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Solara-50w-115w-Morgan45-289x180Which type of Solar Panel is best for Marine Installations?

Monocrystalline; Polycrystalline; SunPower®

3 - 4 - 5 Remember these numbers. These are the factors by which you need to divide the rated wattage of a panel to give an estimate of the amp/hr daily yield, at 12v, that a panel might produce dependent on the type of cell.

    • For a panel with SunPower® cells, divide the watt rating by 3.
    • For a panel with regular monocrystalline silicon cells, divide the watt rating by 4.
    • For panels with polycrystalline cells, divide the watt rating by 5.

Example: If we have one of each type of panel, all rated at 100 watts, then the one with SunPower® cells will produce somewhere in the region of 33 amp/hrs per day; the one with regular mono cells will produce around 25 amp/hrs a day; and the panel with poly cells will produce 20 amp/hrs or so a day. (Note: This is assuming a good solar day, using a MPPT controller, and with a full time load). So why the difference?

SunPower® back-contact solar cells are currently the highest efficiency cells available for use in everyday applications at 22%+ rated efficiency. But they can be expensive and difficult to purchase. Genuine high-grade SunPower® cells are only sold to prestigous manufacturers in a few select markets, and significant quantities, a minimum 10,000 cells at a time, must be purchased by the manufacturer to ensure a workable price for their customers.


It has recently come to light that some back-contact cells made by SunPower® have found their way on to secondary markets and into panels originating primarily in China. Rejected by SunPower®when they failed their rigoroustesting, these faulty cells cannot carry the prestigious SunPower® name due to their substandard quality. Unfortunatley, some promotional material we’ve seen brazenly disregards this fact.


Be wary of any solar panel with back-contact cells that does not explicitly make reference to the cells as being genuine, high-grade SunPower® cells.

While the claim may be that this type of panel has superior shade resistance, the results can be very different.

smaller cell size
FACT
: The smaller the cell size, the less shading it requires to be 100% shaded. The same amount of shade that completely covers a small cell might only cover ¼ of a larger cell. Larger cells are better for shade resistance.

solar cells shaded
FACT
: If just one cell is 100% hard shaded, the output from that series string of cells will be zero, or close to zero.

FACT
: Silicon solar cells consume power as well as produce power. If a cell is shaded, it will consume power from the other cells in the series string, resulting in diminished output and heating up of the shaded cell.

solar panels in parallel
FACT
: If several series strings of cells are connected in parallel without the installation of blocking diodes, the power from good strings will feed into a shaded string resulting in diminished panel output and the chance of cells burning.

blocking diodes
FACT
: Blocking diodes prevent back-feeding in parallel circuits, but reduce voltage output by 0.7v.

solar-panel-6
FACT
: By-Pass diodes prevent cell burning ("Hot-spots") and do not consume any power or diminish panel output.

FACT
: The more cells and electrical connections there are on a panel, the more output-reducing series and shunt resistance the panel will exhibit, and the more chance of a connection failure, particularly if the panel is flexed.

solar cells in series
For best results, look for a panel with large, high efficiency cells, in one series string, with by-pass diodes protecting any string of 50w or greater.

blocking diode drawing
Blocking diodes must be installed on circuits or panels connected in parallel.

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).

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