Generators are rated in Watts (or kiloWatts/kW), which represents the amount of power they are capable of producing. So, before you can determine the right generator size to run your air conditioner, you’ll first need to figure out the power usage (Wattage) of the air conditioner.
In this article, I’ll first discuss the power usage of air conditioners, provide some estimates, show you a couple of methods to determine it with accuracy, and even explain how you can reduce this power usage, allowing you to use a smaller generator to run your air conditioner.
Once we have those details covered, I’ll show you how to use the power usage of your air conditioner and that of other appliances to calculate the size of the generator that you need.
If you’re in a hurry, I’ve also included a calculator that does all these calculations for you and estimates the size of the generator you need to run your appliances, including your air conditioner.
Finally, I’ll also discuss the rate and efficiency at which your generator may consume fuel.
By the end of this article, you’ll have a solid understanding of the basics, allowing you to select a generator that efficiently runs your air conditioner and other appliances.
Let’s jump in.
I get commissions for purchases made through links in this post.
How much power does your air conditioner use?
The power usage of an air conditioner, measured in Watts, denotes how quickly the appliance consumes electricity.
The wattage of your air conditioner depends on various factors like its type, age, and efficiency, but it primarily correlates with the cooling/heating capacity of the AC, indicated by a BTU (British Thermal Units) rating or tonnage.
As the tonnage or BTU rating increases, so does the power requirement of the air conditioner.
However, it is important to keep in mind that for most air conditioners, there are 2 distinct Wattage ratings that need to be taken into consideration:
- The Running Wattage of the air conditioner: This represents the number of Watts the A/C consumes while operating normally.
- The Starting Wattage of the air conditioner: This is the maximum amount of power that your air conditioner might use, and it represents the initial surge in power that most AC units require during startup.
Typically, the Starting Wattage of an air conditioner only lasts for a moment, but as a rule of thumb, it can potentially be as high as 6 times the Running Wattage of the AC.
Potential Starting Wattage (Watts) = 6 x Running Wattage (Watts)
For example, if an air conditioner uses 1000 Watts of power when it’s running, it may potentially require as much as 6000 Watts during startup.
When sizing a generator that can run your air conditioner, the “Peak” wattage of the generator should generally be greater than the Starting wattage of the air conditioner.
Of course, the “Running” wattage of the generator should also be greater than the running wattage of the air conditioner, plus the combined running wattages of the other appliances that’ll be running on the generator.
Before I explain this in detail, let’s first discuss the Starting wattage of your air conditioner and how to accurately determine it.
To give you an initial overview, the following table categorizes air conditioners by their BTU ratings and estimates their running wattages and potential starting wattages:
| BTUs/Tons | Est. Running Wattage (Watts) | Est. Potential Starting Wattage (Watts) |
| 5000 BTU | 500 Watts | 2500 – 3000 Watts |
| 6000 BTU | 600 Watts | 3000 – 3500 Watts |
| 7000 BTU | 700 Watts | 3500 – 4200 Watts |
| 8000 BTU | 800 Watts | 4000 – 4800 Watts |
| 10000 BTU | 1000 Watts | 5000 – 6000 Watts |
| 12000 BTU (1 ton) | 1200 Watts | 6000 – 7000 Watts |
| 13500 BTU | 1300 Watts | 6500 – 8000 Watts |
| 14000 BTU | 1400 Watts | 7000 – 8500 Watts |
| 15000 BTU | 1500 Watts | 7500 – 9000 Watts |
| 18000 BTU (1.5 tons) | 1800 Watts | 9000 – 10500 Watts |
| 24000 BTU (2 tons) | 2400 Watts | 12000 – 14000 Watts |
| 27000 BTU | 2700 Watts | 13500 – 16000 Watts |
| 30000 BTU (2.5 tons) | 3000 Watts | 15000 – 18000 Watts |
| 36000 BTU (3 tons) | 3600 Watts | 18000 – 21000 Watts |
| 48000 BTU (4 tons) | 4800 Watts | 24000 – 28000 Watts |
| 60000 BTU (5 tons) | 6000 Watts | 30000 – 35000 Watts |
Please note the following:
- Some air conditioners are equipped with inverter technology, and will not require as much power to start up. Instead, these air conditioners will gradually increase their power usage until they reach their running power.
