What Size Generator Do You Need To Run An Air Conditioner?

Choosing the right generator to run an air conditioner isn't just about matching the generator to the AC unit's tonnage or BTU rating. You must consider the AC's running electrical demand, compressor startup requirements, and other appliances you plan to operate simultaneously.

A small window or portable AC may require relatively little generator capacity. A central air conditioner, on the other hand, can place a much greater demand on a generator when its compressor starts.

Start with the AC manufacturer's electrical specifications, then compare the total expected load with the generator's running and surge ratings.

How Much Generator Power Does an Air Conditioner Need?

There isn't one generator size that works for every air conditioner. Electrical demand varies by model, efficiency, compressor design, and operating conditions.

Start by identifying the AC's running electrical demand and startup requirement. Then account for the running loads and startup demands of any other equipment you plan to use at the same time.

Use the AC manufacturer's electrical specifications whenever available. Generic wattage figures can provide context, but they should not replace the requirements of your specific unit.

How to Calculate the Right Generator Size

To determine what size generator you need, work through the following steps.

Check Your AC's Running Wattage

Running wattage is the electrical power an air conditioner requires during normal operation. Check the equipment nameplate, owner's manual, or manufacturer documentation for:

• Rated voltage
• Amperage or running current
• Rated power or electrical input
• Other electrical ratings provided by the manufacturer

If the manufacturer provides a wattage figure, use it for generator size. If only voltage and current are available, volts x amps can provide a basic estimate of electrical demand, but this should not be treated as the AC's exact running wattage. Manufacturer specifications take precedence.

Check the Starting or Surge Requirement

When an air conditioner's compressor first starts up, it draws significantly more electrical current than it does during normal operation.

This temporary electrical demand is commonly described as starting or startup demand. Generator manufacturers may express the corresponding generator capability as starting or surge watts.

Don't assume startup demand is always two or three times the running wattage. If the manufacturer provides starting-current, locked-rotor-current, or other startup specs, use that information to assess generator capacity.

Account for Other Appliances

If you plan to run other equipment during an outage, include the loads you expect to use at the same time as the AC:

Common examples include:

• Refrigerator or freezer
• Lights
• Television and internet equipment
• Microwave
• Water pump
• Fans
• Computers

Refrigerators and pumps are motor-driven appliances. Like air conditioners, they can draw additional power during startup. Account for their startup requirements when evaluating the generator's surge capacity.

Compare the Load With the Generator's Ratings

Generators commonly list two capacity figures:

• Running or rated watts: The power the generator can supply continuously.
• Starting or surge watts: The additional temporary output available for starting certain loads.

The generator's running rating must cover the total operating load. Its surge rating must handle the startup demands of motor-driven appliances.

Before choosing a generator, make sure you have:

• Your AC's running electrical requirement
• Its starting or surge requirement
• The running and startup requirements of other essential loads
• The generator's continuous/running rating
• The generator's surge rating

This comparison is more useful than matching the generator's advertised wattage to the AC's running wattage alone.

Example: Calculating Generator Capacity for an AC

Suppose your air conditioner requires 3,000 watts during normal operation. Now add another 1,500 watts for other essential appliances you plan to run at the same time.

Your expected running load would be:

3,000 W + 1,500 W = 4,500 W

This means the generator needs enough continuous/running capacity to handle the 4,500-watt operating load. You'll also need to check the startup requirements of the AC compressor and any other motor-driven appliances. Compare these startup demands against the generator's available surge capacity to ensure it can handle the initial power draw.

This is an illustrative example only. Your actual generator requirement should be based on the electrical specifications of your AC and other equipment.

AC Unit Wattage: Why BTUs and Tonnage Aren't Enough

Air conditioners are commonly rated by BTU capacity or tonnage. However, these measurements describe cooling capacity, not electrical capacity. This means they don't directly tell you what size generator you need.

Two AC systems with similar cooling capacities can have very different electrical demands. This is due to differences in efficiency, compressor design, voltage, model, and operating conditions.

Always use the specific electrical specifications of your AC unit when sizing a generator. Do not assume that a particular BTU or tonnage rating corresponds to a fixed generator size.

