CHAPTER 1: Capture Energy

Marine Solar Panels

​Understand how marine solar panels convert sunlight into usable electricity and learn how to choose the right panels for your boat.

Introduction

Every reliable marine solar power system begins with one component—the solar panel.
Its job is simple: capture energy from the sun and convert it into electricity.
Choosing the right solar panel, however, involves much more than simply selecting the highest wattage. Panel technology, mounting options, available space, shading, and your boat's daily energy consumption all play important roles.
This guide introduces the fundamentals of marine solar panels and prepares you for designing an efficient, reliable marine power system.

Monocrystalline vs Polycrystalline vs Bifacial Solar Cells

​Not all solar cells are created equal. Understanding the differences between the three main solar cell technologies will help you choose the right panel for your marine power system.
​​​💡 Click any topic below to expand and learn more.

Monocrystalline Solar Cells (Recommended)

High efficiency and best for marine installations.

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Monocrystalline solar cells are the industry standard for high-performance marine solar panels. They offer the highest conversion efficiency, typically ranging from 16% to 25%, with some premium cells exceeding 26% efficiency. This means they convert a greater percentage of sunlight into usable electrical energy than other cell types.
For boats, where mounting space is often limited, higher efficiency means more power generated from the same amount of deck or arch space.

​There are two common types of monocrystalline cells:

• Back-contact cells (such as SunPower® cells) place all electrical contacts on the back of the cell. This eliminates front-side wiring, allowing more sunlight to reach the active surface and improving efficiency.
• Front-contact cells are the most widely used monocrystalline design. While slightly less efficient than back-contact cells, they still provide excellent performance and are commonly found in quality marine solar panels.

Polycrystalline Solar Cells

Lower cost but lower efficiency.

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Polycrystalline solar cells are manufactured using multiple silicon crystals instead of a single crystal. They are generally less expensive to produce but also less efficient, meaning they generate less power for the same panel size.
While polycrystalline panels remain a practical option for some land-based installations, they are typically less desirable for marine applications where available mounting space is limited and maximizing power output is important.

• Cell efficiency typically 12% to 18%
• Less expensive than monocrystalline ($2 - $5 per watt)

Bifacial Solar Cells

Can collect reflected light but rarely beneficial on boats.

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Bifacial solar panels are designed to collect sunlight from both the front and back of the panel. Under ideal conditions, reflected light can increase energy production by up to 20%.
However, in most marine installations, the benefits are limited. Boat-mounted panels are often affected by shading from radar arches, masts, rigging, and other onboard equipment, reducing the amount of reflected light reaching the back of the panel. Based on our experience, bifacial panels rarely provide a significant performance advantage in typical marine environments.

• Power generation on both sides of the cells using reflected light (up to 20% gain)
• Limited application in the marine environment

Cell Efficiency is the amount of light energy converted to electrical energy.

Solar Cell Quality Grades: Grade A or B or C?

All solar cells are not created equal.
After production, each cell is tested under standardized lighting conditions and graded according to its electrical output. The distribution of performance is a Bell Curve with most cells being a B grade.
Manufacturers commonly classify cells into performance grades such as A+, A, B, and C.
For marine applications, Grade A or A+ cells are the preferred choice because they deliver the highest power output and the greatest efficiency. Since available installation space on a boat is often limited, maximizing power production per square inch is critical.
At Custom Marine Products, we primarily use premium Grade A+ SunPower® solar cells because of their exceptional efficiency and consistent performance. While manufacturers may offer lower-cost versions using lower-grade cells, investing in higher-quality cells results in greater long-term energy production and overall system performance.​

• Grade A+ is desirable on a boat where space is limited.
• Grade A will typically perform above rating in full sun light.
• Grade B is typically used for residential and solar farms.

We want maximum output per square inch.
​SunPower makes four grades of solar cells.

Rigid, Semi-Rigid, or Semi-Flexible Solar Panels?

Choosing the right type of marine solar panel is about more than just power output. The way you plan to mount your panels, the available space on your boat, and how the installation area is used all play an important role.
​At Custom Marine Products, we offer rigid, semi-rigid, and semi-flexible solar panels. Thanks to advances in solar cell technology, all three types deliver produce about the same amount of power.

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Rigid Solar Panels
Rigid solar panels are the traditional glass panels mounted within a durable aluminum frame. They are the most robust option and are designed for long-term installations.
Best for:

• Arches
• Stern poles
• Davits
• Custom mounting frames

Advantages:

• Robust, life span of 18 years or more
• Extremely durable and weather resistant

If your boat has dedicated mounting structures, rigid panels are generally the preferred choice for maximum durability and long-term reliability.

