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.