Very basically, an amp rating is how much electrical current a wire or a switch can handle consistently over a period of time. This is true whether it is in a house or an automobile. But automobile electrical systems are different because it is considered a closed system with the battery being the source of power.
The amp rating on a car battery is how long it can supply a current. Batteries are rated in amp-hours. The amp-hour rating then is how long the battery can deliver the required electrical current. A battery with one amp-hour rating can provide one amp of power for one hour. It can provide two amps for half an hour, and so on.
If that was all there was to it, your car battery would not last long at all, and you would constantly have to buy new batteries. The alternator is a key part of the system. It generates electric power as the vehicle runs, and recharges the battery as well. The amperage rating for the battery is how long it can sustain delivering a current. The alternator manages that power and restores it to the battery.
Alternators too then must have an amperage rating. An Aftermarket replacement alternator also has ratings so you can get the proper amount of power. If you have too many items on a circuit, it can overwork the alternator and make it wear out faster than it should. If you have to run a lot of extra things, you can get a professional alternator or a high output alternator.
The alternator charges your battery while the motor is running, and it produces extra power to help run the vehicle and anything else on that electrical system. Alternators should last a very long time. If an alternator stops, the vehicle will still run for a short time until the battery itself is depleted.
If you must replace your alternator, you will have to decide between original equipment and aftermarket. Some say aftermarket replacement alternator may not have the same wattage and may not deliver the electricity as well as equipment designed for that specific car would. Some aftermarket replacement alternator parts are as good as original equipment, and some are not. There is an immediate cost saving with aftermarket. The important thing is to know who is making it and what their reputation is.
Professional alternator, or one that has been refurbished, is a cross between original and aftermarket. This can be a good option to save a little money by buying one that has been refurbished.
Cars are made with alternators with amperage ratings that fit that car’s needs. If you start adding a lot of things, or high-end sound systems you may need a High Output Alternator to handle the extra load. Trucks that pull trailers could also fall into this category. Putting too much stress on an alternator can make it wear out faster than it should, so if you need the extra power, a High Output Alternator could save money in the long run.
To make these choices, you need to know the amps you need to power your vehicle. Once the battery starts the car, the alternator takes over and must deliver the amps it is rated for.