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Semi-Rigid Solar Panels
Semi-rigid solar panels combine durability with a low-profile design. Approximately ¼-inch thick, they feature a tough protective surface that is strong enough to walk on, making them an excellent solution for areas that receive regular foot traffic.
Best for:

• Cabin tops
• Deck surfaces
• Area where you're going to be walking

Advantages:

• Life span of 11 years or more
• Walkable, non-slip surface
• Low-profile appearance
• Durable construction for marine environments

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Semi-Flexible Solar Panels
Semi-flexible solar panels are lightweight and can bend to follow gently curved surfaces. Their flexibility makes them particularly well suited for installations where traditional rigid panels are impractical.
Best for:

• Canvas biminis
• Dodger tops
• Slightly curved rigid surfaces

Advantages:

• Lightweight construction
• Flexible installation options
• Life span of 7 years or more
• Easy to mount where weight is a concern

Although earlier generations of flexible panels produced significantly less power, today's high-quality semi-flexible panels use the same premium solar cells found in rigid panels. As a result, their electrical performance is now very similar, making them an excellent choice when flexibility and weight savings are priorities.

​All CMPower panels use premium solar cells and have comparable performance within 5%.

Marine Solar Panels vs. Commercial Solar Panels

​Why choosing a panel designed for the marine environment matters.
Although marine and commercial solar panels may look similar, they are built for very different environments. Saltwater, vibration, limited mounting space, and marine battery systems place unique demands on a solar panel. Understanding these differences will help you choose a system that delivers reliable performance for years to come.

Feature	Marine Solar Panels	Commercial Solar Panels
Junction Box	Filled with inert silicone to prevent moisture and corrosion.	Typically not filled with inert material.
Frame Construction	Rigid panels have strong frames and extra sealants.	Frames designed for rack mounting.
Solar Cell Quality	Panels have high power generation – Grade A+ cells.	Panel cells are typically Grade A- to B.
Electrical Output	Optimized for 12V, 24V and 48V marine battery systems.	Typically 60-80V for residential grid-tied systems.
Panel Size	Designed to fit limited space available on boats.	Larger panel sizes intended for rooftops.
Shading Performance	Some models are wired to accommodate shading.	Generally not optimized for marine shading conditions.
Warranty	Designed and warranted for marine installations.	Warranty may be void for marine installations.

Understanding Solar Panel Specifications

Learn to Read a Solar Panel Specification Label
Every solar panel includes a specification label, typically found on the back of the panel, that lists its electrical characteristics and performance ratings. While these numbers may seem technical at first, they provide valuable information for comparing panels and designing a reliable marine solar system.
In this chapter, we'll introduce the most common specifications you'll encounter. Later sections will explain how each one affects real-world performance and how to use them when selecting a solar panel for your boat.

Specification	Symbol	What It Means	Example
Nominal Peak Power	Pmax	Maximum power point (Watts at STC)	130 W
Nominal Voltage	Vmp	Voltage at maximum output power	22.3 V
Nominal Current	Imp	Current at maximum output power	5.8 A
Open Circuit Voltage	Voc	Maximum voltage output with no load	26.3 V
Short Circuit Current	Isc	Maximum current (amps) output at zero voltage	6.8 A
Cell Efficiency	%	Amount of light energy converted to electrical energy	24.7%
Cell Manufacturer		The company that manufactures the individual solar cells inside the panel	SunPower
Panel Efficiency	%	The efficiency of the complete panel after construction losses and spacing between cells
Temperature Coefficient	% / °C	% degradation per °C

Understanding the Performance Curves

Most solar panel specification sheets also include two performance graphs:

I-V Curve (Current-Voltage Curve)
The I-V Curve shows how the panel's voltage and current change under different operating conditions. It identifies two important values:

• Voc (Open Circuit Voltage): the highest voltage when no load is connected.
• Isc (Short Circuit Current): the highest current when the output is shorted.

​P-V Curve (Power-Voltage Curve)
The P-V Curve shows how much power the panel produces at different voltages.
The highest point on this curve is called the Maximum Power Point (MPP), the operating point where the panel generates its maximum power. At this point:

• Voltage = Vmp
• Current = Imp
• Power = Pmax

Modern MPPT charge controllers continuously track this point to maximize charging efficiency.

