Guide to onboard power / electrical systems

A well-functioning electrical system is one of the most important things onboard. Whether you go sailing on weekends, anchor for longer periods or use your boat without shore power, it is essential to keep your batteries, charging, cables and power consumption in check. In this guide, I’ll go through the most important parts of an onboard electrical system to help you choose the right solution.

Why is a good electrical system important?

Onboard power is about more than just comfort. It is also about reliability, safety and avoiding problems when you’re out on the water. A properly configured system provides a stable power supply for lights, navigation, refrigeration, pumps, charging electronics and, if needed, a diesel heater, inverter or other 230V power supply.

Many problems arise because the battery bank is too small, the cables are undersized or the charging setup does not match the power consumption. That’s why it’s important to view the entire system as a complete solution rather than as individual components.

1. The battery bank – the heart of your electrical system

Your battery bank is your energy storage. This is where power is stored until it is needed onboard. When you switch on the lights, fridge, instruments or charge your phone and computer, the battery bank supplies the energy.

The most common battery types are lead-acid, AGM, gel and LiFePO4 lithium. These days, many people choose lithium because it offers high usable capacity, low weight, fast charging and a significantly longer lifespan.

Common battery types

• Lead-acid: a classic, low-cost option, but heavy and with low usable capacity
• AGM: maintenance-free and still widely used in many boats
• Gel: a stable battery type for certain installations
• LiFePO4 lithium: high performance, low weight and a very long lifespan

2. Charging – how to supply power to your system

Batteries are only as good as the charging they receive. That’s why you need to consider how the battery bank will be charged in practice. For most boats, the best solution is a combination of several charging sources.

Common charging sources

• Solar panels: ideal for keeping batteries topped up and covering daily power consumption
• Shore power: charging in the marina using a battery charger
• Engine: charging while sailing or motoring, often via a DC-DC charger
• Wind generator: an additional source on some boats, especially during longer stays without shore power

Solar panels are often one of the most valuable investments in a modern electrical system. With the right MPPT controller, you can get more energy from the panel and charge the battery bank more efficiently.

If you have lithium batteries, it is especially important to choose chargers and charging equipment that are compatible with the battery type. An incorrect charging profile or setup can result in poor performance and, in the worst case, damage the battery.

3. Cables, fuses and power distribution

Even the best batteries and chargers won’t help much if power isn’t distributed correctly. Cables, fuses, connections and power distribution are essential for both safety and functionality.

Cables that are too thin cause voltage drop, heat build-up and poor performance. This can mean that equipment performs worse than expected or that the batteries do not charge properly.

What to focus on

• Correct cable size for the current and cable length
• Fuses correctly positioned close to the power source
• A proper main switch and clear power distribution
• Correct cable terminals and connections, with protection against corrosion
• A battery monitor or shunt to track power consumption and charging

4. 12V, 24V and 230V – what’s the difference?

Most boats run primarily on 12V DC, while some larger boats use 24V. This is the voltage used by most onboard equipment, such as lights, pumps, fridges, instruments and diesel heaters.

230V AC is typically used for equipment such as coffee machines, power tools, chargers, household appliances and other standard household devices. If you want to use 230V without shore power, you’ll need an inverter to convert the battery voltage to 230V.

The inverter should be selected based on both the load and the starting current. Many appliances draw more power when starting than during normal operation, so it’s important not to look only at the rated power.

5. How much power do you actually use?

One of the most important questions in any electrical system is how much power you actually use. Without a clear overview, it’s difficult to choose the right battery capacity, solar setup and charger.

Typical onboard power consumers

• Fridge or refrigerator
• Diesel heater
• Cabin lights and reading lamps
• Chartplotter, depth sounder and other navigation equipment
• USB charging, phones, tablets and computers
• Pumps, autopilot and other permanently installed equipment

Once you add up your daily power consumption, it becomes much easier to choose the right battery bank and charging solution. It also gives you a much clearer idea of whether solar alone will be enough, or whether you should also have a shore power charger, engine charging or both.

Common mistakes in onboard electrical systems

• A battery bank that is too small for daily power consumption
• Undersized cables causing excessive voltage drop
• Missing or incorrectly positioned fuses
• A charger or controller that is not compatible with the battery type
• No monitoring of battery status and power consumption
• Buying individual components without considering the system as a complete solution

How to build a robust electrical system

A robust electrical system starts with your actual needs. How long do you want to be able to stay without shore power? What equipment needs to run every day? And do you only need power for the essentials, or would you also like a high level of comfort with, for example, an inverter, a larger fridge or more charging options?

Once you know your power consumption, it makes sense to choose the battery bank, charging setup, solar panels, inverter and cables as a complete solution. This means better performance, fewer problems and better value for money in the long term.

The most important thing is not necessarily to buy as much as possible, but to make the right choices from the start.

Need help finding the right solution?

I’m happy to help you find the right solution for your battery bank, charging, inverter, cables and onboard power distribution, so you get an electrical system suited to your boat and the way you use it.

If you’re about to build or upgrade your system, you can also take a look at my calculators: