Alternator Load & Charger Sizing
Use this free calculator to estimate your vehicle’s alternator headroom and pick a safe DC-DC charger size. It’s built for Australian 4WD, caravan/RV and marine setups and accounts for alternator rating, a safety margin, base vehicle loads and existing accessories.
Quick tips: alternator output drops at idle and in heat. Modern smart alternators vary voltage, so a DC-DC charger is often recommended to protect batteries and the vehicle’s electronics.
Alternator Load & Charger Sizing
Rule of thumb: leave a margin so headlights, fans, and ECU loads never push the alternator to 100% for long periods.
How to use
- Enter your alternator rating (A) and choose a safety margin (start at 20%).
- Add the vehicle’s base electrical load (ECU, fans, lights) plus any accessories already fitted.
- Include existing charger loads if applicable.
- Review the results: safe budget, current load, headroom, and a suggested maximum charger size.
What affects alternator output?
- RPM: less output at idle; allow extra headroom for touring and traffic.
- Temperature: hot WA/Outback conditions reduce output and lifespan.
- Smart alternators: variable voltage; DC-DC chargers maintain proper battery charging.
- Accessories: fridges, light bars, winches and compressors add to base load.
Next steps
Shop DC-DC chargers & cabling with fast Australia-wide shipping, or book an installation with our licensed auto electricians in Mandurah and the Peel Region.
Results are estimates only. Confirm limits with your vehicle and charger manufacturers and follow relevant Australian standards.