closer to 10kW. Add margin if you live at elevation or in extreme heat.

FAQ

Can a 10,000W generator run central air?

Often yes for a 2–3 ton system if a soft starter is installed and other loads are managed. Without a soft starter, most 3-ton units need 12,000–15,000W of starting capacity.

What size standby generator do I need for central AC and the whole house?

Most average homes with a 3-ton system need 18–22kW; larger homes with 4–5 ton systems or multiple units usually need 22–26kW or load management.

Bottom line

Central AC typically needs 12,000W+ of starting capacity without help. A soft starter can cut that by several kilowatts, making large portables viable for 2–3 ton systems. For whole-home comfort with minimal management, a 18–26kW standby is the reliable answer.