Something for nothing Solar panels will give you free power, but better than that they can save the life of your batteries
Solar panels are just about the lowest maintenance way of trickle charging your batteries onboard a boat. But before we start talking about types and fitting, it's worth looking at the science behind them.
They are made of a semi-conductive material, the most common being silicon, which contains electrons. Normally they're quite happy just to sit there until photons in the suns rays hit the solar cells, then the electrons absorb the energy and transform into conduction electrons. If the energy from the photons is great enough, the electrons become free, and carry an electric charge through a circuit to the destination, a battery or piece of electrical equipment.
So let's talk panels. The three most common types are rigid panel, semi-flexible and flexible. The basic rule of thumb is the stiffer the panel the more efficient and cheaper it will be.
Rigid panels are industry standard units, exactly the same as those you see on rooftops all over the world. They come in a robust aluminium frame and are sealed under a sheet of glass. They produce the greatest power with the smallest footprint. But on the roof of a boat their bulky unwieldy shape and size mean you have to take care not to stand on, or drop things on, them. You can, though, mount them on moveable brackets to maximise their sight of the sun.
Semi-flexible panels are made of a very similar material, but rather than being sealed under glass they have a clear resin placed over the panel which reduces the amount of light they can absorb. These panels are used extensively on boats because they have a hard-wearing surface that's almost unbreakable. But you will need approx 30 percent more space to achieve the output of a solid panel, together with a 40 percent larger wallet.
Flexible panels are very specialised and unless you want to wrap them round your bow, mould them into an interesting shape or want to take them off and roll them up, it's probably not worth venturing down this route. Again they are less efficient than either the rigid panel or semi-flexible type, but in specialised applications they can be just the thing.
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As with most things in life, the bigger the better is the basic rule of thumb with panels. Nearly all panels are rated in watts. This wattage rating is based on the average daily output in summer months. To convert this to amps for charging and usage, divide the watts by the battery voltage; for example a 50W panel will put 4.1Ah per day into a 12v battery. In winter this output can be reduced drastically, and even on the hottest day of the summer if even one section of the panel is blocked by leaves or a lazy deck hand leaving a mop covering part of the panel, the unit will become almost redundant.
Most people want to fit solar panels to reduce engine running times for charging purposes or top up batteries when shore power is not available.
The silent eater of amp hours on a boat is normally the fridge. A 12v modern fridge is rated at around one amp per hour, which means you have to produce 24Ah of power to offset this use. That means fitting a panel or panels rated at 290 watts, and that's only in summer; in winter you'll probably need four times as many panels.
As you can see, they are limited in power but everything helps. Where they really come into their own is keeping the battery bank topped up when you are not using the boat. All batteries naturally discharge when not in use and a battery left flat for even a few weeks will cripple its capacity, so the �300 you spend on a panel could save you twice that if you have to change your domestic battery bank. As a minimum you should be looking for an 80W panel to keep the batteries topped up over the winter and in summer it will give you some power to spare.
Once you have a panel, it is advisable not to connect it directly to the batteries. Solar panels can produce high voltages up to 24v which could, over a matter of a couple of weeks in the summer, boil the batteries dry. Always fit a solar regulator matched to the size of your panel. These control the voltage and will dump any power the panel produces that could harm the batteries or electronics onboard.
So, maybe �300 to buy a panel plus the cost of the regulator, but after that you get something for nothing.