Gosh, so many of you have been asking about deck mounting of solar panels. How can you be blamed? Now your sleek racing machine can have iridescent scales, crystals of silicon, stealing fire from Heaven to give you POWER! And here I am, claiming to be an advocate of engineering aesthetics, telling you to mount some gawky thing on your stern, flapping in the wind, trailing cords and cables like the tentacles of a jellyfish flung through the air by some fat kid at the beach. Am I crazy? Maybe, but my take home message is simply this:

If you'd like to deck-mount your panels, first check that you are rich.

I say this because for equal power you will probably need at least twice as many solar panels as if they were mounted on a gimbal at the stern. Certainly, deck-mounting can sometimes be a good option (some people are rich!), but either way, you'll get more out of your solar installation if you understand some of the tradeoffs.

Face the Sun

You know this. Good 'ol cosine at work. If your panel is 10 degrees off from the sun, you lose less than 2% of available power. 30 degrees drops 15%, and by 60 degrees you're down by half. So even if your gimbal is limited in range, you can keep your stern panel running near maximum all day long by spending 30 seconds every few hours to adjust the angle. In contrast, your deck panel will often be at a very oblique angle to the sun.

Shady Business

Another no-brainer, right? Then why do we see so many panels on the cabin top, right under the middle of the mainsail? This guarantees that your panels will be in the shade almost 50% of the time. And unless your panels are shade-tolerant, shading even just two cells will generally kill the output of the entire panel.

Trouble Afoot

I cringe whenever I see someone stepping on a solar panel. It's true, I had some very bad personal experiences with semi-flexible panels a few years ago, and I hope things have gotten better, but still... Solar cells are flexible just like glass, and the electrical connections to the aluminum plating of the cells can be a weak point. And there's the more obvious problem of simply nicking the encapsulant and letting the ocean in to do its dirty work.

Mucho Calor

Electronics hate heat, and solar panels are no exception. For every degree Celsius rise in temperature, a solar panel will lose about 0.4% of its voltage, and nearly as much in available power. Naturally, the balsa or foam core sandwich that makes many a sailboat deck is a superlative insulator. This fact ensures that your deck-mounted panel will cook off a sizeable portion of the power you paid for by possibly doubling the temperature rise over the stern-mounted configuration, where the panel can dissipate heat through its back side.

I should mention that conventional charge controllers throw this available power away, and are largely unaffected by temperature unless the panels become so hot that the maximum power point approaches or falls below the battery voltage, at which point the power output will fall off rapidly. Hopefully though, if you're reading this, I've convinced you elsewhere to invest in our GV series charge controllers with Maximum Power Point Tracking for your next or current solar installation. With MPPT, you will always win (10-30% typical) over a conventional controller, but it pays to be more careful about temperature.

And for those who still won't say no, a company called Sunware advertises panels for deck mounting made with 39 or 40 cells, instead of the usual 36, to counteract the voltage reduction from heating.

If You Still want to Deck-Mount...

Understand that you will have to be very conservative with your estimates of power output, due to all the factors working against deck-mounted panels. In the future, I would like to do some tests so I have accurate numbers to preach. For now though, I can only estimate that a deck-mounted panel will do half the work or less than a panel mounted on the stern and kept pointed roughly at the sun.

Keep the darn thing away from the mainsail. Larger boats generally have proportionally shorter booms, and less cramped decks, and will often have some good panel mounting areas behind the cockpit. Multihulls have it easy, and are also good candidates for deck-mounting because windage is a greater concern.

Smaller boats should seriously consider a deck-unmounted panel. This is a small panel stored below that is brought out during fair weather and clipped or tied on a good location on deck. This system provides essentially all of the advantages of stern-mounting without the stern-mounting.

And certainly, for the ultimate in weight and windage at any cost, deck-mounting isthe way to go. The same cells that go into Genasun's High Performance Solar Panels can be bonded directly to the deck of a sailboat, resulting in an astounding 90% reduction in weight over conventional panels. Contact us for details.

In closing...

Now you will be better equipped to make decisions about your solar installation, wherever you decide to mount your panels. And maybe it's less of a mystery why S/V Genasun has a big panel at the stern, and nothing in those empty spots on the cabin top.


-Alex, 11/2006