Choosing the Right Generator Size to Power Air Conditioner

by Anna

Power outages are an inconvenient reality in many parts of the world, and they can be particularly uncomfortable during hot summer months when air conditioning becomes a necessity. In such situations, having a generator to power your air conditioner can be a lifesaver. However, selecting the right generator size to run your AC is crucial to ensure it functions efficiently and safely. In this comprehensive guide, we will explore the factors that determine the appropriate generator size for your air conditioner, helping you make an informed decision and stay cool during power interruptions.


Understanding Your Air Conditioner’s Power Requirements

Before delving into generator sizing, it’s essential to understand the power requirements of your air conditioner. Air conditioners are rated in terms of their cooling capacity, typically measured in British Thermal Units (BTUs). The power consumption of an air conditioner depends on its size, efficiency, and the outdoor temperature. You can usually find the AC’s power consumption in watts or amps on the unit’s nameplate or in the user manual.


The key information you need to know about your air conditioner includes:

Cooling Capacity (BTUs): This indicates how much heat the AC can remove from a room in an hour. It’s a primary factor in determining the size of the generator needed.

Voltage (V): The standard voltage for most residential air conditioners is 120V or 240V, depending on the unit’s size and type.

Current (Amps): The current (in amps) required by your air conditioner when running at full capacity.

Starting (Surge) Current: Air conditioners often require a surge of additional power when they start up. This surge can be several times the running current and is critical to consider when sizing a generator.

Factors Influencing Generator Sizing

To choose the right generator size to run your air conditioner, you must consider several factors:

1.AC Size and Capacity: The cooling capacity of your air conditioner (in BTUs) is the most critical factor in determining the generator size. Smaller AC units require less power, while larger ones demand more. For example, a 12,000 BTU window unit will require less power than a central AC system with a 36,000 BTU capacity.

2. Starting Surge: As mentioned earlier, air conditioners draw a surge of power when they start up. This surge can be up to three times the running current. It’s crucial to account for this surge when sizing your generator.

3. Generator Type: Different types of generators have varying capacities. Portable generators are available in a range of sizes, while standby generators are more powerful and suitable for whole-house backup. The type of generator you choose will impact its ability to run your air conditioner.

4. Other Appliances: Consider what other appliances you may want to power simultaneously with your air conditioner during an outage. These appliances, such as lights, refrigerators, or essential electronics, will add to the total power requirement.

5. Climate: The outdoor temperature can affect your air conditioner’s efficiency and power consumption. AC units work harder in extreme heat, so if you live in a hot climate, you may need a slightly larger generator to account for increased demand.

Calculating Generator Size

Now that you have the necessary information about your air conditioner and have considered the influencing factors, you can calculate the generator size needed. To do this, follow these steps:

Determine Running Watts: Find the running power consumption of your air conditioner in watts. This information can usually be found on the unit’s nameplate or in the user manual. If it’s given in amps, use the formula: Watts = Volts x Amps.

Calculate Starting Watts: Calculate the starting (surge) power required by your air conditioner. This is typically three times the running watts. For example, if your AC requires 1,500 running watts, you’ll need 4,500 starting watts.

Add Other Loads: If you plan to power other appliances or devices simultaneously with your air conditioner, calculate their power consumption in watts and add it to the starting watts of your AC.

Choose the Right Generator Size: Look for a generator that can provide the total power calculated in step 3. Generators are rated by their running and starting wattage capacities. Select a generator that meets or exceeds your requirements.

It’s essential to choose a generator that not only matches your air conditioner’s power needs but also leaves some room for additional loads and potential fluctuations in power demand.

Generator Types and Their Suitability for Air Conditioners

There are two main types of generators to consider: portable generators and standby generators. Each has its advantages and limitations when it comes to running air conditioners.

Portable Generators:


Affordability: Portable generators are typically more budget-friendly than standby generators.

Versatility: You can use a portable generator for various purposes beyond powering your air conditioner.

Portability: As the name suggests, these generators are easy to move around and store.


Limited Capacity: Portable generators have lower power capacities, making them suitable for smaller air conditioners.

Manual Start: You need to manually start and connect a portable generator during an outage.

Noise: Portable generators tend to be noisier than standby generators.

Portable generators are suitable for powering smaller window units or single-room air conditioners during short outages.

Standby Generators:


High Capacity: Standby generators have larger power capacities and can support whole-house backup, including central air conditioning systems.

Automatic Operation: Standby generators can start automatically when a power outage occurs, ensuring uninterrupted cooling.

Quieter Operation: Standby generators are quieter than many portable models.


Cost: Standby generators are more expensive to purchase and install than portable generators.

Professional Installation: They require professional installation, which adds to the initial cost.

Standby generators are the preferred choice for homeowners looking for seamless and reliable backup power, including for larger air conditioning systems.

FAQs: Common Questions about Generator Sizing for Air Conditioners

1. Can I run a central air conditioner on a portable generator?

While it’s possible to run a central air conditioner on a portable generator, it’s important to ensure that the generator has a sufficient capacity to handle the starting surge and continuous power needs of the AC unit. Smaller window units or ductless mini-split systems are generally more compatible with portable generators.

2. How do I know if my generator can handle multiple appliances along with my air conditioner?

To determine if your generator can handle multiple appliances and your air conditioner simultaneously, calculate the total power consumption of all the devices you plan to run during a power outage. Ensure that the generator’s running and starting wattage capacities exceed this total.

3. What’s the difference between rated, running, and starting watts on a generator?

Rated Watts: This is the continuous power output that a generator can provide under normal operating conditions. It represents the generator’s sustained capacity.

Running Watts: This is the power required to operate your air conditioner or other appliances once they are running steadily.

Starting Watts: This is the additional power required by some appliances, including air conditioners, when they start up. Starting watts can be significantly higher than running watts for a brief period.

4. Is it possible to connect multiple generators to power my air conditioner if one is not enough?

Yes, it’s possible to connect multiple generators in parallel to increase your overall power capacity. However, this requires specialized equipment and expertise to ensure that the generators synchronize properly. It’s recommended to consult with a professional if you plan to use multiple generators.

5. Do I need to perform maintenance on my generator to ensure it runs smoothly with my air conditioner?

Yes, regular maintenance is essential to keep your generator in optimal condition. This includes oil changes, air filter replacements, and periodic checks of the generator’s components. Neglecting maintenance can lead to performance issues when you need the generator most.


Selecting the right generator size to power your air conditioner is essential for maintaining comfort during power outages, especially in hot climates. Understanding your air conditioner’s power requirements, calculating the necessary generator size, and considering factors like starting surges and additional loads are crucial steps in the process. Whether you opt for a portable generator for smaller AC units or a standby generator for whole-house backup, proper sizing ensures that your air conditioner operates efficiently and reliably when you need it most.


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