What Size Generator to Run Different HP Air Compressors? Explained!

What Size Generator to Run Different HP Air Compressors? From power tools to tire inflators, an air compressor is a common name for a variety of industries. However, running electric air compressor requires a certain amount of power, and calculating that can be confusing sometimes.

So, what size generator to run different hp electric air compressor?

The size of the generator needed depends on the horsepower of the compressor, as well as the starting and running wattage requirements of the specific model. For example, a 1 HP compressor will require a minimum of 4500 watts of starting and 1500 watts of running power. There is a very easy way to calculate the required amount of power.

I’ve explained two different calculation methods with a detailed power requirement guide so that you can help yourself. So, let’s jump in!

How to Calculate the Wattage of Your Compressor?

An air compressor works by drawing in compressed air, compressing it to increase its pressure, and storing it for various applications through the release of pressurized air when needed

Calculating the wattage of your electric air compressor is the first step to finding out what size generator you might need to run your electric air compressor. While picking generator wattage is a very significant criterion as this directly governs the longevity a portable generator can provide.

There are usually two methods of calculating the wattage and they are the following.

First Method: Calculating Using Voltage and Amperage

This method is the more accurate and mathematically accurate method. So let’s jump into the process.

Find Out Voltage and Amperage of Your Compressor:

This can be easily found on the provided manual by the manufacturer or on the body of your air compressor.

Calculate the Running Wattage:

To find the required running wattage, you need to multiply the amperage, voltage, and electrical power factor of your compressor. For example, if you have a 1HP 120V 10 A single-phase motor then the calculation will be as follows.

Running Wattage = Volts x Amps x Power Factor
= 120V x 10 x 0.8
= 960 watts  

(The power factor is 0.8 because it is a commonly used value for single phase motor)

We should keep some headroom above the theoretical wattage because no system is an “Ideal Carnot System” and can not be 100% efficient. As a result the wattage range, in this case, should be around 1000-1200 watts. However, generally, a 1.2-1.5 kW generator is commonly used when choosing a generator to power a 1HP compressor.

Calculate the Starting Wattage:

Once you’ve found out the running wattage, now you need to find out the required starting wattage of the compressor. Compressors usually use capacitor start motors. 

And there are two types of them, one is G type and the other one is L type. So, find out which one is yours. If your motor is G type then you need to multiply the running wattage by 3 to get the starting wattage. On the other hand, if your air compressor has an L-type motor then you need to multiply the running wattage by 6.

Second Method: Calculating Using Horsepower

This is an easier but less precise method as this requires you to know about the HP to watt conversion only. Check the following steps.

Calculate the Current Draw:

In this step we need to calculate how much current the air compressor draws according to its HP. As we all know 1 horsepower is equal to 745.7 Watts. The formula to calculate the drawn amperage is as follows 

Current (A) = (Horsepower x 746) / (Voltage x Power Factor)

Calculate the Running Wattage:

To calculate the running wattage you can follow the previously mentioned method. However, here I’m repeating the formula again.

Running Wattage = Voltage x Current x Power Factor

Voltage and amperage requirements. Calculate the Starting Wattage:

This process is the same as the previously mentioned method of calculating the starting wattage. So follow that one.

Now I’m going to elaborately explain the requirements of generator size to run different horsepower air compressors.

What Size Generator Do You Need to Run Different HP AC?

I’ve explained thoroughly the general procedure of calculating the required generator size to run an AC. 

To make the calculation uniform and understandable we have taken a single-phase AC operating on a 115 V and 230 V power supply with a power factor of 0.8 and calculated wattage for different horsepower ratings.

Air Compressor PowerApproximate current draws (amperage)Generator Running Wattage Required (watts)Starting Wattage Required for G type Motors 
Starting Wattage Required for L type Motors 
115 V230 V115 V230 V
0.5 hp9.85901.692027055520
1 hp1681472147244168832
1.5 hp201018401840552011040
2 hp241222082208662413248
2.5 hp301527602760828016560
3 hp341731283128938418768
4 hp4023368042321269625392
5 hp5628515251521545630912
6 hp6432588858881766435328
6.5 hp6937634868082042440848

Well, now you have the idea but do keep in mind that while picking the generator keep some headroom as no system can be 100% efficient. Also, I’ve taken the amp draw data from an HP to amp draw conversion chart.

