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All aboard: How to choose and size solar equipment for your boat
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All aboard: How to choose and size solar equipment for your boat

10 mins 02 Apr 2021
Just a while ago a former Navy officer sailed all the way from South Africa to Brazil on a small boat, relying entirely on solar energy. Before you set out to repeat his voyage or make your own route, here is what you need to know about a marine solar installation and how to size it properly.

Talking Size, Shape, Space

Solar panels for boats come in different shapes and sizes. They don't have to be powerful: among sailors, as well as boondockers, small 100W modules are popular. Flexible solar panels are also used more often than in residential systems, since a boat can have difficult configurations and it can be hard to install rigid crystalline modules there.

You can put panels on a deck, canvas or a stern rail of your boat. Think in advance about the positioning of PV modules: there isn't much unshaded space on board. Solar panels on boats are often shaded by masts or sails and it greatly diminishes their power output.

Number of solar panels depends on your energy consumption

To figure how many panels you need, calculate your daily energy consumption. Write down the power ratings of all the devices and the number of hours in use. Then multiply the power rating and hours for each device. Finally, sum the energy needs of every device together.

Solar panels perform at their maximum capacity only during peak sun hours. So, once you have the daily energy consumption of your boat, divide it by the number of peak sun hours in your region. What you'll get as a result is the power of the solar array that you need. For more details, check out our detailed guide on calculating solar power needed to travel the seas.

Don't go overboard with solar panels for your boat – you don't need too many, you need just enough. In residential systems, there is always something to do with excess energy from panels: you can sell it to the commercial grid, or even use it to charge your electric car. Out in the open sea you will just throw away that surplus energy. So, with too many panels you may find yourself having spent more money than needed. Plus, extra panels take up space which is really valuable on a boat.

Daily energy consumption × 3 = battery capacity

If your boat doesn't have a house battery yet or you want a new one, think about its size ahead. The amount of your daily energy consumption when on the boat comes in handy here. As a rule of thumb, a house battery capacity should be 3-4 times bigger than your daily energy consumption.

There are several types of batteries on the market, the popular being lithium-ion and lead-acid deep-cycle batteries.

  • Lead-acid battery is a traditional choice for a boat battery bank. Its capacity is measured in Amp-hour (mAh). To see if the battery can cover your daily energy needs, multiply Amp-hour by the battery voltage, which is usually 12V. It is not recommended to discharge lead-acid batteries below 50%.

  • Lithium-ion batteries rose to glory in the last few years due to the drop in the cost. They are still expensive, but better than lead-acid batteries all around. For example, they handle deep discharge much better, but it is still advised not to discharge it below 20-10% on a regular basis. The capacity of a lithium-ion battery is usually measured in Kilowatt-hour (kWh).

Charge controller protects your house battery

Every solar installation with a battery needs a charge controller, unless you are using very small panels that don't charge your energy storage fully. A charge controller saves your battery from overcharging and deep discharge, and also protects it from high voltage of PV modules. Without a charge controller you risk shortening the lifetime of your house battery.

There are two types of controllers:

  • PWM-controllers are cheap and reliable, but not very efficient. They only make use of 70-80% of the energy coming from panels. What's more, they work well only in the systems where the voltage of panels is slightly higher than the voltage of a battery. You can get one controller for around $20–$100. Overall, it's a good choice for marine solar panel systems, since they are usually small.

  • MPPT-controllers are efficient. When the panels' voltage greatly surpasses the one of the battery, an MPPT controller converts that extra voltage into current for the battery. It fits any kind of solar installation and helps use the full power potential of the panels. Their cost, however, can be as high as $1000, and they last a bit less than PWM regulators, from 10 to 15 years.
To size a charge controller for your system, look at its amps size and maximum voltage (VDC). The minimum amps of the controller for your system are the power of your solar array divided by the voltage of the battery. The maximum input voltage is the sum of VOCs (open circuit voltage) of all panels. The open circuit voltage of a panel, as well as the power, is always listed in its specifications.

Add an inverter for AC devices

Not every marine solar system needs an inverter. The lights and even a fridge work fine on the direct current that comes from panels and a battery. But to use a laptop or microwave on a boat, you would need an inverter to turn DC to AC.

The size of an inverter is measured in watts and generally should be close to the power rating of your solar array. For example, if you have three 300W panels, go for a small 1000W inverter. If an inverter is too small, it won't be able to handle the workload and you'll be losing a lot of energy. Check in advance whether your inverter is compatible with the battery's voltage.

Another choice to make is between a pure sine wave inverter and a modified sine wave inverter. The main difference between them is that a modified sine wave inverter is slightly cheaper, but doesn't work with high-tech, precise electronics. For example, you always need a pure sine inverter for modern TVs, electronic clocks and timers. Microwaves and fridges with an AC motor also prefer pure sine wave inverters, while laptops are usually fine with either one.

Why go solar for marine

  • Solar panels are silent and smell-less energy generators
  • Panels are almost maintenance-free and work even when you are away from a boat
  • Solar panels pay for themselves in 7-8 years on average
  • PV modules don't need a lot of space and you can put them on a mast, stern rail or even a sail of your boat. Sometimes it's enough to install only one powerful solar panel for the boat battery to get fully charged.
  • Using renewable energy contributes to fight against global warming
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Illustrations – Marina Fionova