Two Systems, One Boat

Almost every narrowboat, cruiser and riverboat in the UK runs on a simple idea: store energy in batteries at 12 volts, then use it for lights, pumps and gadgets. Alongside that sits a second system — 230 volts AC, the same as a household socket — which you either plug into on a mooring or create on board with an inverter. Understanding the difference between the two is the first step towards a boat that behaves itself.

The 12-volt side is the one that matters most. It runs your bilge pump, water pump, fridge, LED lighting, heating controls and phone chargers. Lose it and the boat becomes uncomfortable very quickly. The 230-volt side is a convenience layer: a washing machine, a microwave, a laptop power supply. Treat the 12-volt system as the priority and the 230-volt side as a guest.

Your Battery Bank: The Heart of the Matter

Batteries are measured in amp-hours (Ah). A typical leisure bank might be 400Ah, made up of four 100Ah batteries wired together. The golden rule with lead-acid and AGM batteries is never to discharge them below 50 per cent — so 400Ah really gives you about 200Ah of usable energy. Many beginners learn this the expensive way.

• Leisure batteries are designed for slow, deep discharge. They are not the same as car starter batteries, which hate being run flat.
• AGM and gel types tolerate deeper cycling and need slightly different charging voltages.
• Lithium iron phosphate batteries can be discharged to around 80 per cent, charge faster and last longer, but they cost considerably more and need charging equipment configured for them.

Aim to keep the bank topped up rather than run it down and refill it. Shallow, frequent cycles are kinder than occasional deep ones. Also check the water levels monthly if you have open-cell lead-acid batteries, and keep terminals clean and tight — a loose connection causes heat, voltage drop and, occasionally, a nasty surprise.

Charging: Four Ways to Put Energy Back

You will usually have more than one charging source, and they can work together.

• The engine alternator is the workhorse. Cruising for three or four hours a day can put a serious amount of charge back, especially with an uprated alternator and an external regulator.
• Shore power through a battery charger is the simplest and cheapest way to arrive at fully charged batteries. Many boaters plug in whenever a mooring offers it.
• Solar panels are excellent for keeping the bank topped up in summer and adequate in winter if you have enough wattage. An MPPT controller squeezes more from the panels than a cheaper PWM unit.
• Petrol or diesel generators are noisy, heavy and thirsty, but useful in winter when solar fades and the engine is not running.

Whichever you use, remember that batteries accept charge quickly at first and then slow down. Getting from 80 per cent to 100 per cent takes far longer than the first 80 per cent, and skipping that final absorption stage gradually destroys capacity.

Inverters and Shore Power

An inverter turns 12 volts DC into 230 volts AC. Two things matter when choosing one. First, size it for the peak load, not the average — a microwave might briefly demand 1,500 watts or more. Second, choose a pure sine wave inverter rather than a modified sine wave type. Motors run cooler, chargers work properly and sensitive electronics last longer.

Shore power is straightforward until it is not. Use a proper marine-grade cable with an RCD, check the polarity of the supply before connecting, and consider a galvanic isolator to protect your hull from stray currents in the marina. If in doubt about your boat's earth arrangements, pay someone qualified to look at it.

Living Within Your Means

The practical skill of boat electrics is knowing what you use. A 40-watt fridge running continuously consumes roughly 80Ah a day in warm weather. A few hours of laptop charging, some LED lights and a water pump add more. Add it all up before you decide how long you can stay off shore power.

• Fit a shunt-based battery monitor so you can see amps in, amps out and state of charge. Voltage readings alone tell you very little under load.
• Convert halogen bulbs to LEDs — the saving is dramatic.
• Run heavy loads such as the washing machine or immersion heater while the engine is running or you are on shore power.
• Keep a simple log for a week. It teaches you more than any manual.

Common Mistakes and Sensible Habits

Undersized cables are the quiet killer of boat electrics. Long runs at 12 volts suffer voltage drop, so cables need to be thicker than you would expect. Use proper crimped terminals, not twisted wires, and keep connections dry.

Do not mix old and new batteries in the same bank; the weak one drags the rest down. Do not rely on your engine's starter battery for domestics. Do not assume a battery is full because the charger says so — give it time. And never work on the 230-volt side unless you genuinely know what you are doing.

Finally, get into a routine: check the monitor each morning, charge when you drop below 60 per cent, water the batteries monthly and clean the terminals twice a year. Do that, and your lights, pumps and devices will keep working quietly in the middle of nowhere, which is exactly the point of boating.