Sailboat Solar Panel Sizing

[silviris]

A lot of technical information in the last few posts, but I get the general idea. Good point about the boom shading the panels using them as the bimini. Looks like the rear of the boat is going to be the solar panel real estate. Fortunately, the Searunner has plenty of mostly unused space there, so it's not a big deal.

[sailorboy1]

Originally Posted by silviris

Greetings to all!

I will be acquiring a Brown Searunner 34 in the next couple of months and I'm looking around at solar panel options, trying to price out what I'll need. I've been doing a lot of research, but without the actual equipment in front of me it's hard to get real numbers to figure out the required solar panel wattage to keep the batteries topped off.

I'm trying (and not succeeding) to minimalistic about things that use power. For example, anything water related will use manual foot pumps, and the toilet will probably be a 5 gallon MSD port-a-potty so I can still do the "hands off" pump out whenever possible, but also prevent people from screwing up the plumbing and save myself from having to use an energy stealing electric pump and macerator. I considered manual toilets with a holding tank, but I don't think the wife will go for that! Cooking will be propane and AC will be the lovely San Diego weather. :P

As far as what I DO want to have on the boat that draws power, there are a few creature comforts I'd really like to get on board. I'm basically just looking for an experienced solar panel user to recommend total wattage to keep the batteries charged using the following devices:

1) A small mini-fridge with freezer (turned off during the week, but may be used more for extended trips. Gotta keep those steaks from spoiling!)
2) Recharging for two iPads (movies!)
3) Recharging for two smart phones
4) Recharging for laptop (schoolwork at anchor, maybe a movie or two when the iPads are busy)
5) Marine GPS/chartplotter (any recommendations?)
6) Running lights
7) A box fan (helps me sleep at night, but can probably go without if it's too much draw)
8) MAYBE a microwave? Kind of lazy with the cooking sometimes.....

I know this is a very general question and for right I'm just looking for a very general answer so I can start figuring out my budget. I would also appreciate recommended vendors and models.

Thanks!

Dave

I'm not going to go all technical. If it was me I would plan on having:

400-500 AH of batteries
240-300W of solar

This will keep you charged up and even catch the batteries back up if you have a few days of clouds, and that is even if you leave the frig ON during the week when you aren't on the boat. All while keeping that batteries above 50% state of charge.

PS - I wouldn't worry about the power to run a fresh water pump. The reason to consider a foot pump is to save water not power (mine water pump will run maybe every 6 hours for 20 seconds if no one is using any water).

[Mark Johnson]

Originally Posted by silviris

Due to the shading issue, do you recommend multiple smaller panels or the one large panel like I was originally thinking? Do the larger panels compensate for shade? I know some panels are wired to bypass non-functioning cells to help with shade problems, but I'm not sure on the terminology.....I think it was amorphous something or other.

Thanks!

Silviris:
I have been solar self sufficient since my first multihull project in the 70s, and have over 40 years of experience with installing systems on boats such as this, as both vocation and avocation... If you look back on my past posts, (just click on "past posts by Mark Johnson")... There are about 50 pages on this subject, as I applied to this very design boat.

You should get by easily with just 300W of solar panels, IF you conserve as much as I suggested. In our case, this is WITH SSB/Ham, stereo/TV/DVD entertainment center, 20 lights, water maker, pressure water, inverter, 10 fans, RADAR, Auto pilot, laptop nav, vacuum panel refrigeration, GPS, weather FAX etc, etc... We live in comfort!

Since refrigeration is over half of a days energy consumption, having it SUPER insulated, (with vacuum panels = to 7" of foam), will save you thousands of dollars, VS just buying more solar panels. Remember, they're big, can be in the way, and are heavy, so you don't want too many on your small tri.

The perfect setup for your boat is to place them spread out, as I did on Delphys, with the aft one on a rack that articulates. (I only tilt it rarely, if it was REALLY cloudy, and I need to).

My panels are a 35 & 55W on the front cabin corners, with an 85W on the sterncastle deck, and the aft 110W panel is on the rack. By locating them far apart, when one is shaded, (more often than not), the others are still in the Sun.

