Solar

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After several discussions recently with customers contemplating adding solar to their electrically powered vessels, there is still a lot of confusion about exactly how much power can be realistically expected from solar panels. I have an inkling that these customers are so honed in to watts and kilowatts from their dealings with propulsion that they assume that all watts are equal.

But solar watts are a different animal.

The watt is a measure of power and is normally derived electrically from multiplying volts times amps (W = V x A), or amps squared times resistance (W = I² x R).

So, using simple math, if we have a 5,000 watt (5 kilowatt, or 5kW) DC electric propulsion motor running on 100 volts, we would expect it to be drawing 50 amps at full load. If the same sized motor was designed to run at 50 volts then the current draw would be double that at 100 amps.

For reasons of wire sizing and cost of ancillary equipment we’d want to keep the amps as low as possible, so the higher the voltage the better (except for having highly lethal voltages in damp environments). As you can see, the numbers are all very simply calculated and it’s all cut and dried, and it had better stay very dry indeed!

Solar watts are different, and I’m referring here to the wattage rating of the panel(s).

rain barrel dreamstime m 39561177 427x640There seems to be many misconceptions regarding solar panels, particularly concerning marine installations on vessels. I’m thinking that maybe it’s because we all see solar panels on houses and assume that what applies to roof-top installations also applies to boats. Wrong!

Here’s some solar facts. Typical solar panels with silicon cells will:

• Produce electrical power whenever they are exposed to light, and this is proportional. Small amount of light - small power output.
• Produce full voltage even in very low light, often even indoors, but only if they are disconnected and not being asked to produce any power. Potential voltage output depends mainly on the number of cells. Each cell typically produces around 0.6 to 0.7 volts.
• Produce full current output only if there is a big enough load, the sunlight is good enough, and shading is non-existent. Potential current output depends on the size and type of cells.

In general; the quantity of cells determines voltage; the quality of cells determines amperage.

Now, what we are after in a boat solar application is to grab whatever power we can whenever there is available sunlight, and that means from dawn right through to dusk, not just at noontime. I often hear customers say that there is absolutely no shading at the back of their sailboat, but then when it is suggested that maybe in early to mid morning or mid to late afternoon, the presence of a humongous mast and associated rigging might shade a stern-mounted panel, they will humbly agree. From that sort of response one could deduce that many boaters are under the assumption that solar panels will really only work for a couple of hours each side of noon. Not so Horatio.

When you want solar on your boat... You Want Solar On Your Boat! Now, where you put it depends on what real estate you have available that is relatively free of potential shading and yet still gives you room to move around.

Those of you with a sail boat need to be aware of the shadows cast by your mast, boom, and rigging. If you have a power boat, be aware of the shadows cast by your radar, arch, antennas, water toys, helicopter, etc.

So, let’s look at what others have done.

1. Dingy davits are popular locations for either glass panels or lighter weight semi-flexible panels. The davits themselves should be specifically designed and fabricated to carry the extra weight. Notice the clever person who put lightweight panels in-line with the davits to allow easy access to the dingy and swim ladder.
Davit solar panel

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A while ago I debunked some common misconceptions about solar panels in another blog here, but it seems I missed one that has come to light several times recently, i.e. the myth that to have proper, serious, useable solar power on a boat one must use residential panels, as marine solar panels are just itsy-bitsy wimpy battery top-up trickle-chargers.

Nothing could be further from the truth!

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