Maintenance
Add up electrical draw from fuel pumps, fans, stereo, lighting, and accessories in amps and watts. Compare to alternator rated and idle output with configurable headroom so the battery stays charged.
Factory alternators are sized for factory loads. Add dual fuel pumps, electric fans, big stereo, LED light bars, and EFI and you can draw more amps at idle than the alternator produces — the battery supplies the deficit until voltage sags.
This alternator load calculator sums constant draws in amps and watts÷volts, compares to rated and idle alternator output, and applies configurable headroom (often 25–30%).
A good outcome: you know whether you need a high-output alternator, a second battery, or reduced idle load before the headlights dim at a stoplight.
List everything that can run at the same time — not peak stereo burst on a 2-second bass hit unless you size for that.
Use alternator idle output from the spec sheet, not the 100A advertisement at 2,000 rpm.
Watts ÷ system voltage (≈13.8–14.4 running) converts accessories rated in watts to amps.
Total amps = sum of loads + (watts ÷ volts)
Recommended = Total × (1 + headroom%)
Shops commonly add 20–30% headroom so the alternator is not pinned at 100% duty.
Total amps = Σ amps + Σ(watts ÷ volts). Recommended = total × (1 + headroom%).
Does not model duty cycle of fans or PWM stereo — if in doubt, add loads as always-on for sizing.
ECU 15 A, dual SPAL fans 35 A, fuel pump 12 A, headlights 10 A, stereo 1,200 W at 13.8 V (~87 A) → total near 160 A if everything ran at once. A 140 A alternator with 80 A at idle is undersized without fan staging or a higher-idle alternator.
Rated amps are at 2,000+ RPM; at idle your alternator may produce half the brochure number. Fans staged on relays, PWM fuel pumps, and stereo on separate battery help.
Corroded grounds and undersized charge wires cause voltage drop that looks like a weak alternator — load-test at idle with everything on.
Stock cars often draw 40–60 A with lights and HVAC; builds with big fans and EFI can exceed 100 A — size for idle output, not peak rating.
A second battery helps short burst loads but charging still depends on alternator output over time.
Below about 13.0 V sustained with engine running suggests charging deficit — 13.8–14.4 is typical target under load.
Pair this calculator with these related tools in the garage — same session, no signup.