I installed Schotky blocking diodes in the + wire of each panel to prevent backflow into the shaded panel. The "bypass" diodes that comes within the panels, contrary to popular belief, do not do this. They only prevent backflow within the panel. The small V loss from these Schotky blocking diodes is a small price to pay imo, being that one panel will usually be shaded. It will work OK either way, however...

All panels should be SINGLE CRYSTAL type, as they are more space efficient. They are 18-19V "House" panels, and can bear some weight, but should not be walked on. Mine have endured thousands of nights on the hook over 17 years of cruising, (mostly tropical), through hail storms or even winds up to cat 4 hurricane. I do suggest they be a bit OFF the deck to keep them cool, and that you make a set of "Starboard" mounting rails for each one, with beveled ends, to prevent snags from sheets & such. It protects toes too!

The best deal on a good 3 stage + equalize regulator, is through the "back to the land" web sites, although mine came from S.A.L.T. in Marathon Fl. These charge controllers have adjustments for each stage, to match them to your batteries' requirements. This assures a VERY long battery life!

For house batteries, I suggest a pair of "wet" Trojan 6V L-14s, for 340 Ah of storage. With "Hydro Caps", they will just need water a few times per year, and the shallow 40 or so Ah/day cycles, along with a full "3 stage" charge to 100% daily, will make them last with great reliability, for over a decade!

The most important part of a boats electrical system, is the wiring harness! If you use finely stranded, tinned & double jacketed marine wire, it will hold up. Avoid all but the best tinned "ring" connectors on the ends of the wires, and do not use the factory sleeves on them. I remove them, slide on an adhesive lined heat shrink, crimp the connector, and then apply a SMALL drop of solder to the end of the sleeve. (Not enough to wick up the wire). Only then, apply the heat shrink tubing! This way, after decades of the marine environment, they will have very little resistance. MOST production boats which are not wired this way, (and I've worked on hundreds), start loosing a good connection after 15 years. Then after 20 or 25 years max, there are high resistance "ghosts" throughout the wiring harness, and the entire boat needs to be re-wired!

If you make a totally sealed (= submersible) wiring harness, with one size over on all of the wires, (#12 is fine, except on the SSB & refer, which get #8) then it will NEVER be a problem. If something craps out, and the batteries are OK, then it is the unit on the bitter end.

Your boat, like mine, is VERY weight sensitive and the spaces are tight. Energy consumptive monohull solutions do not apply here... For this reason I suggest you make it all "just right"... as well as go small, energy efficient, & simple on every aspect of your boat's systems, OR do without, as the designers originally intended. Either one will work.

Btw... If you subscribe to "Conversations With Jim Brown" Available @ OutRig Media, I talk at length about this subject with Jim.

Best of luck with it,
Mark

[silviris]

Thanks, Mark! Lot's of great information. The info about filtering out your posts will make sifting through it all much easier.

[Mark Johnson]

Originally Posted by silviris

Thanks, Mark! Lot's of great information. The info about filtering out your posts will make sifting through it all much easier.

Glad to be of service! You just click on MARK JOHNSON next to any of my posts, then from the menu that pops up, select, "Past Posts by Mark Johnson". You can then scroll down the titles to the pertinent subjects.

Congratulations btw... you will be the proud owner of one of the best boats ever designed, in my admittedly biased opinion.

Regards,
Mark

[sailorchic34]

Originally Posted by s/v Jedi

For silicone panels, the voltage drop is about 2.2mV per degree Celcius. So, if you go from 28 degrees to 48 degrees which is 20 degree difference, the voltage goes down 20x2.2=44mV which is 0.044V. In F: from 82.4 to 118.4F = a difference of 36F. So it is 0.044V for a 36F. When it would have been 1V per 20F which is 0.05V/degree with 0.6V at 68F then at 80F the output voltage would be 0V. So for 20 degrees F, it is about 0.025V drop. That's 40 times as good as you feared

The output of 36-cell 12V nominal panels is 21.6V (36*0.6). 17V specs are for panels under load, at their typical highest power output. For our rough math on power needed and generated, we can just forget about the temperature coefficient, which makes it easier for us :thumbs:

Here we go again. That's .44mv PER CELL not total for 36 or 72 cells. The cells are in series for 36 cells. So the .44 mv is subtracted for each cell in series. So its about 1.5V drop at 20 degrees C (.44mv x36).

