Sailboat Solar Panel Sizing

[Roy M]

silvirus, you can forget about the anchorage. The moorings are bad enough if you aren't in Americas Cup harbor. Any vessel, unattended, will find that there are folks cruising about helping themselves to gear that isn't glued or welded down. Solar panels are particularly prone to this type of disappearance.

As for your other issues, again, go to the Searunner thread. You can either reinvent the wheel, at considerable waste of time and money, or learn from those who have your exact model and size Searunner, and have all gone through the same process of decision making as you are embarking upon.

Congratulations on the marina slip. In San Diego you will quickly find that it will make your experience much more enjoyable. Not having to row out to the boat to do every single maintenance project, weekend sail, or simply show off the boat, will save you many hours of your life. Having the security of leaving the boat and knowing, when a big storm approaches, that someone nearby will be watching out for it is priceless. I learned this the hard way. I launched my Searunner in 1978 and could find no marina space that could handle a multihull. I anchored for a very short time, was boarded by local "pirates", and quickly doubled my efforts to find a permanent home. I have lived aboard for over 35 years, leaving only for summer cruises. Now, as I prepare to retire, I am looking forward to returning from cruises, at intervals, to the same end-tie at my yacht club, kind of like a condo. In the long run, it's cheaper and much more convenient than owning a piece of dirt. If cost is an issue, San Diego may not be the best locale for you, but then, you already know how nice it is, so......

[sailorchic34]

You need two 6 volt batteries wired in series to give you 12V at 242 amps. A single 6V battery can't supply 12V. Also be sure to use a MPPT controller with your 300 watt panels.

I would location the panels on the aft outer ama's away from the mast, boom and stays as much as possible.

[Mark Johnson]

Silviris,
Welcome to the clan. I too have a SR 34. If you want solar self sufficiency details, go look at my numerous past posts on the subject, or feel free contact me for specifics.

Here is the short version:
Just like with self sufficient homes, money spent on energy conservation will go about 3X further than money spent on energy production.

Go for low A. requirements on EVERYTHING you install, like:

Fans? Bora or Hella

Lights? LED or Alpenglo fluorescent

Anchor light? LEDs

Refrigeration? Air cooled DC electric, set for refrigeration only, and with a SUPER insulated box.

Inverter? Go with a small one for charging ni-cad batteries and such. Forget a microwave, they are EASY to live without. Also, run small electronics like phones, laptops, printers, etc. on 12V car adapters, rather than crank up that wasteful inverter.

It goes on and on... We have cruised all over, with 12 years as full time liveaboards. We remained 100% solar self sufficient, 98% of the time, and consume an average of 40 Ah/day. This is supplied with our 270Ws of solar panels, (total), and since we are normally charged up before noon, we still get there on totally cloudy days.

By charging through 3 steps, to "full", every day, our batteries last > 10 years, and the Link 10 monitor re-zeros daily. This keeps it spot on accurate!

It can all be done... Read back for the long version, it is extensive!

Best of luck,
Mark

[settingsun]

Some usage numbers from our experience: on our boat we use 120-180 amps a day with a DC fridge, two laptops, fans, movie watching on a TV with surround-sound and subwoofer. If we add a standalone freezer it bumps up to 180-250 a day... We turn the inverter on when we wake up and turn it off before bed...

If we don't use the freezer, we never need to plug in with our 555 watts of solar, even in mid-winter (sunny Mexico). If we turn on the freezer, we need to run the engine every 3-4 days for a few hours for supplemental charging.

We've been living in Mexico on solar exclusively since mid-October this year. For 10 weeks our alternator was dead, so we we had no option for supplemental charging, and then discovered that with no freezer we didn't need any, so we've kept it off...

I don't think you'll see better than the rated specs in San Diego. In general, you'll see worse than the rated specs because the sun is at an angle to the panels all day long. You might see the rated spec around June 21st at noon for an hour!! The rated specs assume the sun is perpendicular to the panel, which it never really is...

Some other numbers -- We lived in a trailer for 1 year on solar power doing the minimalist thing: few lights at night, just 1-2 hours of laptop and TV a day, no microwave... 130 watts was sufficient in spring, summer, fall to support that. The trailer had no navigation electronics, of course... In Dec. Jan. Feb. we had to switch to oil lamps at night, minimize TV and laptop use, and we still were very borderline all the time.

Since that first year (2007), we've had either 490 watts of solar (trailer) or 555 watts (sailboat) and we have plugged in extremely rarely... The boat has probably been plugged in for 5 months total since we moved aboard in January, 2010.

