1. Voltage drop: ΔU = k · L · I / (κ · A), with k = 2 for a separate return conductor (1 for chassis ground), L = one-way length. The conductivity of copper κ = 56 m/(Ω·mm²) applies at 20 °C; the calculator corrects it for the expected conductor temperature in operation (warm copper has more resistance). The reference is the battery’s nominal voltage: LiFePO4 12.8 / 25.6 / 51.2 V, lead-acid/AGM 12 / 24 / 48 V. Charging or float voltages (e.g. 13.6 V) are deliberately not used, because the cable must also be sufficient with a low battery and under load.

2. Current-carrying capacity (heating): Guide values for single-core copper cable, in free air, 30 °C, based on DIN VDE 0298-4, corrected with the standard factors for ambient temperature and bundling. Heat-resistant cables are deliberately calculated with the base values of PVC cable and only rated better via the more favourable temperature factors – that errs on the safe side. Multi-core cables: count every loaded conductor in the bundle.

3. Fuse: I_load ≤ I_fuse ≤ I_cable must apply. For continuous loads the fuse is rated at least 125 % of the load current (fuses should only be loaded to about 80 % continuously). The fuse protects the cable, not the device. It must be rated for DC and for the system voltage.

4. Result: The smallest standard cross-section that meets both voltage drop and current capacity (incl. fuse).

Installation tips: Place the fuse as close as possible to the battery or power source; the section before it is unprotected, so keep it short and protect it against chafing. Lithium batteries (LiFePO4) deliver very high short-circuit currents – use a main fuse with sufficient breaking capacity (e.g. Class T or NH). Many blade fuses are only rated up to 32 V: at 48 V only use fuses with a suitable voltage rating (e.g. 58 V or higher). Motors (winch, compressor) have high inrush currents – use a slow-blow fuse or follow the manufacturer’s specs. Only connect fine-stranded cables with matching ferrules or crimped lugs. AWG values are the next larger common AWG, never smaller than the metric cross-section.

Consumption: Wh/day = power (W) × hours × quantity. 230 V devices are divided by the inverter efficiency, plus its idle consumption × switched-on time. Ah = Wh / battery nominal voltage.

Battery: usable energy = capacity (Ah) × nominal voltage × usable share. Guide values: LiFePO4 approx. 80–90 %, lead-acid/AGM approx. 50 % (deeper discharge greatly shortens its life).

Solar: yield/day ≈ Wp × peak sun hours × 75 %. The 75 % covers typical losses (heat, charge controller, cables, flat mounting, light shading). Peak sun hours (hours at full rated power per day) are rough averages for flat-mounted panels – individual days can be much lower, and close to zero when cloudy.

Driving: charge/day ≈ charge current × driving time × nominal voltage. Full charge current only flows while the battery isn’t nearly full.

Autonomy: days = usable energy / (consumption − charge). If charging exceeds consumption, the balance is positive.