step 1: Find out how much RMS wattage your amplifier will produce based on the load presented by your speakers. (The load depends on base speaker ohm rating, # of voice coils, and the way they are wired.)
step 2: Divide this number by 12 (volts)
step 3: Take the resulting number and multiply by the factor below, based on the amplifier type:
Class AB: *1.4
Class T or D: *1.2
step 4: Add the totals together if there is more than one amplifier in the system
step 5: add your stock alternator's HOT current rating to this figure.
The total you get is the amount of current your car and audio system will draw at peak output.
Most vehicle electrical systems are 12.5VDC at idle, and between 13.8 and 14.4VDC above about 900rpm but we use 12 volts as the standard for these calculations because most amplifiers today use a regulated power supply, so they get 12VDC regardless of input voltage, so...
1,200 watts divided by 12 volts equals 100 Amperes of current.
If this is a class D amplifier, it'll be ~80% efficiend, so we'll add 20% of 100A to the total, and we get about 120A of current demand continuously at peak output.
This means that the electrical system of the vehicle will need to produce an extra 120 Amperes of electrical current to power your amplifier and subwoofer at peak levels. Keep in mind that the factory electrical system is designed to produce enough charging power (alternator and battery) for the vehicle's stock equipment, and was not designed to accomodate high-powered audio systems.
The biggest mistake made by consumers (and many installers) is failing to beef up the charging system to handle the extra load of the audio system. First, you need to understand how the electrical system operates. This must be one of the most mis-understood systems of the entire vehicle, so here's a brief summary:
Turning the ignition key begins the process of cranking the motor. The battery supplies the power to get things started. Once the engine is running, the electrical burden is shifted over to the alternator, and the battery then goes into a charging state, and only functions to filter and stablize the DC voltage from the alternator. The alternator uses the engine's mechanical power to produce electrical (AC) current. The AC current is passed through a rectifier and changed to DC current by the DC voltage regulator to smooth out and set the voltage rails for the car . The alternator also has the duty of recharging the battery after starting the vehicle by providing a forward bias voltage higher than that of the battery.
Everything works perfectly so long as the power requirements of the vehicle do not exceed the capabilities of the alternator. If the peak output is surpassed due to excessive load, then power will be pulled from the battery. If the alternator and battery combined cannot meet the demand, then the vehicle's voltage rails, and subsequently the electrical devices are diminished (dimming lights, spark plug misfires, audio distortion and amplifier clipping, or even the car stalling.)
The first place to look to determine if your charging system is up to the task is the alternator itself. If possible, look for the HOT RATING on the alternator. IF you can't see it easily, call a local auto-parts store or car dealership and ask them to look up the stock alternator size, or rating for your vehicle. The hot rating will tell you the amount of power the alternator will produce once the engine reaches it's operating temperature (this is a lower rating than the cold rating). I suggest using your stock alternator unless you experience problems. That's how you know if you need to upgrade, since there is no concrete way to tell if a stock alternator has enough reserve to handle your additional burdens. Now, if you do need a new and larger alternator, after finding the stock rating, then allow your alternator about 10 Amperes credit or buffer area.
More information here:
http://www.bcae1.com/charging.htm
http://www.bcae1.com/charging2.htm
