alternator hp draw
 

I found this formula to calculate how much hp is required to run your alternator.

amps x volts = watts
745.7 watts = 1 hp
hp x 15% = hp loss
hp +hp loss = total hp used
example
55 amp x 14.9v = 819.5 watts
819.5 watts / 745.7 = 1.098 hp
1.098 hp x 15% = 1.26 hp

Dunno where you got that but most alternators are not 85% efficient from my reading. They're more like 50-60%. So, you should really multiply by 1.5 to get your final hp loss to the alternator. Of course, this only calculates the max hp loss to the alternator. You need to find the current draw in normal driving to get an accurate number, and of course that'll vary depending on the electrical load.

...also, don't forget that ELECTRICAL POWER is needed to energize the FIELD WINDING to create/control the magnetic field that's generating the output voltage...that's another "LOSS."

And you would want then to figure what percentage this is of the power your engine generates in order to accelerate your vehicle or hold steady state, no? If at freeways speeds you only needed say 30hp but your alt is drawing off 1.5 or more, you have maybe at least 5% going to power generation instead of vehicular mobility, no?

if the efficiency rate is 60 % that would make maximum draw on a 55 amp alt 1.53 hp. now of course you would only be using that much if you were placing a heavy drain on you battery. under normal driving, the alternator would not be using much hp. but these numbers help us to see why 10% better mpg is possible with an alternator delete. when you figure an engine doesn't produce much hp under 2,000 rpm, lets say 15 hp in my 1300 cc crx, 1.5 hp would make that 10% very plausible.

or another way, normal driving you are drawing 15 to 20 amps for fuel pump, computer and ignition.

which ain't much.

Good information, my car uses a 120 amp alternator so my losses are large, approximately 3.5 hp which when cruising on the hwy with the alternator cranking to recharge a low battery, my losses could be as hi as 25%.
Yesterday I split my 30 watt panel in two(it was 2 panels on a hinge) and put 1 panel on my Beetle's huge dashboard. The panel will keep my battery fully charged and aid the alternator when the car is in use.
I am pleased with the benefits of it.

lets say your starter draws 500 amps for 4 seconds when starting.

your alternator would need to produce 120 amps for 16 seconds to recharge it.

then it draws the same 10 to 15 amps every other cars takes.

Assuming my battery is fully charged from my solar panel
when I cranked the starter you would be correct !
Which is why I have the panel, 15 watts of 14.2 volts at 1 amp is very close and maybe the same on most days as a battery tender junior, which charges at 14.2 volts & 750 ma.

I think running without the alternator for shorter trips is a very good idea. Using a portable PV array to help keep it topped off is also a good idea, but, I wonder if it's worth the cost and trouble? I think it may be if you remain parked for long enough stretches to provide a substantial charging. How long that period of time is, I have no idea. Depends on the size of the array and the strength of the sunlight.

I think a better idea is active management of when the alternator is used and use of deep cycle batteries. A simple way would be to toggle it on with the brake light signal. An accurate voltmeter would tell you when your charge was getting low enough to need to go to full time alternator use.