- The Starting Wattage of your air conditioner will usually be lower than the estimates provided in the table. However, these estimates represent the worst-case scenario and serve as a basis for determining the wattage of the generator required.
How to determine the Starting Watts of your air conditioner?
The easiest way to determine the “Potential” starting watts of your air conditioner is to use an LRA rating that is usually provided on its nameplate.
LRA, or Locked Rotor Amps, represents the maximum amount of current that the compressor inside the air conditioner may require to start up. Using this specification, you can calculate the maximum amount of power that your air conditioner may require:
Starting Wattage (Watts) = LRA (Amps) x Voltage (Volts)
For example, let’s take a look at the following nameplate from a 36000 BTU (3 Ton) central air conditioner:
On the nameplate, notice that the manufacturer specifies “208/230 Volts” as the Voltage of the air conditioner, and 77 Amps as the Locked Rotor Amperage (LRA) of the compressor.
Without getting into too much detail, the 208/230 Voltage rating signifies that the air conditioner can operate at both 208 and 230 Volts. To calculate the maximum amount of power that the AC may require, we’ll use 230 as its Voltage.
The Starting Wattage of the air conditioner can then be calculated as follows:
Starting Wattage (Watts) = LRA (Amps) x Voltage (Volts)
Starting Wattage (Watts) = 77 Amps x 230 Volts
Starting Wattage (Watts) = 17710 Watts
According to our calculation, this 3-ton air conditioner may require up to 17710 Watts of power to start up in some cases.
Please note that this particular air conditioner might not always require the full 17710 Watts of power to start up, however, in a worst-case scenario where the compressor’s rotor is indeed fully “locked”, 77 Amps of current will be required to “unlock” it, and its power usage will surge to 17710 Watts.
When sizing equipment that should be able to start and run the air conditioner, such as an inverter, or in this case, a generator, it should be done based on the worst-case scenario. This way, any potential future failures are avoided.
For some air conditioners, the LRA is not specified on the nameplate. In such a case, your best option would be to measure the starting wattage of the air conditioner using an electricity metering device.
For smaller “plug and play” air conditioners, an electricity meter such as the Poniie PN200 or the P3 P4400 Kill-A-Watt can be plugged between the air conditioner and the electrical outlet to get Wattage readings.
Once everything is set, turn on the air conditioner and wait for the compressor to kick in, the electricity metering device will then display the starting wattage of the AC.
Here’s an example of a Kill-A-Watt meter being used to measure the wattage of a 5000 BTU window air conditioner (link):
In the video, notice that the Wattage briefly surges to about 2000 Watts when the compressor kicks in, which is typical for air conditioners of this size.
For central air conditioners that operate on a dedicated 230V circuit, a clamp meter can be used to determine the starting current of the air conditioner. This current can then be multiplied by the voltage of the unit (230 Volts) to determine the starting wattage of the AC.
If you’re okay with estimates for now, you can use the following rule of thumb to estimate the potential starting wattage of your air conditioner:
Potential Starting Wattage (Watts) = Air Conditioner’s Capacity (BTUs) x 0.6
or
Potential Starting Wattage (Watts) = Air Conditioner’s Capacity (Tons) x 7000
How to reduce the Starting Watts of your air conditioner?
The starting watts of your air conditioner might not be a big deal if you’re running the AC on grid power, or even on an inverter, however, when trying to size a generator that can run the air conditioner, these initial surges in power usage become a problem.
For example, a 2000-watt generator will run a 13500 BTU RV air conditioner, which only requires around 1300 Watts of power to run. But, will the generator be able to start it in the first place?
