A [Brief] Guide To Charging Boat Batteries With Solar Panels

This article discusses the principles that must be followed when installing solar panels on a boat.

We also highlight factors to consider when choosing the solar panels, batteries, and other components that make up a solar battery charger system.

The rise of electric outboard (and inboard) marine motors has created a new dictionary with previously foreign expressions becoming part of everyday language.

The trend toward electric power and renewable energy will not go away, and we must keep ourselves up to date to ensure we are not left behind.

🔢 What Are The Main Things To Consider To Set Up The System?

Several questions must be answered before designing your boat’s solar battery chargers and systems.

  • The daily electrical consumption on the boat
  • Systems that are powered by the batteries
  • The size and type of batteries
  • The geographical location of the boat

What Is The Daily Electrical Consumption On The Boat?

Daily consumption is one of the factors needed to calculate solar equipment specifications.

It will be entirely dependent on the type of boat. It will range from a small standalone system used to recharge the battery in a small electric outboard motor, with related accessories, to a complete system used to power a live-aboard cruiser.

A small solar battery charging system could be used to recharge the batteries while the boat is being trailered, stored, moored, or even while the boats are trolling while fishing.

Which Systems On The Boat Are Powered By The Batteries?

To calculate the consumption, you need to know the three primary measurements of the electrical system.

  • Voltage
  • Amperage
  • Watts

You then tabulate each power consumer and work out the total power the system needs to supply enough power.

The table may resemble the following list (This would be a typical list of items powered by a system on a cruiser).

Electrical ApplianceWatts NeededVoltsThe Amperage Of Each ConsumerSurge Amps
Electrical Motor25,000Peak 27,50035070 (Cruising)8 (Additional used at Full Speed)
Windlass Motor4,800480100
Sail Furler96048020
Bilge Pump48024200
13,500 BTU Air- conditioner35002201616
Microwave Oven80048120
Cell Phone Charger241220
Computer (Charger)21021010
Coffee Makers1350220612
Water Heater1440120120
Total Watts39,415 170183

What Size And Type Of Batteries Are Being Charged?

The normal amperage and the surge amperage should be added together, which in this instance, means the system requires the capability of handling 353 amps.

There are several reasons why these are not absolute values.

1️⃣ They include the “surge” amperage in regular daily use. The surge amperage is required by specific power appliances (such as the fridge), after which the amperage drops to the nominal value.

2️⃣ The electric motor will rarely be used at full power.

3️⃣ It is unlikely that the electric motor will be powered directly from the solar system or utilized continually. The only power needed, therefore, is for the battery bank, as opposed to the motor.

4️⃣ Realistically, not all this equipment will run simultaneously. Accordingly, the system will be sized on the components running simultaneously.

This may look like the list below.

Electrical ApplianceWatts / Hour NeededVoltsThe Amperage Of Each ConsumerSurge Amps
Electrical Motor (Battery Charger)1,00035070 (Cruising)8 (Additional used at Full Speed)
Navigation equipment6120.50
Radios (HF/ Satellite)12012100
Bilge Pump48024200
Cell Phone Charger241220
Computers (Charger)21021010
Total Watts2,065 170183

Based on the motor size, the boat will use a 350V battery.

Where Is The Geographical Location Of The Boat?

The next factor to consider is the average amount and intensity of sunlight that you receive each day.

The following factors affect this.

1️⃣ The geographical position.

2️⃣ The quality of sunlight (air pollution, cloud cover, and dust affect this)

3️⃣ The typical weather conditions.

All of this generates the number of effective sunlight hours every day. This value changes across the country.

The renewable energy resource data center has interactive maps providing effective sunlight hours across the country.

How Many Panels Do You Need?

The formula to calculate the number of solar panels you need is straightforward (Hourly watt requirement / the panel’s wattage)

We will assume the following.

  • Hourly wattage used by the system (as worked out in the list above).
  • Panel Size 400W 

The answer is, therefore, 2,065/400 = 5 panels.

We dive deeper into this topic in another article how much solar is needed for a boat?

⚠️ What To Be Aware Of Before Buying Solar Panels?

For the uninitiated, buying solar panels can be a daunting task.

Technology is advancing at an impressive rate, and the capability of solar panels is increasing rapidly.

The Technology

There are six types of solar panels:

1️⃣ Monocrystalline solar panels

2️⃣ Polycrystalline solar panels

3️⃣ Passivated Emitter and Rear Cell (PERC) panels

4️⃣ Thin-film Cadmium Telluride (CdTe)

5️⃣ Thin-film Amorphous Silicon (a-Si)

6️⃣ Thin-film Copper Indium Gallium Selenide (CIGS)

The Monocrystalline Solar Panel

These panels are made from one piece of pure silicon crystal. The silicon crystal is cut into “wafers,” which gives them a distinctive black color.

