Here is some info from a forum that may be of interest.

"ALTERNATORS _David Harwood_ One horsepower equals 746 watts. A
perfectly efficient alternator would require one horsepower of
mechanical input to produce 746 watts, but since there are friction
and eddy current losses it takes more than one hp to produce 746
watts. Dividing 746 by the 13.8 volts that most regulators are set up
for, one perfectly converted hp would equal about 54 amps. So a 55 amp
alternator that is actually putting out 55 amps will reduce the
engine's output by something more than one hp. But as the charge is
restored to the battery, the current goes down until it the battery is
fully charged and the alternator supplies only the minimal current
needed to run the instrumentation and spark a gas engine. At that
point the engine is able to put out its rated power. If you now turn
on the stereo/GPS/depth sounder/cabin fan etc, the alternator must
supply the current for those consumers, so the hp at the shaft will go
down. The advantage of a larger alternator is that it recharges the
battery faster (or a bigger one in the same time). One horsepower
equals 746 watts. A perfectly efficient alternator would require one
horsepower of mechanical input to produce 746 watts, but since there
are friction and eddy current losses it takes more than one hp to
produce 746 watts. Dividing 746 by the 13.8 volts that most regulators
are set up for, one perfectly converted hp would equal about 54 amps.
So a 55 amp alternator that is actually putting out 55 amps will
reduce the engine's output by something more than one hp. But as the
charge is restored to the battery, the current goes down until it the
battery is fully charged and the alternator supplies only the minimal
current needed to run the instrumentation and spark a gas engine. At
that point the engine is able to put out its rated power. If you now
turn on the stereo/GPS/depth sounder/cabin fan etc, the alternator
must supply the current for those consumers, so the hp at the shaft
will go down. The advantage of a larger alternator is that it
recharges the battery faster (or a bigger one in the same time). _Ted
Andresen, /Gypsy/, #75_ Just wanted to insert a change I made to my
alternator circuit that may be of value to others. When I installed
the alternator, I put a 50 amp meter in the output leg to measure the
charging current. I was surprised to find that my 50 amp alternator
rarely charged the batteries above 10 amps for more than a few
minutes. Then it would decrease to 5 or 8 amps depending on the number
of amp-hours I had drawn out of the battery on the previous day's
sail. I normally use 10 amp-hours at night. At 5 amps it would take 2
hours to put the 10 amp-hours back into the batteries. I did not like
listening to the engine, so I found a way to get more current out of
the alternator. I ran an extra line from the +12V side of the battery
through a 20 ohm 50 watt rheostat directly into the alternator's field
windings. That's where the voltage regulator also connects to the
alternator. Then I could adjust the rheostat and crank up the current
to 10 amps. I don't think it made much difference to the batteries,
since they didn't bubble (hydrogen gas) or get warm. Instead, of
charging them at 60 watts for 2 hours, I was able to charge them at
120 watts in half the time."