What Size Generator to Run 3 Ton AC Unit? (Detailed Guide)

Welcome! If you’re wondering what size generator you need to power a 3 ton air conditioning unit, you’ve come to the right place. Having the correctly sized generator is crucial to keeping your AC running during a power outage.

In this comprehensive guide, we’ll cover everything you need to know to choose the right generator for your 3 ton AC unit. We’ll look at key factors like starting and running watt requirements, choose between portable and standby generators, recommended brands and models, cost considerations, and proper installation and maintenance.

Whether you’re looking to buy your first generator or upgrade an existing unit, you’ll find the information you need here to make an informed decision. By the end, you’ll know exactly what size generator you need to keep your home cool when the power goes out.

Let’s get started!

What is a 3 Ton AC Unit?

Before looking at generators, it’s helpful to understand what we mean by a “3 ton” air conditioner. The “ton” refers to the amount of heat the AC can remove from your home in one hour.

One ton of cooling is equal to 12,000 British Thermal Units (BTUs) per hour. So a 3 ton AC has a cooling capacity of 36,000 BTUs per hour.

This cooling capacity is suitable for a home between 1,500 and 2,500 square feet. It can effectively cool that much space on hot summer days, providing a comfortable indoor environment.

The higher the tonnage, the more powerful and effective the AC unit. While 3 tons is typical for most homes, smaller places may only require 1.5-2 tons. Larger homes might need a 4-5 ton unit or multiple AC systems.

Now that you know what a 3 ton AC entails, let’s look at choosing the right size generator.

What Size Generator to Run 3 Ton AC Unit?

Generator Sizing for a 3 Ton AC

Sizing a generator properly comes down to having enough “watts” to power your AC unit and any other essentials you want to run during an outage.

Watts are a measure of power output. To choose a generator, you mainly need to pay attention to two watt measurements:

  • Starting (or surge) watts – The initial power surge needed to start the AC compressor.
  • Running (or continuous) watts – The ongoing power required once the AC is running.

Let’s look closer at each:

Starting Watts

When your air conditioner first turns on, the compressor motor needs an extra jolt of power to get going. This causes a brief power surge, which can be double (or more) the AC’s running watts.

For a 3 ton AC unit, the starting wattage usually falls between 4500-6000 watts. This surge only lasts a few seconds, but your generator needs to be rated to handle it.

Tip: The starting watts are sometimes listed in the AC specs. If not, you can roughly estimate it at 2-3 times the running watts.

Running Watts

Once the AC is up and running, the generator must provide enough continuous power to keep it going. This running watt draw is much lower than the start-up surge.

For a 3 ton AC, the running watts are typically 1500-2000 watts. This depends on the SEER rating and compressor efficiency. Higher SEER units run more efficiently and use less power.

To choose the right generator, add up the running watts of all essentials you want to power. This is your required continuous (or running) wattage.

Do You Need a Portable or Standby Generator?

Now that you know the basic power requirements, the next decision is what type of generator to get:

Portable Generators:

  • Smaller and lighter for easy transport
  • Typically 5000-7500 running watts
  • Need to be manually started and connected
  • Run on gasoline

Standby Generators:

  • Permanently installed outside your home
  • Available in both gas and diesel models
  • Automatic start when power goes out
  • Can provide power for a whole home
  • More powerful – up to 15,000+ running watts

For most homes, a portable gas generator in the 5000-7500 watt range will provide enough power to run a 3 ton AC and some other lights and appliances. This is the most affordable and convenient option.

However, a permanently installed standby generator can automatically power your entire home during an extended outage. If you want whole home coverage, a 14kW standby unit is usually sufficient for a 3 ton AC.

Recommended Generator Brands and Models

Here are some recommended portable and standby generator models suitable for a 3 ton AC from top manufacturers:

Portable Generators

  • Westinghouse iGen4500 – 3700 running watts, electric start
  • Generac GP5500 – 5500 running watts, gas powered
  • Champion 3500-Watt – 3000 running watts, remote start option
  • Honda EU7000is – 5500 running watts, inverter generator
  • Rainier R12000DF – 9500 running watts, dual fuel

Standby Generators

  • Generac 22kW – 20kW running power, air-cooled gas engine
  • Kohler 20kW – 18kW running power, liquid-cooled diesel
  • Champion 18kW – 16kW running power, electric start
  • Cummins 17kW – 15kW running power, weatherproof enclosure
  • Briggs & Stratton 40346 – 19.5kW running power, Wi-Fi enabled

Consult an electrician if wiring a standby unit, and always carefully follow installation directions.

