Taking the mechanical fan off the engine will certainly lighten the load, but probably not enough to notice. The nice thing about electric fans is they are efficient and safe (simply cannot get fingers near the blades). That being said, most electric fans draw enough current that you will need an alternator to keep up with it. A typical one wire 12V GM style alternator will put out 63 amps max, where as a generator usually puts out around 8 amps. A 14in or 16in electric fan will likely draw over 10 amps. The alternator will not only charge the battery faster and run your electric fan, but will also have capacity to run other electrical equipment such as fuel pumps, water pumps, lights, etc. if you wish.