Photovoltaic Panels: Recharge the Onboard Batteries with the Sun

The sailboat is an ecological means of transport by definition. Sailing is not just a hobby or a sport but also a lifestyle and philosophy. I have always been fascinated by achieving energy independence on a boat.

During stops for a few days at anchor, I have always resented having to turn on the engine daily for an hour or two to recharge the onboard batteries. And it’s not just an engine wear problem, I assure you!

A couple of years ago, I equipped myself with an 85-watt photovoltaic panel to see if I could reduce the recharges with the engine alternator. With great satisfaction, installing that single solar panel guarantees me, on average, those 35-40 Ah per day sufficient to cope with consumption.

Ultimately, I stayed at anchor in sunny weather for up to a week without starting the engine. But let’s go step by step. You need to know some things before rushing to buy a photovoltaic panel.

First of all, power is needed. It is, therefore, necessary to calculate how much our daily electricity consumption amounts to. (click on the link to read the article that explains how to do it)

Photovoltaic Solar Panels onboard

Once we have established how much our consumption is, we need to choose a photovoltaic panel of adequate power.

Let’s take an example. Our calculations show that our daily electricity requirement is 50 Ah (Ampere Hour).

Consider that a photovoltaic panel, if correctly oriented, on a completely clear day produces, on average, about 40-45 Ah for every 100 Watts of nominal power.

Installing a 60 Watt panel will produce about 25 Ah per day (60% of 40 Ah), which is half of your daily requirement. Better than nothing, right?

Which photovoltaic panel to choose? Where to buy solar panels?

The photovoltaic panels on the market are basically of three types:

Monocrystalline Silicon Photovoltaic Panels

These are the most expensive panels but have the highest efficiency, i.e., the best ability to transform light into electrical energy. The efficiency of solar panels of this type on the market today is measured at around 15%.

This means that from a surface of 1 square meter of panel exposed perpendicularly to the sun on a clear day, we will obtain a power of about 150 Watts of electrical power.

This panel type must always be correctly exposed to the sun and not be obscured, even partially, for any reason.


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A small shadow (from the boom, the mast, or even simply from a shroud) on one of the individual cells that make up the panel is enough to reduce energy production drastically.

Polycrystalline Silicon Photovoltaic Panels

They are produced substantially similarly to the monocrystalline ones. Still, in this case, the silicon is not as “pure,” and therefore, the efficiency drops slightly (around 13%), but also, the price per Watt generated is a little lower. Even the polycrystalline silicon panels cannot bear the slightest shade due to a significant decrease in the energy produced.

Photovoltaic panels in amorphous silicon have a different construction technology compared to the previous types, with some advantages and disadvantages.

First, the efficiency, i.e., the yield, is much lower (almost half) compared to the two previous types: 6-10% compared to 13-15%. This means that to generate the same power. I will have to install larger surfaces (almost double).

Normally on a sailboat, this is a big handicap. Still, on the other hand, we must also consider that amorphous silicon panels can be produced in very thin films and glued onto supports suitable for installation on irregular surfaces (such as deckhouses or decks). And even be trampled on.

Another positive feature of this amorphous silicon technology is that it has a higher yield in the event of harmful exposure to the sun, partially shaded panels, or partly cloudy weather (about 20% more than mono or polycrystalline panels exposed to the same light conditions).

If the price per Watt produced is 10-20% lower than the first two types, this should be enough to seriously consider their use, provided you have enough space for installation.

High-Performance Poly or Monocrystalline Silicon Flexible Solar Panels

For the installation of solar panels on sailing or motor boats, or on campers, today it is highly advisable to use these flexible panels which, being composed of monocrystalline or polycrystalline silicon cells, offer high efficiency, and therefore many watts in little space, as well as a lightness unthinkable until a few years ago. Think that a 100W panel weighs just over 1 kg. Practically the same weight as a sheet of cardboard of the same size.

Panels with 40 V and above outputs are more suitable for producing electricity for domestic systems, in which the final use voltage will be 220 V alternating.

Today these panels can also be used to recharge 12V batteries, but in this case, it will be necessary to use a charge controller with the MPPT algorithm, which, although certainly better, will have a higher cost. But it could be money well spent.

Solar Panels on Water

Naturally, the 17 Volt voltage will have to be brought to more suitable values for recharging the batteries, so I recommend you: do not attach the panel directly to the battery poles because you would risk burning it in a short time (the battery would go “boiling,” as used to say)

Instead, I advise you to equip yourself with a good voltage regulator. There are many types on the market at prices ranging from 20-30 euros up to 150-200 and even beyond.

Around 100-120 euros, you can find excellent ones with electronics capable of charging the battery up to values very close to 100% of its nominal capacity and then switching to the charge maintenance state, thus avoiding potential damage to the battery.

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These devices are also equipped with an LCD that provides very useful information, such as the current that the panel is generating, the one that is entering the battery, the voltage of the battery itself, and more. The picture shows you what I’m using in my solar panel system.

Saving too much on a voltage regulator can sometimes mean wasting part of the precious energy that the panel is producing or, in the worst case, shortening the life of our batteries.

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