Understanding The Power Ratings and Ohm’s Law In-Car Audio Systems
Last updated: May 2026
Understanding power ratings and Ohm's Law is essential for anyone building a car audio system. This guide explains watts, impedance, and how they affect your alternator requirements.
Watts vs. RMS Watts
When shopping for car audio equipment, you'll see peak wattage and RMS wattage. RMS (Root Mean Square) is the only number that matters — it represents continuous power output. Peak wattage is a marketing number. A 1,000W RMS amplifier is a legitimate 1,000W amp. A 1,000W peak / 200W RMS amp is a 200W amp.
Understanding Ohms and Impedance
Impedance (Ω = ohms) is the resistance of your speaker or subwoofer. Amplifiers produce different power at different impedance loads:
- Lower ohms = more power from the amplifier (but more heat)
- Higher ohms = less power but more stable operation
- Most car audio amplifiers are rated at 4Ω, 2Ω, and 1Ω (some)
Series vs Parallel Wiring
When wiring multiple speakers:
- Parallel wiring halves impedance per doubling of speakers: two 4Ω subs parallel = 2Ω
- Series wiring doubles impedance: two 4Ω subs series = 8Ω
Full wiring guide: How to Wire Subwoofers: Step-by-Step Guide
Power Draw and Your Alternator
Every watt of audio power comes from your vehicle's electrical system. The conversion: Watts ÷ 12V × efficiency factor (1.2) = amps drawn from battery/alternator.
A 1,000W RMS system draws ~100 amps. Your stock alternator likely produces 120–180 amps total. After factory loads, there's little or no headroom for serious audio. See: How Many Amps Do I Need? The Alternator Sizing Guide