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Difference between electric and air impact wrench
Caution
Always use an impact wrench with impact sockets so as to avoid breaking, damaging, or stripping nuts and fasteners.
Electric Impact Wrench
Electric Impact Wrench
An electric and pneumatic/air impact wrench often comes handy because its powerful torque feature enables this tool to remove hard-to-turn nuts and bolts. Although electric as well as air impact wrenches perform the same basic function of extracting stripped or stubborn lug nuts and bolts from their sockets, the rate of torque delivered by each tool makes all the difference, and leads to efficiency disparities between the two motors. This Buzzle guide discusses the differences between electric and air impact wrenches.
Air impact wrench
Air impact wrench
Electric vs. Air Impact Wrench
Functionality
An electric impact wrench is powered via an electric plug point through an extension cord, or via 18-28 volts lithium-ion batteries. Since people prefer using cordless electric impact wrenches, only a handful of manufacturers continue to sell corded varieties.

An air impact wrench is powered through compressed air. The power of such a wrench is determined by the power supplied by its air compressor.
Drive Sizes and Torque
Drive sizes help in ascertaining the rate of torque offered by the wrench, and subsequently, higher drive size produces greater amount of torque. An electric wrench comes with adjustable power options which makes it easier for the user to monitor and control the level of power and torque while working.

Air impact wrenches are known to offer more torque with every impact as compared to electric impact wrenches. Unfortunately, most pneumatic impact wrenches do not come with adjustable power modes, which makes it increasingly difficult to control the torque. Therefore, in situations where lesser torque and power is needed, a pneumatic wrench may end up breaking or damaging the nut or bolt being worked upon.
Power-to-weight Ratio
An electric impact wrench holds its main power source and other components within its frame. Thereby, increasing the size, length, and weight of the entire device. This is another reason why, some corded impact wrenches come with an additional handle to provide more support to the user and reduce the bulkiness of the tool. Therefore, corded and cordless impact wrenches provide comparatively lesser power-to-weight ratio than pneumatic wrenches.

A pneumatic impact wrench is comparatively lighter, smaller in size, easier to hold, and more powerful than electric models, because its main power source comes from an external motor in the air compressor.
Size of Device
Corded impact wrenches are found only in two sizes of 0.375 and 0.50-inch, cordless impact wrenches are manufactured in sizes 0.25, 0.375, and 0.50-inch.

As mentioned earlier, pneumatic impact wrenches are lighter, more powerful, and smaller in size. These can be found in size such 0.25, 0.375, 0.50, 0.75, and 1-inch.
Portability and Convenience
Without constant power supply, a corded impact wrench would prove useless as well. However, a cordless battery-operated impact wrench would get the job done in a matter of minutes in such a situation.

Similarly, a pneumatic wrench requires constant supply of compressed air to function optimally, such a tool cannot be used in emergency situations that warrant the use of an impact wrench. Unless the driver with a flat tire carried a portable air compressor, it would be impossible to change a flat tire with the help of a pneumatic impact wrench.
Professional Use vs. DIY
An pneumatic impact wrench is often considered as the auto mechanic's traditional tool, and it is predominantly used in automobile assembly lines because of the high speed and torque it provides. For beginner DIYers, an air impact wrench may prove too powerful and difficult to control for simple tasks.

On the other hand, professionals as well as DIY enthusiasts find it easy and comfortable to use electric impact wrenches.
Safety and Cost
The self-contained design of the cordless impact wrench makes it comparatively more expensive than other types of impact wrenches. Secondly, cordless wrenches are powered by batteries, and thus, unlike corded impact wrenches, do not pose any threat of accidentally electrocuting the user.

Although, pneumatic impact wrenches are cheaper than electric models, buying the tool along with an air compressor may prove to be more expensive.
Therefore, if your work entails the need of powerful and quick torque, you may want to consider going in for an air impact wrench. That being said, you will need to keep checking to ensure that the air compressor has adequate pressure to function optimally. On the other hand, a cordless wrench allows the user to reach tight spots that a corded pneumatic or impact wrench does not allow.