Yes the open voltage is 21 volts but that's not usable as no current is flowing..

[silviris]

Any recommendations on what kind of fridge to get comercially? I'm not confident enough in my very limited craftsmanship to build one myself. Also, most of the 12v stuff I've ever seen was meant to work off of those cigarette lighter plugs, or you could use a plug in inverter. How does this work on boats? I don't imagine you install a bunch of cigarette lighters all around :P

[Mark Johnson]

Originally Posted by silviris

Any recommendations on what kind of fridge to get comercially? I'm not confident enough in my very limited craftsmanship to build one myself. Also, most of the 12v stuff I've ever seen was meant to work off of those cigarette lighter plugs, or you could use a plug in inverter. How does this work on boats? I don't imagine you install a bunch of cigarette lighters all around :P

A PLUG IN INVERTER IS FOR VERY SMALL LOADS, NOT A REFRIGERATOR. You can, however, get a 12V DC cooler...

I do in fact have about 10 of these 12V cigarette lighter plugs mounted all about the boat. I use them to plug in the TV, Laptop, etc. using factory available "step up" car adapters... Most of these devices actually run on from 17 to 19V DC, (some even 12V), and you can get this directly from the boat's DC system by using these adapters. This avoids the need for an inverter in most cases. (I still use one to re-charge my drill/driver batteries). I also use these 12V outlets for fans on a portable base, NiCad chargers for the hand held VHF, etc.

The 12V outlets are plastic, and not meant for cigarette lighters, or any heavy load. IF you want to go with a commercially available DC compressor cooler, (the portable type), than I would get a special made outlet made for a larger load. There are many available.

Bear in mind... This type of cooler will change your power requirements upward, a lot! Even though small, they are barely insulated... (Like 2" thick).

IF your planned use for the boat is daysailing & weekending, and you have limited boatbuilding skills, then my previous posts to you may be overkill. It is highly skilled stuff, and would cost a LOT to hire it done.

Perhaps your best bet is to mount one large solar panel on a rack, (110 - 130W), and use a pair of larger Trojan L-16S. (These two 6V batteries are 360 Ah rated, and when you hook them together in series, you get the same 360 Ah, but @ 12V, rather than 6V). They have twice the cycle lifespan of even their own brand of large deep cycle 12V batteries!

You could always start small & add panels later, as needed. This part way scenario is for more limited weekend use, assuming you set out with full batteries from the dockside charger, and also that you will return with the batteries a max of 1/2 way down. The solar panel you have will just cut in half the rate of decline. Then the batteries can re-charge over the week.

Of coarse, the 10+ year lifespan of the batteries may now be half that, due to deeper cycling.

Remember, the refrigerator is MORE than half of the boat's load. The portable type are particularly consumptive too. What about a good cooler and ice? This is more hassle to live with, but getting rid of that refrigerator load, may make just one or two solar panels work fine, without maxing out the batteries.

See... on Searunners, we put our built in refrigerators, or ice boxes, below the footwell sole in the sterncastle. It is below the WL, and the coolest part of the boat. This is partly why a Searunner refrigerator can be so efficient! Building a good one here, is a LOT of work, and takes boatbuilding skills.

A portable cooler will not work here, electric OR otherwise. Wrong shape and won't fit. Also... the compressor's hot exhaust is trapped under there, defeating the purpose.

Things to think about:
IF you don't opt for the full enchilada, solar power wise... You could go about half way there with the solar, and lower your expectations to match your available power.

You could then omit the Refrigerator and use ice, OR use the portable 12V ice box (like a Norcold), perhaps in that rear bunk area. Then accept a 2 or 3 day cruise as the max time before your batteries go too low.

