Also see the alternator overview article.
If you look at the specs for your alternator will be an amp rating, which is a measure of how much electrical current it can make under optimal conditions. This might be 80A for a passenger car, 150A for a cargo van, or >200A for a heavy duty vehicle. You can often find the actual rated output of your alternator in your manual, vehicle specs, or using a VIN lookup tool for your brand of car.
For the purposes of this discussion we will assume a cargo van with an alternator rated 140A.
Alternator output in the short term and longer term is affected by several factors
There is no way to know the actual number of amps you can pull from the alternator in any given condition. A general rule would be that if the output voltage is lower than normal the alternator is stressed.
While paying attention to the above rule, you might use some other rules of thumb:
Alternator voltage output is intimately related to current. In the simplest model the alternator's voltage is controlled2) by a voltage regulator. The regulator uses current to hit the desired voltage setpoint.
If the alternator cannot make sufficient current to stabilize voltage3) then output voltage would drop. Hence the general rule above about voltage drop.
Note: modern alternators may not actually be regulated to a single voltage like 14.0v; see below.
Standard alternators are the simplest model like the 14.0v model in the example above. They are called internally-regulated because the voltage regulator is built into the alternator itself.
Externally-regulated alternators are alternators where the regulation function is physically separated from the alternator. This may have advantages in parts replacement4) and “hacking” the alternator's output. The external regulator may be:
Smart alternators are designed to support fuel mileage efforts. Voltage might plummet during full accelleration (to free up engine power) then spike quite high in other conditions. This wide variance means a DC-DC charger is required. Note: some smart alternators have a setting that allows them to be function like a standard alternator.
Heavy duty / high output alternators - any of the above alternators can be build in heavy duty configuration, which typically connotes:
HD alternators for any particular vehicle are typically bolt-in with no modifications required. They do cost somewhat more. The labor cost would remain the same so some people choose to upgrade the alternator when/if the OEM one fails.
Some older Class A and C RVs came with DUVAC (DUal Voltage Alternator Control)5) setups, which use separate voltage-sensnig wires for the house bank.
…the primary reason for DUVAC is to allow the alternator to “compensate” for the voltage loss (turned into heat) in the old generation diode-based battery isolators. There is approximately a .7 VDC loss through the battery isolator. The sense wire/terminal allows voltage on the battery side of the isolator to control the alternator voltage.9)