​□ Tip:
Don't focus on just one number.
A panel's wattage is only one part of the story. Cell quality, operating voltage, efficiency, construction, and temperature performance all contribute to how well a solar panel performs in real marine conditions.

Factors Affecting Solar Panel Performance

​A solar panel's power rating is measured under ideal laboratory conditions. In real-world marine environments, these five factors determine how much power your system actually produces.

• Light intensity
• Solar cell temperature
• Sun angle of incidence
• Shading
• Quality of the solar cells

​💡 Click any topic below to expand and learn more.

PV Solar Panel Performance – Effect of Light Intensity

The amount of sunlight reaching the panel has the greatest effect on power production. More sunlight means more electricity.

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☑️ What Is Light Intensity?

Light intensity refers to the amount of solar energy reaching the surface of a solar panel. It is the single biggest factor affecting how much electricity a panel can produce. As sunlight decreases due to clouds, haze, smoke, or poor weather, power production decreases accordingly.

☑️ Standard Test Conditions (STC)

Manufacturers rate every solar panel under laboratory conditions known as Standard Test Conditions (STC). These standardized conditions allow different panels to be compared fairly.

• Cell Temperature: 25°C (77°F)
• Light Intensity: 1,000 W/m²
• Sunlight: Directly perpendicular to the panel surface

The wattage printed on the back of a solar panel is measured under these ideal conditions.

☑️ Why It Matters

Solar output changes almost proportionally with available sunlight. If the available sunlight drops from approximately 1,000 W/m² to around 600 W/m², panel output can decrease by nearly 50%.

☑️ Real-World Example

Many boat owners noticed reduced solar charging during periods of heavy smoke from the Canadian wildfires. Although their solar panels were operating normally, the smoke significantly reduced the amount of sunlight reaching the solar cells.

Key Takeaway
Solar panel performance is proportional to light intensity

PV Solar Panel Performance – Effect of Solar Cell Temperature

Solar panels become less efficient as their temperature increases. Cooler panels generate more power.

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☑️ Why Does Temperature Matter?

Solar panels are tested and rated at a cell temperature of 25°C (77°F), known as Standard Test Conditions (STC). In actual marine environments, however, solar cells often operate much hotter than this.
On a sunny day, cell temperatures commonly reach 60°C (140°F) or higher. As the temperature rises, the electrical efficiency of the solar cells decreases, resulting in lower power output.

☑️ Performance Loss from Heat

As solar cells heat up, their voltage drops while current changes very little. The result is a noticeable reduction in the panel's overall power output.
For example, a 130-watt solar panel operating at approximately 140°F (60°C) may only produce about 112 watts, representing nearly a 14% loss from its rated output.

☑️ How to Keep Solar Panels Cooler

Proper mounting can significantly improve airflow beneath a solar panel, helping dissipate heat and improve performance.

• Provide an air gap beneath the panel whenever possible.
• Use mounting systems that promote natural airflow.
• Tilted or elevated panels generally stay cooler than panels mounted directly against a solid surface.
• CMP's walkable panels include a twin-wall polycarbonate backing and J-channel mounting system that allows cooling airflow beneath the panel.

☑️ Additional Benefit of Adjustable Mounts

Tilting and rotating a solar panel not only improves its angle to the sun but also increases airflow underneath the panel. Together, these benefits can substantially improve overall energy production throughout the day.

Key Takeaway
A cooler solar panel is a more efficient solar panel. Proper mounting and ventilation help reduce heat buildup and maximize power output.

PV Solar Panel Performance – Effect of the Sun Angle of Incidence

Solar panels produce the most power when sunlight strikes them directly. As the sun moves away from a perpendicular angle, output decreases.

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☑️ What Is the Sun Angle of Incidence?

The angle of incidence is the angle between the incoming sunlight and a line that is perpendicular (90°) to the surface of the solar panel. Maximum power is produced when sunlight hits the panel as close to 90° as possible.
As this angle becomes steeper, less sunlight reaches the solar cells, reducing the amount of electricity the panel can generate.

☑️ Why It Matters

The effect is more significant than many boat owners realize.

• 30° off perpendicular: approximately 13% power loss.
• 60° off perpendicular: approximately 50% power loss.

This is one reason solar production is often lower during winter months or when panels are mounted flat on a deck without the ability to tilt toward the sun.

☑️ Improving Panel Performance

There are several ways to improve solar output by managing panel temperature and sun angle.

