Solar + Electric Propulsion: How to Size a Combined System for Coastal Cruising

Solar panels and electric propulsion share the same DC bus, which makes integration straightforward in principle but requires careful sizing in practice. The goal is to dimension the solar array so that it meaningfully offsets propulsion energy without over-sizing to the point where excess harvest cannot be absorbed.

For a typical 35 ft coastal cruiser with a 24 kWh 48 V propulsion pack and 2 hours of daily motoring at 5 kW, daily propulsion consumption is approximately 10 kWh. A 2 kWp solar array (four 500 W panels) produces 6–9 kWh per day in the Mediterranean between May and September — offsetting 60–90% of daily propulsion energy in peak season. In northern European waters (UK, Netherlands, Baltic) during summer, the same array produces 4–6 kWh/day.

The integration requires a solar charge controller rated for 48 V battery input. Victron SmartSolar MPPT 150/70 and 250/100 controllers are the most widely used options and integrate natively with the Victron GX monitoring ecosystem. Setting the absorption voltage correctly for LiFePO₄ chemistry (typically 57.6 V for a 48 V bank) and enabling the low-temperature charge cutoff prevents cell damage during early morning charging in cold conditions.

One sizing error to avoid: connecting the solar array directly to the same bus bar as the motor controller without a fuse and disconnect between the MPPT and bus. In a ground fault or short-circuit event, the solar panels will continue supplying current until manually isolated — a fire risk. Always fuse each MPPT output and install an accessible DC disconnect within 30 cm of the battery bus.