The answer is: not always.
As explained above, the starting wattage of a non-inverter air conditioner can potentially go up to 6 times its running wattage. So, the 1300 Watts of running power that a 2000W generator can easily handle, can potentially go up to 7800 Watts of starting power.
If you have a non-inverter air conditioner, you’ll essentially have 2 options, get a big enough generator to accommodate the starting watts of your air conditioner, or install a soft starter device.
A device such as the MICRO-AIR Easystart 364 can reduce the Starting Wattage of your air conditioner by up to 70%, and therefore, also reduce the size of the generator that you need.
By reducing the size of the generator that you need, you’ll not only save on the cost of the generator itself but also on fuel costs. This is because generators are most fuel-efficient at 100% of their Running (normal) load capacity or about 80% of their Maximum (peak) load Capacity.
If the generator operates at a lower load capacity, for example, 20 to 30% of its running load capacity, it will consume more fuel per unit of energy produced (measured in gallons, liters, pounds, or grams per kilowatt-hour, kWh) compared to running at its full capacity (100%).
This can be mitigated by using a generator that matches your running wattage requirements, which would only be possible by reducing the gap between the running wattage and the starting wattage of your loads.
What size generator do you need to run your air conditioner?
Generators have 2 main ratings that you would need to consider:
- Their Rated/Running Wattage: This represents the maximum amount of power that the generator is designed to “continuously” deliver at its output.
- Their Peak/Maximum Wattage: This represents the maximum amount of power that the generator is designed to “briefly” deliver at its output. The Peak Wattage of generators is usually around 110 to 130% of their Running wattage.
As explained above, a generator that could run your air conditioner should have a Running Wattage rating that is equal to or higher than the Running Wattage of the AC, but more importantly, a Peak/Max Wattage rating that is equal to or higher than the Starting Wattage of the AC.
For instance, if your air conditioner requires 7000 Watts to start up, the generator should have a peak wattage rating of 7000 Watts or more, which would typically mean a rated wattage of about 6000 Watts.
However, in such a case, the 6000 Watts that the generator can supply would only be useful when starting the air conditioner, and for the other 99% of the time, you’ll have a generator that is about 5 times oversized.
Unless you plan on justifying the extra wattage of the generator by using it to run other appliances as well, the generator will end up running at low capacity all the time, making it fuel-inefficient.
A solution to this would be to install a soft starter kit, which would reduce the starting wattage of your air conditioner by 60 to 70%, and with that, the size of the generator that you need.
To visualize this, here’s a table that estimates the size of the generator (Peak Wattage) that you would need to run an air conditioner based on its BTU (Tonnage), and whether or not it is equipped with a soft starter:
| BTUs/Tons | Required Generator Peak Wattage without a soft starter | Required Generator Peak Wattage with a soft starter installed |
| 5000 BTU | 3000 Watts+ | 1200 Watts+ |
| 6000 BTU | 3500 Watts+ | 1400 Watts+ |
| 7000 BTU | 4200 Watts+ | 1600 Watts+ |
| 8000 BTU | 4800 Watts+ | 1800 Watts+ |
| 10000 BTU | 6000 Watts+ | 2200 Watts+ |
| 12000 BTU (1 ton) | 7000 Watts+ | 2600 Watts+ |
| 13500 BTU | 8000 Watts+ | 2800 Watts+ |
| 14000 BTU | 8500 Watts+ | 3000 Watt+ |
| 15000 BTU | 9000 Watts+ | 3300 Watts+ |
| 18000 BTU (1.5 tons) | 10500 Watts+ | 3800 Watts+ |
| 24000 BTU (2 tons) | 14000 Watts+ | 5000 Watts+ |
| 27000 BTU | 16000 Watts+ | 5700 Watts+ |
| 30000 BTU (2.5 tons) | 18000 Watts+ | 6500 Watts+ |
| 36000 BTU (3 tons) | 21000 Watts+ | 7500 Watts+ |
| 48000 BTU (4 tons) | 28000 Watts+ | 10000 Watts+ |
| 60000 BTU (5 tons) | 35000 Watts+ | 12500 Watts+ |
Related: What size generator to run a heat pump?
