I did have it set up that way, but I wasn't getting consistant cooling. I changed to on with the key (thru relays of course). I haven't run it that way yet. I have a FoMoCo 2 speed fan from a 3.0 Taurus.
Maybe my temp sensor is not in a good place.
let's say 20 amps draw at 13.7 volts = 274 watts = 0.274KW * 1.34 = 0.367HP
that's how much you lose, not a lot but there is a loss. The mechanical fan would use more, but if you run it all the time the difference is less since your alternator will be using power to provide current for the fan.