Enter your Engine RPM at Idle, Pulley Diameters, and electrical loads — A/C, Fuel Injection, and more — and the Alternator Sizing Calculator figures out your Total Electrical Load, Pulley Ratio, and exactly what Required Alternator Output you need to keep everything running.
Required Alternator Output
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Alternator Shaft RPM at Idle
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Pulley Ratio
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Total Electrical Load
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Recommended Alternator Size
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Add up all your electrical loads in amps, then add 20-30% safety margin. The alternator must produce enough current at idle RPM to power all accessories while maintaining battery charge.
A 2000-watt amplifier draws approximately 167 amps at 12V. Combined with other electrical loads, you'll typically need a 200-250 amp alternator for reliable performance.
No, never connect amplifiers directly to the alternator. Always run power through proper fusing and the battery. The battery acts as a buffer for peak current demands that exceed alternator output.
Alternators need 1800-2000 shaft RPM to produce rated output. Below this speed, output drops significantly. Check your pulley ratio to ensure adequate RPM at engine idle.
An undersized alternator will cause dimming lights, weak bass response, battery drain, and potential damage to electrical components. It can also lead to premature alternator failure.
Yes, if your audio system exceeds 500 watts RMS. High-output alternators provide stable voltage under load, better sound quality, and prevent electrical system strain.
Add 20-30% above your calculated load for safety margin. This accounts for alternator efficiency losses, aging, and unexpected electrical demands.
Alternator shaft RPM is determined by the pulley ratio. If your engine idles at 800 RPM with a 2:1 pulley ratio, the alternator spins at 1600 RPM.