Please note that the generator sizes provided in the table were calculated based on the typical LRA (Locked Rotor Amperage) ratings of these air conditioners, and based on the assumption that no other appliances will be running on the generator when the air conditioner starts up.
If other appliances will be running on the generator as well, the running wattage of these appliances should also be taken into consideration.
The Peak Wattage of the generator should be calculated using the highest starting wattage of your appliances, plus the running wattages of the other appliances that would be simultaneously operating.
For example, let’s say you want the generator to be able to run the following appliances:
- A 15000 BTU RV air conditioner that has a running wattage of 1400 Watts, and a starting wattage of 8000 Watts.
- A TV that has a running wattage of 100 Watts.
- An RV residential fridge that has a running wattage of 250 Watts, and a starting wattage of 1500 Watts.
- A few LED lights which pull a total of 80 Watts when they’re ON.
As long as you turn on the air conditioner and the refrigerator separately, the highest wattage that your generator will have to handle running all of these appliances at the same time is:
Highest Wattage (Watts) = AC’s Starting Watts + TV’s Running Watts + Refrigerator’s Running Watts + LED Lights Watts
Highest Wattage (Watts) = 8000 Watts + 100 Watts + 250 Watts + 80 Watts
Highest Wattage (Watts) = 8430 Watts
In this case, a generator such as the DuroMax XP8500EH would be a perfect fit, as it has a Peak Wattage capacity of 8500 Watts and a Running Wattage capacity of 7000 Watts.
However, let’s say you’ve decided to install a MICRO-AIR Easystart 364 on your RV’s air conditioner, and let’s conservatively assume that you were able to reduce the starting wattage of the AC to about 4000 Watts.
Now, the highest wattage that the generator would have to be able to handle is:
Highest Wattage (Watts) = AC’s Starting Watts + TV’s Running Watts + Refrigerator’s Running Watts + LED Lights Watts
Highest Wattage (Watts) = 4000 Watts + 100 Watts + 250 Watts + 80 Watts
Highest Wattage (Watts) = 4430 Watts
With this upgrade, a smaller generator such as the 4650W Westinghouse generator would be enough for these power requirements, as it offers a peak wattage of 4650 Watts and a running wattage of 3600 Watts.
Now, to save you some time, I’ve put together a calculator that estimates the size (Peak Wattage) of the generator that you’d need to run your air conditioner and other appliances.
All you have to do is choose an appliance from the dropdown menu, and the calculator will estimate the Running Wattage and Starting Wattage of the appliance. However, you can also add your own appliances and submit their power ratings manually.
The calculator will then estimate the Peak Wattage rating of the generator you need based on your list of appliances and their power usage:
Now that you have some idea of the size of generator you’d need, let’s talk about fuel and efficiency.
Fuel consumption and efficiency:
The fuel consumption of a generator to run your air conditioner primarily depends on the energy consumption of the air conditioner, which, in turn, is determined by the size of the air conditioner. The greater the BTU rating or tonnage of the AC, the more fuel the generator will use.
Click here to determine the energy consumption of your appliances.
However, as mentioned above, the rate at which a generator consumes fuel will also depend on how much of its capacity you’re using. The higher the load, the less fuel (Gallons or Pounds) the generator consumes per unit of energy (kWh) that it generates.
For example, a generator that has a Running Wattage rating of 3000 Watts, will consume the least amount of fuel per unit of energy (kWh) when it’s producing 3000 Watts (3 kW) of power.
