Can a generator run central air? Sizing for AC compressors, soft starters and smarter cooling

Summer outages can be dangerous, not just uncomfortable. Ready.gov warns that extreme heat can cause heat-related illness, especially for older adults, young children, and people with chronic conditions. Central air is often the first thing people want to keep running, and the hardest thing for a portable generator to handle. This guide explains the numbers and the options.

Why central AC is hard on generators

A central air conditioner has a large compressor motor in the outdoor unit and a blower in the indoor air handler or furnace. The compressor draws a big inrush current when it starts. The nameplate on the outdoor unit lists:

• RLA (rated load amps) or MCA (minimum circuit ampacity): roughly the running current.
• LRA (locked rotor amps): the current the compressor draws at the instant of starting, often five to eight times the running current.

Multiply amps by voltage, usually 240, to estimate watts. For example, a compressor with an RLA of 14 A and an LRA of 76 A at 240 V runs at roughly 3,400 W but can briefly demand about 18,000 W to start. Few portable generators can supply that surge.

Typical sizes

Rough figures for central air on a generator, before soft-start:

AC size	Approx. running watts (compressor + blower)	Generator size often needed without soft-start
2 ton (24,000 Btu/h)	about 2,500 to 3,000	about 8,000 to 10,000 W
3 ton (36,000 Btu/h)	about 3,500 to 4,000	about 10,000 to 12,000 W
4 ton (48,000 Btu/h)	about 4,500 to 5,500	about 12,000 to 15,000 W
5 ton (60,000 Btu/h)	about 5,500 to 6,500	about 15,000 to 20,000 W

These are generalizations. Efficiency, compressor type, and age all matter. Always check your equipment's nameplate, and ideally ask an HVAC technician to confirm.

Soft-start kits

A soft-start kit is a small electronic device installed in the outdoor unit. It ramps up the compressor's start, cutting the inrush current, often by more than half. With a soft-start, a 3-ton system that might have needed a 10,000+ watt generator can sometimes start on a 6,000 to 8,000-watt unit. Results vary by compressor, so manufacturers usually provide compatibility lists.

Soft-starts should be installed by a qualified HVAC technician. Ask whether installation affects your equipment warranty.

Variable-speed systems and heat pumps

Many newer high-efficiency systems use inverter-driven, variable-speed compressors that ramp up slowly and draw much less starting current. Heat pumps with inverter compressors work the same way. If your system is variable-speed, it may run on a smaller generator than a traditional single-stage unit. Check the manufacturer's documentation for generator guidance, because some variable-speed systems are sensitive to power quality and may require a low-THD generator.

Connecting central AC

Central AC runs on 240 volts and is hard-wired. You need:

• a generator with a 240-volt outlet (an L14-30 or 50-amp);
• a transfer switch or interlock installed by an electrician, with the AC circuit included and protected appropriately.

Don't try to wire the condenser to a generator directly. That's dangerous and usually violates code.

Cooling with less generator

If whole-house AC isn't realistic, these strategies help:

• A window or portable AC in one room. A 5,000 to 8,000 Btu window unit typically runs on 500 to 800 W and starts easily on a small generator. Make one room the cool room for sleeping and for vulnerable family members.
• A mini-split. Ductless inverter mini-splits are efficient and start gently, so they often run well on modest inverter generators.
• Fans and airflow. Fans use little power. Run them at night to pull in cooler air if outdoor temperatures drop.
• Block the heat. Close blinds on sunny windows, and avoid using the oven.
• Run AC in blocks. Cool the house during the hottest hours, then switch to fans and refrigerator duty.

Heat safety during an outage

Ready.gov's extreme heat guidance includes:

• Stay hydrated.
• Know the signs of heat exhaustion and heat stroke.
• Check on neighbors, especially older adults.
• Find cooling centers if your home becomes too hot.

Never sacrifice generator safety for cooling. Keep the generator outdoors, at least 20 feet from windows, doors, and attached garages, and never run it in a garage to keep it out of the sun.

Is a standby generator the answer?

If keeping central air running is essential, for example because of medical needs or extreme climate, a home standby generator with an automatic transfer switch and load management is often the practical solution. Many standby systems can lock out the AC until the generator has capacity, or cycle it with other loads. Get a load calculation from a qualified installer, and ask about soft-start kits even with a standby unit, because they can allow a smaller generator.

Frequently asked questions

Will my 7,500-watt portable generator run my 3-ton AC? Possibly, with a soft-start kit and few other loads, but often not without one. Check the condenser's LRA. If LRA × 240 is far above the generator's starting watts, plan on a soft-start or a smaller cooling option.

Can I run AC on a dual-fuel generator using propane? Yes, but dual-fuel generators usually produce less power on propane, so the margin shrinks. Use the propane rating when sizing.

Do I need to turn the thermostat off before switching to generator power? It's a good habit. Switch to the generator, let the other loads settle, then turn the thermostat back on, so the compressor starts on its own rather than at the same moment as the refrigerator or well pump.

Will running AC on a generator damage the compressor? Low voltage from an overloaded or undersized generator can stress motors. If the generator bogs down when the compressor starts, turn the AC off and reassess. Don't let it keep trying.

Efficiency pays off twice

High-efficiency air conditioners use less energy for the same cooling. When you're on a generator, efficiency means less fuel and possibly a smaller generator. If you're replacing an old system anyway, ask the installer how a variable-speed unit would behave on backup power.