In any event, the amount of HP required to operate a 150 amp alternator at full load is fairly insignificant ... a few horsepower at most.
Correct.
1 horsepower = 745.699872 Watts
Watts / Volts = Amps
745.699872 Watts / 12 volts = 62.141656 amps.
150 amps / 62.141656 amps = 2.413839759918854 horsepower
A 90 amp alternator is 1.448303855951312 horsepower
The difference between an 90 amp and a 150 amp alternator is 0.965535903967542 horsepower, about 1 horsepower.
In boating they use a little metric to calculate your engine's fuel consumption at wide-open-throttle (WOT) as approximately equal to HORSEPOWER/10 = GallonsPerHour (gph).
Then, approximate the fuel consumption at a particular throttle setting as a linear percentage of the wide-open-throttle consumption in RPMs. The results give you gallons per hour at a given RMP. You can more or less translate that as an approximation to fuel economy in a car or truck.
A 300 HP diesel engine on a boat might get gallons per hour of
GPH = (0.4 x 300)/ 7.2 = 105/7.2 = 16.66666666666667 GPH
That's on a boat, though, wide open, about 4400 RPM, no gears, just running. But you can do the same with a 301 HP engine and get
GPH = (0.4 x 301)/ 7.2 = 105/7.2 = 16.72222222222222 GPH
That's a difference of 0.0555555555555522 GPH.
OK, if you get 20 MPG and drive 60 miles in an hour, that's only 3 gallons per hour, instead of the 16+ gallons per hour on a boat. So, the .055 GPH used for the extra 1 horsepower will be far, far less when you're getting 20 MPG at half the RPMs than when you are getting the 3.5 MPG at 4400 RPMs.
It's about 5.7142857 times less fuel for that 1 HP, or 0.0097222222465272 GPH, and at 20 MPG that becomes 0.00016203703744212 MPG.
That's not a tenth of a MPG (0.1) or even onethousandth of a MPG (0.001), it's one tenthousandths of a MPG. And that's what it costs you, at most (if the alternator is cranking out max amps 100% of the time), to go from a 90 amp alternator to a 150 amp alternator.
