Marine Electrical System Design

A marine electrical system has five layers: loads, distribution, sources, storage, and monitoring. Most retrofit projects fail because they treat one layer in isolation: replacing the batteries without updating the charging, or adding solar without rethinking the consumption side. This guide covers the layered design approach.

Layer 1: Loads, start here

Every system design starts by understanding what the vessel actually consumes. Without this, you’re guessing at battery size, solar capacity, and charging requirements.

For each load on the vessel, document:

• Voltage: 12V, 24V, 48V, or AC
• Continuous current draw
• Run hours per day (typical, not maximum)
• Run hours per day (cruising mode, more demanding)
• Whether it’s variable (refrigeration cycles, lighting on/off) or constant

The result is a load table that looks something like:

Load	Voltage	Current	Hours/day	Wh/day
Refrigeration	12V	5A avg	24 (cycling)	1,440
Lighting	12V	2A	5	120
Electronics standby	12V	1.5A	24	432
Electronics in use	12V	4A	4	192
Inverter loads	AC via 12V	8A DC equiv	1	96
Bilge / pumps	12V	1A avg	1	12
Daily total				2,292 Wh

A daily consumption of around 2.3 kWh on a typical 35 ft cruiser is normal. Watermakers, electric cooking, and air conditioning push this dramatically higher.

Layer 2: Distribution, getting power to the loads

Once you know the loads, you can design the distribution. The principles:

• Every load on its own circuit, with its own fuse or breaker sized to the load
• Circuit voltage drop under 3% at maximum current (AS/NZS 3008 cable sizing applies)
• Common ground bonding. All DC negatives ultimately connect to a single bonding point.
• Heavy current paths kept short. Battery to inverter, battery to windlass, battery to alternator.
• DC and AC distribution physically separate. Separate cabinets, separate cable runs where practical.

Common distribution failure modes on retrofits:

• Original wiring undersized for the loads it now carries (especially after adding inverters)
• Fuses oversized “to stop nuisance trips,” which defeats the protection
• Battery bank wiring not properly balanced, causing one battery in the bank to do most of the work
• Negative bonding scattered across multiple points instead of star-bonded to a single bar

Layer 3: Sources, where the power comes from

A modern cruising vessel has three to five charging sources. Each has its own characteristics, limits, and best-use cases.

Shore power

The simplest source. AC from a shore connection, into a charger that produces DC for the battery bank. Available whenever the vessel is in a marina with shore facilities.

Considerations:

• Charger sized to the bank: typically 0.2–0.4C for AGM, up to 0.5C for LiFePO4 (a 200 Ah bank wants a 40–80A charger for AGM, 100A for LiFePO4)
• 240V AC dock supply varies in quality. Galvanic isolation is non-negotiable.
• Multi-stage charging (bulk, absorption, float) matters for AGM; less critical for LiFePO4 with internal BMS

Engine alternator

Charges from the main engine while running. Default on every vessel, but often poorly utilised.

Considerations:

• Stock automotive-style alternators are designed for cars, not for sustained marine charging. They overheat under continuous load.
• External alternator regulation (Balmar, WakeSpeed, others) lets the alternator deliver full output safely
• For lithium banks, an alternator-to-battery DC-DC charger is often the right answer (protects the alternator, gives proper lithium charging profile)
• Maximum useful alternator output at sustained loads is typically 70–80% of nameplate

Solar

The single most useful charging source for cruisers. Charges silently while at anchor, contributes substantially while underway, and reduces engine running time.

Considerations:

• Panel placement matters. Bimini-mounted panels lose 30–50% to shading from rigging compared to clear-sky bench tests.
• MPPT solar controllers are essential. They extract significantly more energy than older PWM controllers.
• Panel orientation on a vessel is variable, so panels are typically wired in parallel (not series) to minimise shading impact on each individual panel
• Typical Gold Coast vessel solar: 600–1,200W on a sailing yacht, 1,500–3,000W on a catamaran

Wind generator

Useful at anchor in consistent wind. Less useful underway (the apparent wind is usually behind the boat). Less useful on the Gold Coast where afternoon sea breezes are short-lived.

Considerations:

• Most useful as a supplement to solar in trade-wind cruising areas
• Noise can be a problem, especially in marina berths
• Usually contributes 5–15% of total daily charging on a typical setup

Hydrogenerator

For boats that sail extensively. Generates power from the prop turning while sailing. Niche but capable, can produce 500W+ continuously while underway.

Generator

For vessels that need to charge fast or run high AC loads independent of shore. Common on larger cruisers, less common on smaller boats.

