About alternators: alternators are regulated to put out more or less constant voltage despite varying load and RPM. Most regulators shoot for around 14.1V these days. But looking at the votage tells you nothing much about the load on the alternator, you need to watch the amerage being put out. That's why some dashboards have ammeters in 'em.
For a given amperage output, the alternator needs a given amount of energy input. That's torque times RPM. At low RPMs and high load, the alternator pulls a lot of torque from your belt drive. At high RPM, the same load is going to need less torque.
Alternators run more efficiently at high RPMs because the current flow within the field winding is lower and therefore things don't get as hot. Plus that crappy little "fan" on the pulley is moving more air to keep things even cooler. Despite that, alternators are terribly inefficnet at best. Most are in the neighborhood of 25%-30%.
Physically bigger alternators with lower amp ratings are generally more efficient. That's not the Ranger's alternator.
An electric fan will be doing the most work when sitting at idle for long periods of time, when the alternator is at the greatest disadvantage. It's likely that using an electric fan setup will shorten the life of your alternator.
Electric motors are also lossy. Going through the pulleys, alternator, then the motor running the fan, you're lucky if you get as much as 20% of your power to actually turn the fan.
Mechanical drive can't be beat for efficiency. For every 1 horsepower put at one end of a shaft, you get 1 horsepower at the other end under constant load. 100%. Belts and pulleys are in the 95%+ neighborhood. A viscous clutch would have to waste near 80% of the power going through it to be as inefficient as an electric fan. Even though it has heat sinks all over it, they're not enough to survive that much heat. Seat-of-the-pants guess, they're 60% or better efficient on average.
The viscous clutch works by fluid drag. The relative rotation of the parts naturally pumps fluid around a circuit inside the clutch. There's a little thermometer on the front of the hub, exposed to heat from the radiator. As it heats up, a valve is opened to let as much fluid as possible move through the clutch, making max drag, and spinning the fan the fastest. But it's also the most energy wasteful mode, so the clutch itself gets pretty hot. When the radiator cools off, the valve closes, causing fluid to be collected in a reservoir, and the clutch can slip freely. This is when the clutch is most efficient.
Right at start up the fan is on at full blast for a few seconds while the fluid is pumped into that reservoir, then you can hear the fan unload and freewheel.
At normal operating temp the clutch will drag a little, but nowhere near max. When you drive down the highway the air rushing through the radiator will actually help to rotate the fan and the clutch will be unloaded and use no power. Just like an electric fan that's "off."
The advantage to an electric fan is that you've got a battery to cushion the amp draw when the fan kicks on, so you don't necessarily notice the fan suddenly loading down the engine. Also, since the fan runs at one speed only, you can use a lighter, flimsier blade, but more efficient. The factory fan has to compromise on the fan blade to make it cheap to build, strong enough not to pop at 5,000rpm, able to take revving up and down all day, and more vibration tolerant. Actually moving air is pretty low on the list of priorities.
I'm sticking with my factory setup.
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