Both Kayakncamper and Madjack are right (to a degree).
Installing a resistor will reduce the current and since power = current X voltage (and the voltage remains the same) the power/current withdrawn from the battery will be less depending upon the resistance of the resistor.
The voltage drop across the resistor does nothing except generate heat, no motion of the fan. Generating heat takes power. This power is wasted.
A pulse width modulation (PWM) speed control shuts the power to the fan on and off at a rate that drives the fan to the desired speed as determined by the position of the control knob. As no current is flowing through a resistor, no heat is being generated and power wasted as heat.
Now, in reality the PWM does generate a bit of heat and some power is "wasted" but the amount of heat generated by the PWM unit is far less than a simple resistor.
More "World according to Gus".
Hope this helps.
The opinions in this post are my own. My comments are directed to those that might like an alternative approach to those already espoused.There is the right way,the wrong way,the USMC way, your way, my way, and the highway.
"I'm impatient with stupidity. My people have learned to live without it." Klaatu-"The Day the Earth Stood Still"
"You can't handle the truth!"-Jack Nicholson "A Few Good Men"
"Some people spend an entire lifetime wondering if they made a difference in the world. The Marines don't have that problem"-Ronald Reagan