Some key points you should know about solar panels:

• With anything other than a very small panel for trickle charging your battery when you're away you need an MPPT controller not a PMW. They're just more efficient.
• When you're calculating the power you expect from your panel take the manufacturer's quote and half it. That will be a much more realistic figure.
• Shading is a big issue on a sailboat. All this business of 'blocking diodes' within the panels stopping shading from killing your output is bul!£*it !
• You can help shading issues by connecting in parallel or having separate controllers for each panel.
• It used to be stated that polycrystalline panels weren’t as efficient as monocrystalline ones but were better in shade. At the moment (2019) better technology means this doesn't always hold true. Most panels are about 20% efficient. That figure is getting better every year.
• Panels work better in lower temperatures. Making sure air can get around the panel and cool it helps efficiency.
• As we move around on boats, tilting panels to always be direct to the sun isn't really an option. How much you loose depends on how off the perpendicular you are, but you will loose about 20% of power being 30 degrees off.

So how much solar do you need? Well, you can work out the draw of everything on board, when and how you use it, then match it to the perfect solar / alternator / generator / battery bank... Err yeah good luck with that!

No, seriously, it's fairly futile trying to crunch those figures because for instance your auto helm may draw 2-3 amps on a reach in flat seas but hit 10-12 amps on a run with a nasty cross swell. It is a good idea to pay attention to these factors as things like good sail balance will help your auto helm no end and reduce its power needs. Also, having a look at what systems use the big amps is a good thing to know. That way when you're struggling for power at anchor somewhere you can work out how many lights you need to turn off to avoid losing your fridge and having to suffer warm beer! (Yes that did happen to me)

Anyway, total power systems is too much to go through on this page as we're talking solar. But, within your boat's system on a cruising yacht the place for solar is hopefully having enough that on sunny days you get your house bank back up to 100%. It's important for batteries to get regular full charges. This is not something you want to do with a generator as the charging period between 80% and 100% on any wet/gel/agm battery has very high internal resistance so very slow charging. Solar is great for this. If you aim for a system that means your alternator is powerful enough to get your overnight low (not lower than 50%  unless you're one of those flash lithium guys!!) back up to around 80% as to raise anchor and motor out to sail. Then with the engine off the solar can take over and do the bit it's best at and alternators and generators really aren’t.

So how much solar you need is as much as you can fit on without huge shading issues or making your boat look like an unseaworthy physics experiment.

Originally I set out to have a dual solar wind power setup, but research I did before hand and talking to people who have wind generators and don't like them, convinced me to just pack the solar on. (wind generators have got a lot quieter and more efficient but it seems they can still be a pain in the backside.)  I have more room on the pushpit rails and maybe some more on the doghouse for solar so if/when I decide we need more power I'll do that first and then maybe add a wind generator. Wind isn't great in trade wind sailing conditions, remember, as apparent wind over the decks is low.

This is the set up for the panels I put up in the video.

The one on the dog house has it's own controller

As I said in the video, I opted for panels in series as I don't have big shading issues so I'm operating  on 62volts at 8.7 amps (in parallel it would have been 31 volts at 17.4 amps)

The two figures on the controller relate to the max allowable current and voltage.  Mine is 150/60 so can take 150 volts and 60amps. Make sure you're not exceeding either of these figures taking into account the way you've connected the panels.

Obviously as Power (Watts) = Volts x Amps the power remains the same however you connect, but putting large current down cables is lossy even if you've been careful to size them well. So you're better off with high voltage and low current (but if you've got shading or panels in vastly different positions i.e. each side of the boat then you're stuck with series because the cells in good light will feed the shaded ones even across panels.

You'll also see that I've fused the positive feed to the battery. This is not because I expect the controller to suddenly spit out huge current and damage my batteries, it won't. But it is important that EVERYTHING coming from the batteries is fused close to the battery. A wire coming loose or getting chafed that's connected to the battery can easily flop around and touch the engine block, or something else that's grounded to negative, and your whole house bank charge will merrily exit through said cable, possible melting it and causing a fire. Fires on boats are bad!

You will see in the video my house bank goes to an adjacent large fuse box via large well protected cables. I took my feed from a bus bar there using a short piece of wire straight to a fuse.

On paper then my system should be capable of delivering 37.46 Amps. I wouldn't bank on getting much more than half of that most of the time. Sharing real world results is part of what we want to do on this website so I will let you know how they perform over time.

This is the best shading explanation I've found online