These are the most durable panels, with the longest expected lifespan.

Because so much silicon is wasted in manufacturing, these are the most expensive solar panels.

These panels have an efficiency rating of 20% (and above) and an expected life span of 25 years.

The Polycrystalline Solar Panel

The manufacturing process involves melting the silicon pieces, which are poured into a mold.

While the manufacturing process is less wasteful, resulting in a lower price, they are less effective than Monocrystalline Solar Panels.

They have an efficiency rating of 17% and an expected life span of 25 years.

Passivated Emitter And Rear Cell (PERC) Panels

While the technology has its foundations in Monocrystalline Solar Panels, the design is improved.

  • It lessens the tendency of the electrons to recombine and restricts their movement,
  • Monocrystalline Solar Panels Light cannot absorb waves over 1,180nm, which causes them to pass through.
  • The back sheet reflects this higher wavelength radiation and prevents them from heating the rear of the panel.

They have an efficiency rating of 25% and an expected life span of 25 years.

Thin-Film Cadmium Telluride (CdTe)

Thin film panels have the advantage that they can be molded onto different surface shapes. However, the downside is that they do not produce the same yield as Crystalline panels.

CdTe panels have the same low-cost production method as Polycrystalline Solar Panels. However, they are relatively inefficient compared to Monocrystalline Solar Panels.

The toxic properties of Cadmium make these panels difficult to recycle.

They have an efficiency rating of 10% and an expected life span of between 10 and 20 years.

Thin-Film Amorphous Silicon (A-Si)

The word Amorphous describes the shapeless nature of these cells. This is because the silicon is not structured on a cellular level.

They have the lowest production cost of all panels. The downside is that they are less efficient.

They have an efficiency rating of 7% and an expected life span of 10 and 20 years.

Thin-Film Copper Indium Gallium Selenide (CIGS)

Copper Indium Gallium Selenide is made up of thin layers of

  • Copper.
  • Indium.
  • Gallium.
  • Selenium.

The components are installed on a glass panel or plastic backing.

They are the most efficient type of thin-film panel, but they are not in the same league as Monocrystalline Solar Panels.

They have an efficiency rating of 14% and an expected life span of 25 years.

Read next: Solar panels for boats | All you need to know

Will It Even Work?

The system must be designed and installed correctly. You will be surprised at how effective it is with sufficient sunlight hours.

The fantastic thing about solar power is that it requires minimal human intervention and starts working with direct sunlight. If clouds appear, they will produce electrical current albeit at a significantly reduced rate.

Some appliances that need 120V power will require an inverter to be included in the system. The rest can be connected directly to the combination of batteries that match the voltage requirements.

🔋 What Type Of Battery Can Be Charged With Solar?

All types of batteries can be charged with solar power. The critical question, however, is the degree to which different batteries can be discharged.

  • Lead Acid Battery – 50%.
  • AGM Batteries – 70%
  • Gel Cell Batteries – 75%
  • Lithium-Ion Batteries – 85%

Other FAQs

Will A Solar Panel Charge A Marine Battery?

Solar panels connected to an inverter or charge controller will recharge any battery in a boat.

The issue is the number of panels required to recharge the respective battery size. For example, a 350-volt battery will take longer to recharge than a 12-volt unit.

Do Solar Trickle Chargers Work For Marine Batteries?

Solar trickle charges are optimal devices to use to charge lead-acid batteries.

As lead acid batteries can only be discharged to 50% of their capacity, a trickle charger will help keep it above this figure.

Gel acid batteries are not suitable for regular charging and discharging cycles; therefore, a trickle charger will keep it topped up.

Do You Need A Special Battery Charger For A Marine Battery?

Depending on the battery type and the electrical consumption on the boat, you will have to decide whether to install a trickle charger or a heavy-duty unit.

A heavy-duty charger requires a larger solar system.

🔑 Key Takeaways

Solar energy is the optimal choice for providing electrical power in a boat. It works automatically when the sun is shining.

So even if the boat is stored, as long as the solar panels are exposed to sunlight, it will continue to charge the batteries.

We covered some key aspects of marine solar, including:

  1. Key considerations when setting up the system
  2. Things to be aware of when buying solar panels
  3. The types of batteries that can be charged with solar

The next step will be calculating the electricity usage on the boat – This evaluation spreadsheet will help you do just that.

Once you have worked out your needs, you should evaluate the number and type of panels required, the inverter capacity (for large voltage consumers), and the charge controller.

I’m the founder and chief editor here at Kite Ship. The electrification of boating is the most exciting thing to happen to the marine industry in a generation! Welcome, and I hope that we can provide the portal you need to dive into the world of electric propulsion and power.