Planning Boat Solar? Read This First to Avoid Costly Mistakes!

When installing solar panels on a boat, efficiency and proper system design matter more than other locations. Yet many boaters repeat common myths, often based on advice from land-based installers. In this article, we will start with one of the biggest myths:

Myth #1: "Over paneling is always good."

What Is Over Paneling?

Over paneling means installing more total solar wattage than your MPPT solar charge controller is rated to handle. For example, a Victron 100/30 MPPT controller is rated for 30 amps. On a 12-volt battery system charging at about 14 volts, this means the controller can handle roughly 420 watts of solar input.

If you install 600 watts of panels on this controller, the controller will self-limit at 30 Amps (approximately 420 Watts). This results in wasted solar energy that could have been captured.

Common Misunderstandings

"My system stays at max longer."
Yes, but the total daily energy output is lower because the system "clips"(maxes out) during peak sunlight hours.

"I am maximizing my solar controller."
In theory, yes, but in practice you sacrifice daily energy production, which is critical on space-limited boats.

"Running controllers at maximum is fine."
Not really, running electronics at their limits often causes overheating and premature failure.

"Big companies recommend over paneling."
True, but those recommendations mostly apply to land-based systems. Boats have unique challenges such as limited space and shading issues.

Real-World Example

A friend owned a Lagoon 500 catamaran with a professionally designed 2.4 kilowatt solar array. Despite that, energy production was low. Why? All MPPT controllers were undersized, leading to clipping for hours every day.

After replacing the MPPT controllers with properly sized models, his daily energy capture almost doubled.

This clearly shows how undersized MPPT controllers can severely limit energy capture, even with a large solar array.

The Numbers Do Not Lie

Typical 1 kilowatt Solar Array on a Caribbean Day: As shown in the graph and chart above, undersized controllers cause significant energy loss despite identical panel wattage.

When Over Paneling Can Work

Over paneling can be beneficial if you have ample space and want to maximize energy in low light or poor sun angles. However, on boats, space is limited and shading is common, making over paneling less practical.

Myth #2: "Bypass Diodes Fix Shading Issues"

Bypass diodes help reduce the impact of shading but they do not eliminate the loss. Even a mast shadow can reduce output by 30 percent or more. Larger shaded areas can cut output by up to 80 percent.

Always install panels to be as shade free as possible.

Myth #3: "Series Wiring Is Always Best"

Series wiring has its advantages:

• Higher voltage means smaller wires for the same voltage drop
• Fewer wires required
• Lower installation cost

Series wiring has disadvantages:

• Series wiring creates potentially dangerous voltages often over 100 volts
• One shaded panel reduces output from all panels in the string
• There is no redundancy

A better solution is to use one MPPT controller per large panel. A study by Siemens found that one MPPT per panel improves yield. On sailboats, it is even more beneficial where shading and panel orientation vary.

Myth #4: "Solar Panels Will Never Achieve Their Rating"

This myth causes people to undervalue proper MPPT sizing. In real world conditions, especially in the tropics with high sun angles, panels often hit 90 to 100 percent of their rated output.

High quality installations can occasionally peak at 20 to 30 percent over panel rating, especially in bursts with cool wind or cloud edge effects.

While peak output is not everything, it is a strong indicator of whether your system is performing efficiently.

Conclusion: Design for Maximum Capture

• Match your controller size to your solar array and leave some headroom
• Do not rely on bypass diodes to fix bad sytem design and installation
• Avoid shading as best as possible. Panels on a stern arch are best

A well designed solar system on a boat should capture every watt possible. Oversizing panels but undersizing controllers just throws away energy you cannot afford to lose.

Sources:

• Springer Study on Bifacial PV and Shading
• Siemens Study on MPPT Efficiency
• Victron MPPT Technical Documentation

Here are some of the relevant parts we use on SV Cosmos:

Victron MPPTs Blue Sea MRBF Fuse Holders

MRBF Fuses Ancor Marine Wire Victron Cerbo GX

Cerbo GX 7" Display Blue Sea Bus Bars

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