• Use adjustable mounts that allow panels to tilt and rotate toward the sun.
• Raise walk-on panels slightly above the mounting surface to improve airflow underneath, helping keep the solar cells cooler.
• Avoid permanent mounting locations where panels spend much of the day facing away from the sun.

☑️ Real-World Example

Custom Marine Products found that combining tilting mounts with improved airflow beneath the panel can significantly increase energy production. In favorable conditions, properly oriented panels can generate up to 40% more power than fixed flat-mounted panels.

Key Takeaway
The closer your solar panel faces the sun at a 90° angle, the more electricity it produces. Tilting the panel and improving airflow are two of the simplest ways to increase daily solar output.

PV Solar Panel Performance – Effect of Shading

Even partial shading can significantly reduce a solar panel's power output. Understanding how panels are wired helps explain why.

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☑️ Why Is Shading So Important?

Shading is one of the biggest challenges for marine solar installations. On sailboats and powerboats alike, masts, booms, antennas, radar arches, and other equipment can cast shadows across a solar panel throughout the day.
Unlike many people expect, shading just a small portion of a panel can reduce the output of the entire panel, not just the shaded area.

☑️ How Traditional Solar Panels Respond to Shading

Most conventional solar panels connect their cells in long series strings. Since every cell in the string carries the same current, shading one section restricts the current flowing through the entire string.
Depending on where the shadow falls, a traditional panel may lose 40% to 80% of its power output.

☑️ Shade-Tolerant Panel Design

To improve performance in real marine environments, some marine solar panels divide the panel into multiple independent sections using half-cut solar cells and bypass diodes.
Instead of behaving as one large panel, the panel operates as several smaller sections. If one section becomes shaded, the remaining sections can continue producing power.

☑️ Benefits of Shade-Tolerant Panels

• Reduces power loss caused by partial shading.
• Allows unshaded portions of the panel to continue generating electricity.
• Especially beneficial on boats where shadows constantly change throughout the day.
• Provides more consistent charging in real-world marine conditions.

Key Takeaway
A small shadow can dramatically reduce the output of a traditional solar panel. Shade-tolerant panel designs help minimize these losses, making them especially valuable on boats where partial shading is common.

PV Solar Panel Performance – Effect of Solar Cell Quality

Not all solar cells are created equal. Higher-quality cells generate more electricity and perform better under real-world marine conditions.

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☑️ Why Does Cell Quality Matter?

The quality of the solar cells is one of the most important factors affecting a panel's overall performance. Even two panels with the same size and wattage rating can produce different amounts of energy depending on the quality of the cells used during manufacturing.
Higher-grade cells convert sunlight into electricity more efficiently, allowing the panel to produce more power throughout the day—especially during the early morning, late afternoon, and under less-than-ideal weather conditions.

☑️ Understanding Solar Cell Grades

After solar cells are manufactured, each one is tested under identical lighting conditions and sorted according to its electrical performance. Manufacturers commonly classify cells into quality grades based on their measured output.

• Grade A+ – Highest output and consistency.
• Grade A – Excellent performance with very small variations.
• Grade A− to B – Lower output and typically used in lower-cost panels.

☑️ Real-World Performance

Higher-quality solar cells begin producing useful power at lower light levels and continue generating more energy throughout the day. They also maximize the available installation space on a boat by producing more power per square foot.
For marine applications, where available mounting space is often limited, maximizing power output from every square inch of panel becomes especially important.

☑️ Choosing the Right Solar Panel

When comparing solar panels, don't focus solely on the advertised wattage. Also consider the quality of the solar cells, panel efficiency, and whether the panel is specifically designed for the marine environment.

Key Takeaway
High-quality solar cells produce more power, perform better in challenging conditions, and maximize energy generation where installation space is limited.

Chapter 1 Summary

​PV solar panel performance is affected by the following:

Performance Factor	Example	Can You Improve It?
Light Intensity	-15% output due to reduced sunlight	✖ Weather dependent
Solar Cell Temperature	-14% at operating temperature	✔ Improve airflow beneath panels
Sun Angle of Incidence	-13% when 30° off perpendicular	✔ Tilt or rotate panels
Shading	-15% power loss depending on shading	✔ Better placement or shade-tolerant panels
Solar Cell Quality	Influences all of the factors above	✔ Choose Grade A/A+ marine cells

Important
The amount of power your solar panels produce is only half of the equation.
The other half is how much charging current your battery bank can actually accept.
This topic is covered in the next chapters on battery charging, charge controllers, and system design.