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

ByMathew F Updated On:

Before I get into talking about how to charge with solar you first need to know how much solar your boat needs overall because installing solar panels on your boat involves more than just mounting hardware and wiring.

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 of a solar battery charger system.

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

The Main System Considerations

To help break this up I’m going to split everything up like so:

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.

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)
Navigation6120.50
Radios12012100
Autopilot10012812
Windlass Motor4,800480100
Sail Furler96048020
Bilge Pump48024200
13,500 BTU Air- conditioner35002201616
Refrigerator10024715
Microwave Oven80048120
Cell Phone Charger241220
Lights252410
Computer (Charger)21021010
Coffee Makers1350220612
Toaster50010040
Water Heater1440120120
Total Watts39,415170183

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.

  • 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.
  • The electric motor will rarely be used at full power.
  • 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.
  • 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
Autopilot10012812
Bilge Pump48024200
Refrigerator10024715
Cell Phone Charger241220
Lights252410
Computers (Charger)21021010
Total Watts2,065170183

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

But what about the number of solar panels then? I cover that in depth in the sizing panels section further down.

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

    The following factors affect this.

    • The geographical position
    • The quality of sunlight (air pollution, cloud cover, and dust affect this)
    • 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.

    Types of Boat Batteries

    Your boat can be equipped with different types of batteries, each designed for specific roles:

    • Starter Batteries: Designed to deliver quick, high-power bursts to start engines. They have a high cranking amperage but are not intended for prolonged discharge.
    • Deep Cycle Batteries: Built to provide a steady amount of current over a long period. They have thicker plates allowing for sustained discharge and recharge, optimal for your solar setup.
    • Dual-Purpose Batteries: These can perform both starting and deep cycle functions, offering a compromise for boats with limited space for multiple batteries.

    💡Pro Tip: Battery capacity, measured in amp-hours (Ah), is key to energy storage.

    Types of Marine Solar Panels

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

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

    There are six types of solar panels that can be either rigid or flexible.

    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 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.

    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 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.

    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.

    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.

    Sizing the Solar Panels

    The size of the solar panel you require hinges on:

    • The energy consumption of your appliances (I covered this earlier)
    • The amount of usable surface area available on your boat

    To size your panels:

    • Calculate the daily power usage in watt-hours (Wh) by listing all your appliances and their consumption.
    • Assess the average sunlight hours your boat will be exposed to, which dictates the total wattage needed from your panels.

    For instance, a fridge drawing 40W for 24 hours needs 960Wh per day. If you expect five peak sun hours, you’d need around 200W of solar panels to cover just the fridge usage. Here’s a simple way to understand the math:

    Total Daily Usage (Wh) / Average Sunlight Hours = Required Solar Wattage

    It is also vital to include a buffer of at least 25% to accommodate for inefficiencies and potential overcast days. So consider a setup totaling approximately 250W to maintain uninterrupted operation of your fridge. Always round up to the nearest panel size available to ensure your needs are met.

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

    Optimizing Solar Charging

    Start by selecting a solar panel with sufficient voltage and power output to match your battery’s requirements.

    A panel should provide a higher voltage than the battery’s full charge voltage to be effective—for a 12-volt battery, for instance, you want a panel that offers around 14 volts under optimal conditions.

    Position your solar panel in a location on your boat where it will receive maximum sunlight. Avoid shaded areas and consider tilt angles that maximize exposure to direct sunlight throughout the day.

    Solar panel performance can falter under extreme temperature conditions. Try to install your panels in a way that allows for air circulation to help regulate temperature and maintain solar power efficiency.

    Check the solar panel output regularly using a charge controller monitor. It should reflect real-time power generation and how effectively your panel charges the battery.

    Be prepared to adjust angles and positions to accommodate for changes in location and seasons. This ensures your panels continue to receive direct sunlight and operate at peak efficiency despite the varying position of the sun.

      Frequently Asked

      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.

      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.

      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.

        Written by:

        Mathew F

        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.