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Solbian solar panels installed on canvas
An increasingly popular question we hear at Boat Shows is "What size solar panel do I need to run the refrigeration on my boat?" Of course, this begs the question "What is your refrigeration's current draw?" which in itself may not have a set answer.

The current consumption of any refrigeration system is meaningless unless all the conditions are specified, i.e. box temperature, ambient temperature, water temperature, compressor speed, voltage, etc. The manufacturer's figures are simply average numbers; some manufacturers, like Frigoboat, attempt to give a true average figure, while others use their figures more as a marketing tool.

Making use of variable compressor speed, as Frigoboat does with the Merlin II and Guardian speed controls, and others do with the Danfoss/Secop AEO control module, brings even higher efficiency and lower overall power consumption in air- and keel-cooled systems, but is counter-productive in a pumped-water system. This is because the pump adds 25% to 35% extra current draw, and so it is best to run the compressor at full speed and get the job done as quickly as possible. The most efficient system is one using the Keel Cooler; no pump, no fan, and variable compressor speed can be used to gain even higher efficiency.

Daily consumption is measured in amp/hours per day, and you can get an idea of what to expect by using a watts meter, like Watts Up, on your refrigerator leads. These meters can tell you how many amp/hours per day a device is using, and it's best to use it over several days to get a daily average. The running consumption will vary dependent on conditions, as will the on/off cycle time, so you will need to be able to come up with an average daily amp/hr draw.

As a general rule of thumb, a solar panel with SunPower® cells will give approximately 1/3 of its rated wattage as a daily yield in amp/hrs. A panel with regular monocrystalline cells will produce about 1/4 of its wattage as daily amp/hrs, and a polycrystalline panel produces around 1/5 of its wattage rating in amp/hrs per day.

Let's take an example:

(as found in Section 6 of the Frigoboat manual)

Glenn-bending-plate-2The F-type flat evaporators may be carefully bent on a minimum 1.5" radius to follow the shape of the icebox. This is best done by holding a section of suitably padded PVC pipe (with an outside diameter of 3.0" or greater) firmly down on the plate, and then carefully bending the section upwards with the palm of your hand.

The stainless steel plates need more force to bend and may require the assistance of a second person. This must be done slowly and with great care to avoid excessive kinking of the channels in the evaporator.

Never attempt to bend a plate downwards over a pipe, as damage may result.

In order to prevent the paint from cracking, the area to be bent should be warmed with a hair dryer or heat gun to approximately 200 degrees F before bending.

There are sections of the evaporator that must not be bent, and these are indicated on the specification sheet.

All F-type evaporators can be mounted with either side facing inwards, including the 380F which has one smooth side and one with channels. F-type plates with a stainless steel cover must be mounted with the stainless side facing inwards.

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.

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