Battery & Alternator Budget
Check whether your alternator and battery can support your amplifiers, and how long you can play with the engine off.
Inputs
Class D ≈ 75–85%, Class AB ≈ 50–60%.
Of full RMS power. Typical music averages 10–20% (about −8 dB).
Real output at your RPM, not just the rating.
Lights, engine, A/C, ECU: 25–50 A is typical.
Lead-acid starting batteries: use ≤ 50% to keep them healthy.
Electrical budget (amps)
How it works
Your audio system takes its power from the alternator while driving and from the battery when the engine is off. This calculator compares what the amplifier draws with what your electrical system can supply.
I_peak = P_rms / (η · V) I_avg = I_peak × (average level %) Headroom = I_alternator − (I_car + I_avg) Play time = Ah × usable% / I_avg
Full-scale RMS power is only demanded by test tones. Music has a high crest factor, so the long-term average draw is far lower than the peak, typically 10–20% of full current. Peaks are handled by the battery (and capacitors, which mostly just help the battery respond faster). If the average demand exceeds the alternator output, the battery slowly discharges while driving.
The classic “1 farad per 1000 W” capacitor rule would suggest about 1.5 F for your system. A capacitor does not create energy: it stores a tiny amount, so a good battery and thick cables matter more. Consider a high-output alternator or a second battery (the “Big 3” wiring upgrade) when headroom is negative.