What Size Generator is Needed for a 3 Ton AC?

Now that we’ve covered the key considerations, what’s the simple answer for powering a 3 ton AC?

For a portable generator, you’ll want at least 5500 running watts. This provides enough power for the AC and a few lights and small appliances. Go up to 7500 watts if you want to run more items.

For whole home coverage with a standby generator, aim for 14-22kW units. The larger 20kW+ models allow you to run nearly everything during an extended outage.

To summarize, the recommended generator sizes for powering a 3 ton AC are:

  • Portable: At least 5500 running watts
  • Standby: 14,000-22,000 running watts

These properly sized generators will have enough power to start and run your 3 ton AC when the electricity goes out.

What Size Generator to Run 3 Ton AC Unit?

Installation and Maintenance

To finish the job, you’ll need to correctly install and maintain your generator:

For portable generators:

  • Only run in well-ventilated outdoor areas
  • Plug appliances directly into the generator
  • Make sure extension cords are properly rated
  • Check oil and fuel levels regularly
  • Consider an hour meter to track maintenance schedules

For standby generators:

  • Professional installation is highly recommended
  • Must comply with local electrical codes
  • Integrate an automatic transfer switch to safely power your home
  • Register with utility company to avoid back-feeding lines
  • Schedule monthly and annual system tests
  • Check fluid levels, belts, and hoses
  • Replace fuel and change oil as directed

Proper generator installation and care ensures safe, reliable performance when you need it most during a power outage.

Cost of Generators for 3 Ton AC

Generator cost varies widely based on type, size, and features. Here are some typical price ranges:


  • 3,000-5,000 watts: $300 – $1,000
  • 5,000-8,000 watts: $800 – $2,000


  • 10-12kW: $2,000 – $4,000
  • 14-22kW: $3,000 – $6,000

Standby generators are a major investment. Get quotes from several licensed electricians to find the best value.

There are ways to save, especially on portable models:

  • Buy during off-season months for sales
  • Consider a refurbished or used generator
  • Go for a basic model without premium features
  • Use a smaller unit and conserve energy during an outage

With some smart shopping, you can find an affordable generator to keep your 3 ton AC running.

Frequently Asked Questions (FAQ’s)

Here are answers to some frequently asked questions:

How long will a 5500W generator run a 3 ton AC?

With a 5500 watt portable generator, you can expect 6-8 hours of run time for a 3 ton AC unit. This covers most shorter power outages.

What happens if I use too small of a generator?

An undersized generator will struggle and likely shut down when trying to start the AC’s compressor. It may also overload and be damaged if used long term.

Can I connect a generator to my home’s electrical panel?

You should not back feed a portable generator directly into your home wiring. To safely integrate a standby generator, install an automatic transfer switch with the help of an electrician.

How close should a generator be to an AC unit?

Portable generators should be kept at least 15 feet away from AC systems due to carbon monoxide concerns. Install standby generators at least 5 feet away per manufacturer recommendations.

How long will a standby generator run an AC?

Properly sized standby generators can run an air conditioner continuously for as long as needed, automatically turning on when an outage occurs. Fuel usage depends on load. Expect 24 hours on an LPG tank or 35-40 hours on a full tank of diesel or gasoline.


Choosing the right generator ensures you can run your 3 ton AC unit and other essentials when the power goes out. By understanding your starting and running watt needs, installation, maintenance, and properly sizing a portable or standby model, you can stay comfortable through outages.

Remember that an undersized generator won’t be able to start your AC, while an oversized one uses more fuel than necessary. Consult an electrician if considering a whole home standby unit. And always follow manufacturer safety instructions.

We hope this guide has helped determine the best generator size to power your 3 ton central air system. Stay cool when the weather gets hot and the power goes down!

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