Considerations:

• Sized to the largest AC load it needs to support, plus the charging current going to batteries
• Ideally runs less than 1 hour per day on cruising mode. Extended runtime burns fuel and wears the engine.
• LiFePO4 banks dramatically reduce generator runtime (because they accept charge faster)

Layer 4: Storage, the battery bank

The battery bank is the buffer between sources and loads. Sized correctly, it lets loads run uninterrupted while sources contribute when they can.

Sizing the bank:

• Daily consumption × autonomy days × derating factor
• For AGM: 2× consumption (50% depth of discharge limit) × 1–2 days autonomy = 2–4× daily kWh
• For LiFePO4: 1.2× consumption (80% depth of discharge) × 2–3 days autonomy = 2.4–3.6× daily kWh

A vessel consuming 2.3 kWh/day:

• AGM bank: 4.6–9.2 kWh of nameplate capacity (380–760 Ah at 12V)
• LiFePO4 bank: 5.5–8 kWh of nameplate capacity (460–680 Ah at 12V), but with 80%+ usable

For more on the AGM vs LiFePO4 trade-off, see LiFePO4 vs AGM Marine Batteries.

Layer 5: Monitoring, what’s happening right now

Without monitoring, the rest of the system is opaque. You don’t know what loads are doing, what charging is achieving, or what the battery is actually at.

A modern vessel monitoring setup:

• Battery monitor with shunt (Victron BMV-712 or Smart Shunt). Coulomb counting, accurate state of charge, voltage and current trends.
• Solar controller with comms (Victron MPPT, others). Production data, fault history.
• Inverter/charger with comms (Victron MultiPlus or Quattro). Load and charge data.
• Tank monitors for fuel and water (optional but useful)
• Cerbo GX or Ekrano GX as the data aggregator
• VRM portal for remote monitoring from anywhere

The Victron VRM portal is the dominant marine monitoring solution because it integrates everything into one view, accessible from anywhere with internet, with permanent data history. It’s also free for systems with modest data needs.

For more on what the data tells you, see Solar Monitoring Data Explained.

How vessel-to-villa integration works

For owners with both a Gold Coast home and a vessel, Iron and Air’s standard integration delivers:

• Vessel Cerbo GX feeding VRM as primary monitoring
• VPN tunnel between vessel and home network
• Home Assistant on shore subscribing to vessel data
• Shore dashboard showing vessel status alongside home status
• Alerts triggered by vessel events (low battery, shore power lost, bilge pump activity)
• Same monitoring infrastructure during cruising periods (vessel becomes mobile, dashboard travels)

The home and vessel run as a single coordinated system rather than two separate ones. See the services overview for how this is delivered.

Compliance and standards

Marine electrical work in Queensland is governed by:

• AS/NZS 3000: wiring rules (where they apply to AC mains on vessels)
• AS/NZS 3004.2: electrical installations on boats
• AS/NZS 5139: battery systems for electrical energy storage
• AS/NZS 4509: stand-alone power systems (relevant where vessel is treated as off-grid)
• AMSA regulations: applicable for commercial vessels and some larger recreational vessels

All Iron and Air work is done to these standards, with photo documentation and test results retained for warranty and insurance purposes.

Common design mistakes

The patterns we see most often on retrofit assessments:

• Battery bank sized for AGM nameplate, then converted to LiFePO4 without resizing. Result: unnecessarily large lithium bank, money wasted.
• Solar added without checking shading. Result: panels produce 30–50% of nameplate even at noon.
• Single inverter/charger for both house bank and AC distribution. Result: failure of one component takes down everything.
• Charging sources not coordinated. Result: alternator and solar fight each other for who’s charging the bank, neither delivers full output.
• Negative bonding scattered. Result: ground loops, voltage offsets, electronic interference, hard-to-diagnose faults.
• No monitoring. Result: no idea why the system isn’t performing, no data to support warranty claims.

Common questions

Can I design this myself? You can do the analysis yourself. Many cruisers do. The actual installation, particularly anything involving AC mains or grid-tied equipment, requires appropriate licensing. A System Audit produces the analysis as a deliverable you keep regardless of whether Iron and Air does the install.

What’s the most cost-effective starting point? A LiFePO4 upgrade with proper monitoring delivers the largest single improvement on most vessels. Solar additions deliver the next biggest. Inverter/charger upgrades come third. Engine alternator upgrades come last unless the existing alternator is genuinely undersized.

My boat is older. Is the existing wiring usable? Often. Older copper marine cable in good condition (no corrosion at terminations, no insulation damage, properly sized) is fine. The issues are usually at the terminations, the fuse arrangements, and the bonding rather than the cable itself. A System Audit identifies which existing wiring stays and what needs replacement.

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