What Size Generator Do I Need to Run an Air Conditioner?

To run central air conditioning, you will need a generator in the 5kW to 10kW range to run only the AC, or one between 14kW to 22 kW for whole-home power. Depending on your system's tonnage (size) and energy efficiency (SEER rating), central air conditioners typically only draw 1,500 to 5,000 watts while running, but startup can often surge to 6,000 to 9,000+ watts.

Generally speaking, you’ll need a whole-home generator to power your central AC. Portable generator may be able to handle a smaller window unit or mini-split but typically can’t deliver the surge wattage a central AC compressor needs to start.

In this guide, we’ll explain starting vs running watts, why a whole-home standby generator is recommended for homeowners, and answer frequently asked questions.

Why AC Is the Hardest Load in Your House

In South Florida, specifically, your air conditioning system is likely to be your highest priority load. Summer storm season temperatures frequently reach the upper 90s and produce “feels like” temperatures in the triple digits.

Your AC, especially if it is central air, requires a lot of power to function. When thinking about whether a generator will power your AC, we must consider both starting and running watts. But what are starting vs running watts?

Simply put, starting watts are the burst of extra power needed for a couple of seconds to start a motor and get its compressor running from a dead stop. Running watts are what it takes to maintain normal operation once it’s settled. That’s why a generator that’s well above an AC’s normal running wattage may still fail to start it. The momentary spike can trip up the generator.

Starting watts for a central AC system can easily surpass 7,500 watts without a soft starter system. A good rule of thumb is that typical starting watts are 2 to 3 times the running watts.

Generator Size by Air Conditioner Size

Let’s look at the recommended generator size by AC size. For this table, we based the figures from the estimate that 1 ton of cooling capacity equals 1,000 watts of electricity usage and that notion that starting watts are 2 to 3 times higher than the running watts. The recommended generator size was calculated based on the estimate of starting watts with a 20% safety buffer.

This table should help you choose the right generator size for your home.

Can a Portable Generator Run Central AC?

In most cases, no, a portable generator cannot run central AC without help. Most standard central AC systems need a surge of 5,000-10,000+ starting watts to get the compressor going, and most portable generators are only 3,000 to 8,000 watts.

Especially if you consider running a refrigerator or your lighting system, a portable generator is not enough. Forcing it usually just trips the generator's breaker or can damage the AC’s compressor.

The exception is a portable generator may be able to power a window unit or mini-split since these systems draw way less power to start. That’s why they are a realistic backup cooling solution during an outage even if your central AC isn’t.

If you are dead set on using a portable generator instead of a standby generator, a soft start device can make it possible. Soft starters cut compressors’ starting watt spikes by 60-70%, which brings a central AC system’s surge down to a reasonable level for a large portable generator. A soft start device can be a good option for those who already own a large portable generator and don’t want to buy another generator just for the AC.

The Whole-Home Answer: Sized Once, Runs Everything

For most Florida homeowners, a whole-home standby generator is the best solution. Whole-home generators are equipped with enough power to run your entire home during outages. Unlike portable units, standby generators are permanently installed outside of your home and normally use an automatic transfer switch to seamlessly transition from utility power to backup power.

Whole-home generators can power all the essential systems, such as lighting, HVAC, refrigeration, and well pumps all at once. This means you don’t have to prioritize which systems to run or keep track of electrical wattage because whole-home generators are professionally sized to handle your home’s full electrical load.

They also typically run on liquid propane or natural gas, and can connect into your existing natural gas line. In this case, you don’t have to ever worry about refueling mid-outage or storing fuel. Since whole-home generators can handle your home’s essential system, you also don’t have to worry about overloading the unit or juggling extension cords during a storm.

Reliable Power Systems provides professional generator services, including installation, maintenance, and repair throughout South Florida. Our team offers fast, friendly service to homeowners looking for dependable backup power solutions.

Check out our whole-home generator cost calculator or schedule a free sizing assessment today! Call us at (561) 408-5147!