It's not either/or... Mix N match will work too. I will say, that IF you want to go full solar some day, you figure your average daily need, and install enough panels to supply TWICE that need. This way, they work well enough even on cloudy days, to bring you up to 100% before sunset. With our system, we are usually to full/100%, through ALL 3 stages, by 11:00 AM! On VERY overcast days, it may take all day, but we still get there.

Hope this helps,
Mark

[silviris]

I have a lot of variables that make planning things out really difficult. I'm getting a severance pay from the military for a medical discharge, which is what is allowing me to buy the boat in the first place. The purchase price of the boat is going to clean out my planned budget and more, but I really like the boat and can't pass it up. I'm having to make some compromises in other areas to make the overall budget work out.

The boat still needs to be re-painted and have lighting and a better toilet installed...and that's in addition to the over-my-budget purchase price. I also want to get that hard cockpit cover worked out before I spend any money on something to make the boat cruise longer.....a family history of melanoma compels me to make it a priority! Upon further reflection, it's probably going to be a while before I even start looking at any kind of solar power...

There's also the fact that since I'm leaving the military and starting school soon, I'm starting all over career-wise and probably won't have much time for anything longer than a few days at a time anyway for the next dozen or so years. I would love to be able to just cruise for months at a time exploring the world, but I don't know of too many jobs that allow you to do that and still get paid. And it's going to be a long time before I can save enough to do it "retired"

Even if I did have the income or savings, it's going to be at least 16 years until the kids are out of the house, so I'm stuck either way :P

[s/v Jedi]

Originally Posted by sailorchic34

Here we go again. That's .44mv PER CELL not total for 36 or 72 cells. The cells are in series for 36 cells. So the .44 mv is subtracted for each cell in series. So its about 1.5V drop at 20 degrees C (.44mv x36).

Yes the open voltage is 21 volts but that's not usable as no current is flowing..

Yes, per cell. Just like the 0.6V is per cell. We were discussing cell voltage and temp effect on that.

[sailorchic34]

Originally Posted by s/v Jedi

Yes, per cell. Just like the 0.6V is per cell. We were discussing cell voltage and temp effect on that.

Oh sorry My original remark about roughly 1V drop per 20 degrees F rise, well I was talking about per panel, not per cell. Easier to work with voltage drop per panel per 20 degree rise, as that's whats really important. Though I can see where you could take it per cell. Sorry I changed gears in the middle back there. It must be the blonde RPN math I use..

So in any event the .6V cell voltage will drop by roughly 0.044v per 20 degrees C rise. On the plus side as the temperature drops below 68 degrees the voltage will rise. Which helps a bit in winter.

So when your panel temperature is oh say 128 degrees F, there is roughly a 3V drop across each panel. So when the panel would be putting out 17V at 68 degrees it would only be putting out ~14V at peak current at 128 degree F. That's an 18 percent drop in panel or system output.

Lucky the sun shines more in the summer.

[sailorboy1]

In the real world should we be concerned about the temperature/voltage relationship that we can not do anything about .................I think not!

[sailorchic34]

Originally Posted by Don L

In the real world should we be concerned about the temperature/voltage relationship that we can not do anything about .................I think not!

Well its nice to understand the relationship, so that when your panel output drops by 20% or so in the summer, you know what happening.

[s/v Jedi]

Originally Posted by sailorchic34

Well its nice to understand the relationship, so that when your panel output drops by 20% or so in the summer, you know what happening.

Okay. Does anybody claim that panel output drops in the summer, due to temperature or any other seasonal factor?

[sailorchic34]

Originally Posted by s/v Jedi

Okay. Does anybody claim that panel output drops in the summer, due to temperature or any other seasonal factor?

Well I do. Its part of the properties of the solar cell after all. That temperature coeffenciency thingy

See link
Temperature - UQ Solar Photovoltaic Data - The University of Queensland, Australia

and
http://dasp.mem.odu.edu:8080/~solar_...%20of%20PV.pdf

Though because there are more hours of daylight the efficiency loss due to temperature is not as bad as it might otherwise be.