What should I consider when choosing an electrical system for my boat?

Choose your boat's electrical system to suit its daily energy demand, the time spent away from charging and the equipment already fitted. For most small cruising boats, this means a 12V domestic bank, a separate engine-start battery, correctly sized and fused wiring, and charging sources matched to the batteries.

Check consumption and existing wiring

Start with the wiring diagram, battery labels, alternator rating and condition of the main cables. These show what you can keep and expose faults a bigger battery cannot fix: corroded terminals, undersized cables and poor charging connections.

List each consumer's current draw and daily running time. Multiply amps by hours to get amp-hours, keeping all calculations at the same system voltage:

• Fridge: 4A for eight accumulated hours of compressor running = 32Ah.
• Instruments: 2A for eight hours = 16Ah.
• Autopilot: an average 3A for eight hours = 24Ah.
• Lights, pumps and device charging: allow another 15Ah.

The total is 87Ah a day at 12V, roughly 1kWh. Actual fridge duty cycle and autopilot demand vary considerably with temperature, sea state and steering balance. A shunt-based battery monitor measures current entering and leaving the bank, replacing estimates with measured consumption.

Choose voltage and batteries together

Keeping 12V usually makes sense on an existing small yacht or motorboat. Larger boats often use 24V for domestic services or heavy consumers, sometimes alongside 12V equipment supplied through DC-to-DC converters. At 1,200W, the theoretical current is 100A at 12V or 50A at 24V, before losses. Higher voltage reduces current and cable voltage drop.

Separate domestic consumption from engine starting with a suitable split-charging arrangement. Older boats may have a 1-2-BOTH selector whose position determines both the supplying battery and which battery receives alternator charge. Check the actual wiring before changing it.

• Flooded lead-acid: economical and familiar, but needs ventilation and electrolyte checks where accessible.
• AGM: needs no routine watering, but still requires secure mounting, appropriate ventilation and the correct charging profile.
• LiFePO4: provides more usable capacity for its weight, but needs a compatible battery management system, charging controls and low-temperature charging protection.

For the 87Ah daily example, approximately 200Ah of lead-acid capacity gives one day while keeping discharge near 50%. Around 120Ah of LiFePO4 can cover similar consumption at roughly 80% usable capacity, with little reserve. Add capacity for longer stays and contingencies. Switching to lithium means converting the system: check alternator protection, charge limits and what happens when the battery management system disconnects.

Match charging to boat use

A marina-based boat benefits from a shore charger matched to its battery chemistry and capacity. Solar is usually useful for nights at anchor; frequent motoring makes alternator charging more useful. A nominal 100A alternator will not necessarily deliver 100A continuously into the domestic bank.