All together we've spent nearly 2000 nights with solar power on either a trailer or our sailboat since we started traveling full-time by RV and sailboat 6 years ago...

We have golf cart batteries on the trailer and the rating is for 2 in series -- ours are Trojans rated for 220 ah... it takes 2 in series to get that 220 ah. So four of them is 440 ah… They have worked just fine.

As for battery bank capacity, the batteries are just temporary storage, like a sink. What matters is how fast water flows into the sink from the faucet (solar panels) and how fast it drains out of the sink (your usage). A rule of thumb we've been told (and seems to work) is to match your solar watt capacity to your battery bank capacity -- about 500 watts and about 500 ah… or 200 / 200 or 800 / 800. Another rule of thumb - your overall daily ah usage should be about ¼ to ⅓ of your battery bank ah capacity.

So if you use about 150-200 ah a day, a 600 ah battery bank and a 600 watt solar array is a good balance.

We have a bunch of those round push button plastic LED lights in closets and dark nooks and crannies. Can't say they provide much light or that they last very well... but worth a shot!! Maybe if you get better quality ones... ours were cheapie $4 ones and the push button stopped working, the light was dim...

Good luck. We have loved living on solar for the past 6 years...

[silviris]

Thanks for all the great advice guys! Looking forward to starting with all of it!

zeehag: I figured as much about walking on the panels and wasn't really planning on it...just sort of hoping to conserve space.

roy: I don't have the boat yet, so won't be able to figure out slips until I get it. The guy I'm buying it from is retired navy, but he has the boat on a slip at Chula Vista Yacht Marina.....I may be able to transfer the slip. I will definitely be reading through that SeaRunner thread...i've come across a few of it's posts in my research but haven't read all the way through it yet...it's a lot of pages! I do worry about theft, and I wasn't aware that the moorings weren't watched. You'd think since you have to pay to park it there they'd have some sort of security there, but I guess that's asking too much. I wasn't considering the free anchorage.

marc: not quite a part of the clan just yet....waiting until I get my money to buy the boat in a couple of months. I will definitely be looking for low power options for things and my list was mostly "best case scenario" kind of stuff. I know I probably won't be able to do the microwave or fan (only big box fans make enough noise to drown out the ringing in my ears) so I'll probably just suffer through those inconveniences. Already planning on going LED on everything. I'll definitely need a lot more info on solar and batteries before I commit.

settingsun: all that gives me some hope that it'll all work out in the end, but reinforces my suspicion that I'll end up needing two 300 watt panels in the end. Then again, since this will be a primarily weekending boat, maybe that will be enough.

I'll be reading through the searunner thread for a while, but before i go:

Is everyone using "marine specific" panels, or does that matter with this application? I've seen 80 watt panels going for $500 plus dollars from some marine specific sites, and then I've seen 300 watt panels for $300 from "wholesaler" websites, and you can actually buy single panels that way. I know most everything on a boat needs to be "marine-proofed", but going with those 80 watt "marine" panels will push the price up into the thousands to get 300 watts.

[settingsun]

Silviris - We used regular solar panels from Kyocera. They survive blizzards on houses. We have three 24 volt 185 watt panels. Sure wish we'd gotten three 220's or 240's instead.

No need to go "marine." We also used outdoor rated cable not marine cable. It is not as flexible as that really lovely marine cable, but it was also a heck of a lot cheaper. We got it from our solar panel store, Northern Arizona Wind and Sun in Flagstaff Arizona. They ship worldwide and we've gotten everything from them. Their salesmen are very knowledgeable. My husband Mark did a gorgeous installation, encasing all the exposed wire in wire loom and running all the wires inside the tubing of our arch so no wiring is visible anywhere.

We stopped doing funky things to conserve power after that first year in our first trailer. We didn't want to feel like we were camping in a tent. So we switched to LED lightbulbs in some places, including anchor light and running lights, and bought two very nice LED reading lamps. But the rest of our lighting is the stock fluorescent lighting that came with our boat.

We use the microwave all the time. We have two inverters - both were factory installed at Hunter. One is a 600 watt pure sine wave and the other is a 2500 watt modified sine wave inverter/charger. We use the pure sine wave for everything, except the microwave.

We don't think about our power use at all. So if we leave a light on by accident for a few hours, no big deal. If we leave the inverter on overnight by accident, no big deal (yikes, it happens all the time!). We wanted our system to have that kind of flexibility and comfort -- not caring if we listened to cool tunes on the stereo full blast for 4 hours or had a quiet day with no music... that's why we're keeping the freezer off -- I'd rather not have to worry about whether I can fool around on the computer for another few hours or have to stop because the batteries can't handle it...