Here’s a graphical comparison of different power requirements for different HP gasoline powered air compressor.

horse power chart

Note: This sizing guide is just an estimate to give you a rough idea while choosing the generator. The actual power requirement will vary with brands, models, capacity, and specifications. So better take the help of the user manual or talk to the manufacturer.

What Size Generator to Run a 1/2 hp AC?

A ½ HP air compressor will require a generator with a wattage of at least 2 kW. As we can see from the table above it requires 1 kW for running and 2 kW for starting. We have rounded up the rough value as there is some need for headrooms. 

Even though this is a very small generator and can be easily fitted into your house, never run the generator inside your house.

What Size Generator to Run a 1 hp Compressor?

To start an AC of 1 horsepower, you will be needing a minimum 4.5 kW generator. The minimum running power is 960 watts, the upper range is almost 1.5 kW. A portable generator of this size will easily be able to power a house during blackouts or regular days.

What Size Generator to Run a 1 hp Air Compressor
Source: ecicomfort.com

What Size Generator to Run a 1.5 hp AC?

A 1.5 HP electric air compressor will need around a 2000 running wattage and 6000 starting wattage generator to support it, minimum. However, this value can rise up to 12000 watts if the motor is a type L motor. 

However, you might be wondering why the starting power is so much more than the running power.

The starting power is almost 3 times the running power because the electric motor requires higher torque while starting than usual running. During the starting phase, the motor requires a surge of current which is way more than the current required in the running phase.

To maintain the generator and prevent any damage to any parts this is crucial that enough wattage is supplied to support the starting stage.

What Size Generator to Run a 2hp Compressor?

A whooping 7000 starting watts is required for a 2 HP air compressor whereas the running wattage is around 2300 watts. Always keep in mind that most compressors require 3x times starting power than the running power.

What Size Generator to Run a 2.5 hp compressor?

You will require a minimum of 8300 watts to start a 2.5 HP AC if the motor is type G. The voltage of 115 V or 230 V will not change the value of the total required running wattage of 2800. Because along with the change in the voltage, the current draw also varies.

What Size Generator to Run a 2.5 hp Air compressor
Source: tooltally.com

What Size Generator to Run a 3 hp AC?

For a 3-Horsepower air compressor you need to get a generator with 3.2 kW running wattage and 9.5 kW starting wattage. But do remember this is for a type G motor and if yours is a type L motor then this value will shoot up to around 19 kW.

What Size Generator to Run a 5 hp Compressor?

For a single phase 5 HP motor with a power rating of 0.8 will require at least 15500 watts to start and 5200 watts to run efficiently. However, the range you should consider is around 20,000 watts as this is a heavy-powered motor and needs a significant amount of headroom.

So that was all about the requirement generator for different horsepower AC. However, it’s better if you calculate as I’ve explained for your compressor as there are many things that can vary from motor to motor requirements.

Frequently Asked Questions (FAQs):

Can I use a portable generator to run my compressor?

Yes, you can use a portable generator to run your compressor. All you need to take care of is that it has enough power to support the power required to start and run the AC.

Can I run 2 compressors together?

You can run 2 compressors if they are matched. However, the two compressors won’t run together most of the time. The second generator will help out only when there is too much load.

Can I run a 240v compressor on a generator?

Yes, obviously you can run a 240V compressor on a generator. All you need to do while calculating the power draw must be factored in precisely. Otherwise, your AC will not run.

Conclusion: What Size Generator to Run Different HP Compressors?

That was all about what size generator to run different hp AC. I’ve tried my best to break the whole concept down into understandable pieces. Hope you now have a much clearer view of this issue. I’ve mentioned this many times in the article but always choose the generator keeping the headroom in your mind.

See you soon! Best of luck!

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About Frank Diaz

Hi everyone, my name is Frank Diaz and I am the face behind GaragestPicks. I studied Mechanical Engineering at Georgia Institute of Technology and worked as a mechanic for over 20 years. I love working on cars and doing DIY projects around the house. I write this blog to share my knowledge and help others fix their cars with most common tools.

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