12 V or 24 V on board

24 volts halves the current for the same power output. This reduces cable cross-sections, voltage drop, and copper costs — but makes everything that is only available in 12-volt expensive. The decision is made early because it affects every single component.

What doubling the voltage does

Power is voltage multiplied by current. If the voltage is doubled, the current halves for the same power. The voltage drop across a line is directly dependent on the current — so it also halves. At the same time, the value considered to be three percent doubles: 0.72 instead of 0.36 volts.

Both together mean that at 24 volts, the same consumer can be connected over the same distance with a quarter of the cross-section. This is not a minor advantage, but the real reason for the higher voltage.

For small consumers, the difference is negligible. From about 1000 watts, it becomes a question of whether the cable can still be laid practically.

When 12 volts remains the right choice

• Small to medium-sized systems. As long as the largest consumer remains below about 1000 watts, the currents are manageable.
• Short distances. In a vehicle or smaller boat, there is rarely more than five meters between the battery and the consumer.
• Device availability. Pumps, lighting, navigation, small devices — the range available in 12 volts is incomparably larger, and replacements are available everywhere.
• Existing system. If the system is present and functional, conversion is rarely economical.
• Engine with a 12-volt electrical system. Starter and engine control determine the level at which the starter battery is connected.

When 24 volts is worthwhile

• Large individual consumers. Bow thrusters, anchor windlasses, inverters from about two kilowatts.
• Long cable runs. From about ten meters between the battery and a large consumer, the cross-section at 12 volts becomes unwieldy.
• Large battery banks. With several hundred ampere-hours, 24 volts simplifies wiring and charging infrastructure.
• Engines with a 24-volt electrical system. This is the norm for larger drives anyway.
• Weight. Halved cross-section also means significantly less copper on board.

NoteA rough practical limit: If a single cable theoretically requires more than 70 square millimeters at 12 volts, it's worth re-evaluating the system question. Such cross-sections are expensive, difficult to lay, and difficult to terminate cleanly.

Both levels in one system

The most common real-world scenario is a mix: 24 volts as the basis for large consumers and the battery bank, 12 volts for everything else that is only available at that voltage. The levels are connected via a DC-DC converter.

Two values are crucial for its design: the continuous current that the 12-volt side actually draws, and the inrush current of the connected devices. Pumps and compressors draw a multiple of their continuous current when starting up — a converter that is too small will then shut down.

WarningA 24-volt device on 12 volts usually won't run at all, a 12-volt device on 24 volts will be destroyed. In mixed systems, both levels should be clearly marked — on the distribution, on the connectors, and in the diagram.

The second option, obtaining 12 volts by tapping the middle of a 24-volt battery bank, works technically but leads to uneven loading of the batteries and a battery bank that is never uniformly charged. It falls into the category of "it works," not "it should be done."

Conversion of an existing system

Converting an existing 12-volt system to 24 volts means: re-wiring the battery bank, replacing or reconfiguring all charging sources, checking each device for its input voltage, and providing a converter for the remaining 12-volt devices. The cables can usually remain — they will even be generously sized.

This is almost only economical if a large part of the system is being renewed anyway, or if a new large consumer is added that cannot be sensibly connected at 12 volts. As a single measure, it rarely pays off.

Typical mistakes

• Decision based on habit. The question should be asked of the largest consumers and longest cable runs, not device selection.
• Converter undersized. Inrush currents of pumps and compressors significantly exceed continuous current.
• Levels not marked. A 12-volt device connected to 24 volts will immediately be defective.
• Center tap used. Leads to uneven loading and a battery bank that is never uniformly charged.
• Converted late. After purchasing the devices, the decision is effectively made.
• Charging infrastructure forgotten. Alternator regulators, solar regulators, and chargers must match the voltage.