If you are new to canal boating, the electrical system can seem mysterious. But it is simpler than you think. Almost every UK narrowboat uses a 12-volt system, just like a car, with a few clever additions to keep your lights on and your fridge cold. This beginner's guide explains the basics so you can cruise with confidence.

The 12-volt system and your two battery banks

Almost every UK canal boat runs its lights, pumps, fridge and phone chargers from a 12-volt DC system. This is safe in a damp steel hull and uses cheap, widely available components. You have two separate battery systems. The engine start battery delivers a huge burst of current for a few seconds. The domestic bank – your leisure batteries – powers everything else when you are moored or cruising. Keeping them separate stops a left-on light from flattening your starter battery. A split charge relay or battery-to-battery charger handles the separation automatically. The 12V system also runs your bilge pump and navigation lights.

Leisure batteries: capacity and care

Leisure batteries are built to be discharged and recharged many times. Most boats wire two to six in parallel to increase capacity, measured in amp-hours (Ah). A typical narrowboat might have a 330Ah domestic bank, say three 110Ah batteries. Never discharge a lead-acid battery below 50% of its capacity – that shortens its life. So from 330Ah you have about 165Ah usable. A 12V fridge draws 3–5 amps, using 80Ah over 24 hours in summer. Add lights and pumps, and charging becomes a daily concern. Flooded, AGM and gel are the three main types. Flooded are cheapest but need water checks. AGM and gel are sealed. Keep all batteries in a bank the same age and type. If one battery fails, replace the whole bank.

Charging: alternator, shore power and solar

Your engine alternator is the main charging source when cruising. A typical narrowboat alternator produces 70–120 amps, but it only charges the domestic bank through a split charge relay or battery-to-battery charger. Alternators taper off, so you need to run the engine for several hours for a full charge. A smart regulator can speed up alternator charging. At a mooring with a shoreline, a multi-stage battery charger does a better job: bulk, absorb, then float. Solar panels are a quiet complement – a 200W array with an MPPT controller can add 40–60Ah per day in summer. A battery monitor with a shunt shows amps in and out, and state of charge as a percentage. That beats reading voltage alone.

Inverters: getting 240 volts on board

An inverter turns 12V DC into 240V AC for laptops, televisions and small appliances. Modified sine wave inverters are cheaper but can buzz audio gear, run motors hot and upset electronics. For anything with a motor or sensitive electronics, choose pure sine wave. A 1000W inverter can run a small microwave, but remember: 1000W at 240V draws about 90–100 amps from your 12V batteries. That empties a typical bank in minutes, and the cables must be very thick. Inverters are typically 85–90% efficient. Most boaters use an inverter for low-power items and rely on gas or the engine for kettles and washing machines.

Fuses, safety and daily habits

Every positive cable leaving a battery must have a fuse or circuit breaker as close to the terminal as possible. If a cable shorts to the hull, the fuse blows and prevents a fire. Use blade fuses for lights and pumps, and ANL or MEGA fuses for inverter cables. Size the fuse to protect the cable, not the device. For a 100A load, use at least 35mm² cable. Keep batteries in a ventilated box – flooded types give off explosive hydrogen. Check voltage in the morning: 12.7V is full, 12.2V is 50%, 11.8V is empty. Switch to LED lights, turn off the inverter when idle, and carry a multimeter and spare fuses. If lights dim when the pump runs, check connections. And if you smell rotten eggs near the batteries, ventilate immediately – that is hydrogen sulphide from overcharging.

• Never use a fuse larger than the cable can carry.
• Keep terminals clean and tight.
• Replace old batteries together.