What Size Generator Do You Need for Different Types of AC?

Different types of air conditioners have different electrical requirements. The examples below highlight how generator-sizing considerations vary by AC type.

Central Air Conditioners

Central AC systems typically demand more generator capacity than individual room units. This is largely due to the high startup demand placed on the compressor.

Check the system's electrical specifications for its operating and startup requirements, then account for other loads that will run at the same time.

Window Air Conditioners

Window AC units generally require less generator capacity than central systems. However, requirements vary by model. Always check the unit's nameplate or manufacturer's specifications for its electrical and startup requirements. Do not rely on BTU rating alone.

Portable Air Conditioners

Portable AC units can vary in electrical demand. Some may operate with smaller generators, but the generator still needs sufficient running and surge capacity for the AC and other loads you plan to use.

Generator safety matters, too. Never operate a fuel-burning portable generator inside a home, garage or other enclosed space. The CDC recommends operating portable generators outdoors and more than 20 feet from windows, doors and vents to reduce the risk of carbon monoxide poisoning.

What Size Generator Do You Need to Run a House With AC?

If you want to run your house with the air conditioner operating during an outage, calculate the generator requirement based on the essential loads you expect to use at the same time.

For example:

Add the expected running loads, then evaluate the highest relevant startup demands against the generator's surge rating.

For whole-house backup, there are additional electrical and installation considerations to keep in mind. A qualified professional can assess your home's electrical system and determine the right generator requirements.

Portable vs. Standby Generators for Air Conditioning

The generator type depends partly on how much of your home you want to power during an outage.

Portable Generators

Portable generators can power selected essential loads, including an AC unit and other appliances. To do this, the generator must have adequate capacity. It must also be connected according to applicable requirements.

Standby Generators

Permanently installed standby generators can provide broader backup coverage through the home's electrical system. They may be used for central AC and multiple household circuits when properly sized and installed.

Can a Soft-Start Device Help an AC Run on a Smaller Generator?

A properly selected soft-start device can reduce the electrical demand associated with compressor startup on compatible air-conditioning systems. This may make it easier for a generator to handle the AC's startup demand.

However, a soft-start device does not eliminate the AC's normal operating load or guarantee that a particular generator will be adequate. The device must also be compatible with the AC system and installed correctly.

Frequently Asked Questions

What size generator do I need to run my air conditioner?

The required generator size depends on your AC's running and startup requirements and the other loads you plan to operate at the same time. Check the manufacturer's specifications and compare the combined demand with the generator's running and surge ratings.

Can a 2,000-watt generator run an air conditioner?

It depends on the specific AC. A 2,000-watt generator may operate some smaller units, but the AC's running and startup requirements must fit within the generator's continuous and surge ratings.

Can a 7,500-watt generator run central air conditioning?

A 7,500-watt generator may power some central AC systems. However, compatibility depends on the system's operating and startup requirements, as well as any other connected loads. Check the generator's running and surge ratings before relying on it for backup power.

Can a generator run a house with central AC?

Yes. A properly sized generator can provide backup power to a home with central AC. The required capacity depends on the AC's electrical demand and the other loads you want to operate simultaneously. Whole-house backup generally requires more capacity than powering the AC and a limited number of essential loads.

Should I choose a generator based on my AC's BTU rating?

No. BTU rating describes cooling capacity, not the AC's exact electrical demand. Use the unit's electrical specifications and startup requirements when assessing generator capacity.

How do I know if my generator is large enough for my AC?

Check the AC's operating and startup requirements. Compare these against the generator's running and surge ratings. Then account for any other equipment you plan to run at the same time. If the generator lacks sufficient capacity, it may overload or shut down rather than operate the AC reliably.

When to Get Professional Help With Generator and AC Sizing

Generator sizing should be based on your AC's actual electrical requirements. It should also account for any other loads you plan to run during an outage. Do not rely on BTU rating or tonnage alone.

If you're unsure about your AC's electrical requirements, consult a qualified professional. This is especially important if you plan to run central air conditioning on a backup generator. A professional can evaluate your system and recommend the right setup.

Need help with your AC? Contact Fahrenheit AC to discuss your system and service needs.