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An essential component in the installation of solar panels on your vessel is a solar charge controller or regulator. This will regulate the voltage and current coming from your solar panels going to your battery. Most solar panels are 16-25 volts, so if there is no voltage regulation the batteries will be damaged from overcharging. Bear in mind that a fully charged 12v battery is around 12.7 volts at rest, but needs around 14.2 to 14.8 volts under charge. A solar panel has to put out at least that much voltage to be of any benefit, but if the panel voltage is not controlled and reduced it will cause serious battery damage.

Another point to keep in mind: solar panels provide power best when cool, under a clear sky, and in full sun; in other words in perfect conditions. But one can't count on that type of weather day after day, so solar panels have to be built to provide that extra voltage for when the sun is low in the sky, there is cloud cover, high temperatures, or heavy haze to ensure your solar output is not compromised. The truth is a 100 watt panel rated at per industry Standard Test Condition (STC) of 77oF and 1Kw/sq m irradiance will put out less watts when its surface temperature is 100oF degrees and when only 800 w/sq m irradiance is available, i.e. when it is in non-standard conditions.

So this is where the right solar charge controller can help. There are basically two types – PWM (Pulse Width Modulation) and MPPT (Maximum Power Point Tracking).

The PWM controller simply limits voltage to a safe charging voltage, and any excess power gets wasted as heat, therefore you lose a lot of power. The MPPT controller uses all available solar energy, nothing is wasted except for minute amount of power used by the electronics. It does this by electronically finding the best mix of volts and amps to get the maximum watts, and then delivering almost all that power to the battery at a safe charging voltage. One big advantage of an MPPT controller is that it will deliver more amps to the battery than the panel produces, while the PWM controllers simply limit the voltage to the battery and the amps remain unchanged.

Because marine installation conditions are so different from land-based installations, certain unique factors need to be considered when setting up a solar array. On boats, there is a high probability that shading from rigging, structures, neighbor boats, etc. will occur at some point during the day and on different solar panels. For this reason, it is recommended to install a single solar controller for each panel as verses setting up multiple panels in series or parallel all to one large controller. In that way, should one panel become shaded and its output lessened to a degree, the other panels on the boat will not be affected. Genasun (create link) specializes in manufacturing high-efficiency MPPT controllers sized for this type of installation and at reasonable cost.

We at Coastal Climate Control have performed many tests comparing PWM and MPPT controllers and can confirm the manufacturers' claim that the MPPT controllers perform consistently about 30% better than PWM controllers in low-light and partial shading situations.

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