Inside every cordless screwdriver is an electric motor.
When the speed control trigger is squeezed, it activates a switch which engages the motor instructing it to draw electrical current from the battery. The further the trigger is pulled back, the faster the speed of the drill.
The motor converts the battery’s electrical energy into mechanical energy giving the rotation required to turn the chuck.
Wattage
The power a motor delivers is measured in watts, and is a combination of torque and speed.
A motor with a higher wattage will be more powerful, and will be able to deliver the required torque at a higher speed.
Most manufacturers won’t state the wattage of a cordless screwdriver’s motor.
This is because cordless screwdrivers, compared to other cordless tools, are not very powerful, and so the motor’s wattage is not considered important.
As a result, you only need to worry about motor wattage if you are looking for a very powerful cordless screwdriver for working with tough materials (such as metal or hardwood) or larger screws.
If this is the case, look for a model with a 100 watt motor or higher.