To give you a general idea, here’s a table that estimates the amount of fuel that a generator uses to produce 1 kWh (kiloWatt-hours) of energy at different percentages of its capacity, and for different types of fuel:
| Fuel Type | Consumption at 25% Running Capacity | Consumption at 50% Running capacity | Consumption at 100% Running capacity |
| Gasoline | 0.28 – 0.32 US Gal/kWh (1 – 1.2 liters/kWh) | 0.22 – 0.25 US Gal/kWh(0.83 – 0.94 liters/kWh) | 0.16 – 0.19 US Gal/kWh(0.6 – 0.72 liters/kWh) |
| Propane | 1.3 – 1.6 lbs/kWh (0.3 – 0.38 US Gal/kWh) | 1 – 1.2 lbs/kWh (0.23 – 0.28 US Gal/kWh) | 0.7 – 0.9 lbs/kWh (0.16 – 0.21 US Gal/kWh) |
| Diesel | 0.11 – 0.13 US Gal/kWh (0.44 – 0.5 liters/kWh) | 0.08 – 0.1 US Gal/kWh (0.32 – 0.38 liters/kWh) | 0.06 – 0.07 US Gal/kWh (0.24 – 0.28 liters/kWh) |
| Natural Gas | 0.7 – 0.9 Cu. m./kWh (25 – 32 Cu. ft./kWh) | 0.5 – 0.65 Cu. m./kWh (18 – 23 Cu. ft./kWh) | 0.39 – 0.48 Cu. m./kWh (14 – 17 Cu. ft./kWh) |
It is worth noting that the values provided in the table are rough estimates, the actual fuel consumption, and efficiency may vary depending on the generator.
It is also important to note that these figures represent the rate at which fuel is consumed (gallons or pounds per kWh). A generator will naturally consume more fuel when it’s producing more power.
For example, let’s say a gas-powered generator that is rated at 10000 running watts is producing 10000 Watts (10kW) of power. Because the generator is operating at 100% load, it will consume between 0.16 and 0.19 Gallons of gas per kWh of energy that it generates.
Since the generator is producing around 10 kWh of energy per hour, it will consume 1.6 to 1.9 Gal/hour.
However, if the generator is only producing 2500 Watts (2.5 kW) of power, it would consume between 0.28 and 0.32 Gallons of gas per kWh of energy that it generates.
Since the generator is producing around 2.5 kWh of energy per hour, it will consume 0.7 to 0.8 Gal/hour.
Now, when the generator was producing four times as much power (10 kWh), it did consume more fuel per hour, but almost only twice as much fuel instead of four times as much.
If you don’t yet grasp the difference between kW(power) and kWh (energy), please click here to learn more.
To further understand how important it is to size the generator correctly, let’s consider the following example:
Case 1:
Let’s say you have a 13500 BTU RV air conditioner that, on average, uses 1300 Watts to run, and consumes around 1000 Watt-hours, equivalent to 1 kWh per hour. Let’s also say this air conditioner is not equipped with a soft starter, and at some point, may require up to 7000 Watts to start up.
For simplicity, we’ll assume that there are no other appliances and that we only need the generator to run the air conditioner. This means that we’d need a generator with a Peak Wattage of 7000 Watts or more.
A generator such as the Westinghouse 7500W generator seems to fulfill these requirements, as it has a Peak Wattage of 7500 Watts and a Running Wattage of 6000 Watts.
When the air conditioner is up and running, it uses 1300 Watts of power, which represents around 22% load for the generator (1300 Watts ÷ 6000 Watts = 22%). Since this is a Gas generator, at 22% load, it will use around 0.33 Gallons of gas per hour.
Case 2:
Now, let’s assume that the same air conditioner is equipped with a MICRO-AIR Easystart 364 and it now only requires 2100 Watts of power to start up.
In this case, we’d only need a smaller generator such as the Honda EU2200I, which has a Peak Wattage rating of 2200 Watts and a Running wattage of 1800 Watts.
Running our RV air conditioner, this generator would be running at around 73% capacity (1300 Watts ÷ 1800 Watts = 73%), and it would consume around 0.2 gallons of gas per hour.
