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To calculate the size of the alternator you need, you’ll need to consider the amperage draw at 12 volts, not the wattage. There are 20 amps in a watt, so your system draws about 150 amps. You should look for an alternator that can produce around 170% of that, so approximately 255 amps. However, it’s always a good idea to oversize the alternator a bit to ensure it can handle your system’s power needs.