The published wattage rating of solar panels is determined by testing with a machine that flashes a light with an intensity of 1,000 watts per square meter on to the panel as if the sun were directly overhead. This flash test, performed under standard temperature and air quality conditions, gives a theoretical maximum power output that might be possible from that panel under the most ideal conditions.

This may sound like a cheap marketing ploy, but in fact is done from a safety aspect, as the cable and safety devices (fuses, breakers, etc.) must be sized in accordance with the maximum power that the panel(s) might produce, especially in multiple panel arrays. Typically, a panel will produce nowhere near its rated output in normal use, except maybe occasionally and then only very briefly. Considering the above, and the multitude of ever-changing conditions in a real deployment, it is not practical to simply use the wattage rating of a solar panel as an indicator of what power output to expect over the course of a complete solar day; i.e. from sun-up to sun-down. So how can we estimate what size panels, and how many, we might need to satisfy our daily amp/hour consumption?

First it is necessary to differentiate between the three most common types of silicon cells in general use.

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.

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 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:

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