You will probably want the 600 watts not the 300 if you cruise for very long periods.

I've written a lot on my website about solar stuff, including four parts of a 6-part tutorial (that I haven't finished yet). In the tutorial I describe "weekending" systems versus "full-time" systems, and in essence you are right -- 300 watts is good for weekending and vacations. Especially since you will probably do some motoring in the course of the weekend or vacation... However, 500+ watts is necessary for full-time living, especially if you want to stay anchored in one spot for weeks or months at a time...

Marine solar installation -- as I just mentioned in another thread, there are two links on that page that are formal papers/explanations of the devastating effect of shade on solar panels... if you can't avoid shade in your installation (i.e., radome or wind generator is nearby and mounted higher), then get more total watts.

RV solar installation - this link describes our two solar installations on RVs... lots of great info that applies to boats too -- it's all the same.

Solar power tutorial - part I - You probably know all this already, but the parts III and IV may have info you find useful... I've written the rough draft of parts V and VI but haven't found the minutes to get them posted!!

[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!

[settingsun]

There may be technology that handles shading, but I haven't seen it. That doesn't mean it doesn't exist, though!!

To my knowledge (having observed it on my boat and trailer with 3-5 year old Kyocera and Mitsubishi panels), the standard solar panels available on the market all shut down when just a portion of one corner or a strip of one side is shaded. The links on my website are technical papers that explain why.

I believe this is a problem for all standard solar panels, as the scientific paper from the Physics Department at the University of Arizona that I link to is addressing the problem of banks of solar panels like you see in Nevada where there are acres of them in rows. Problem is when the sun is low, each row casts partial shade on the row behind, so the panels aren't working at full capacity. The point made in the paper is to spread the rows out a bit.

I think a bigger panel is going to be more of a problem because a 4"x4" square of shade will knock the panel's production down by half, and it is more likely that some part of it will get hit by shade. Our panels are all pretty big, but they are mounted high and away from everything except the mast and boom which you can't escape.

If you're going to go for ~600 watts, why not get three 210's? Our panels are 24 volt which allowed us to run smaller wiring between the panels and the charge controller (though you need fat wire from the controller to the batteries because that is at 12 volts).

[s/v Jedi]

Okay, let me try this from memory so don't hang me up on exact numbers:

If you have a panel with 72 cells and it is 12V nominal, then it has an open output voltage of 21.6V which is 36 cells times 0.6V. This means that there are two parallel strings of 36 cells each, so effectively two parallel panels in one panel.

The voltage of a cell is always 0.6V. The power output depends on it's surface area and it's power efficiency.

The best thing to do is check if you can fit the largest panel possible in the space you have for it and use a high voltage panel. Then connect that to a suitable MPPT controller. Genasun is popular for single-panel. Outback is popular for bigger arrays. Most efficient is many big panels each with it' own MPPT controller. This is also the most expensive solution. Almost everybody ends up with a compromise.

[sailorchic34]

Originally Posted by s/v Jedi

The voltage of a cell is always 0.6V.

Actually the voltage is temperature depended and is ~0.6V at standard temperature of 20 degrees C or 68 degrees F. Above that it drops as the temperature rises. Very roughly it drops 1V for 20 degrees F, more or less depending on type of cell. This is why a 12V panel puts out 17 volts, to allow for the voltage drop when the cells are baking in the sun

On my little boat, I like and installed two smaller panels, one off the back and one off the side. With two it means that most of the time at least one panel is in full sun. Because I can pivot the panels in one plane and alsomove the boom out the other side using the vang as preventer to hold the boom away from the boat (and panels), I can get both panels in full sun for a great part of a day.

Really using large panels or small panels will work it just depends on how you do it. Though I think having the panels located as far from the mast as possible is a great idea

All the panels are pretty much made exactly the same way so why spend more for "Marine" panels.

[Irish rambler]

Just to add a little grist to the mill, I have solar panels and they work great. The 'human temptation' factor comes later once they're installed and working well as you tend to use more electric as you add more comfort to the boat. My personal advice is to fit the biggest practical array you can with a good capacity battery bank and an Adverc alternator controller. Why be uncomfortable ? We're here for a good life, not an uncomfortable one.

[downunder]

Originally Posted by Irish rambler

Just to add a little grist to the mill, I have solar panels and they work great. The 'human temptation' factor comes later once they're installed and working well as you tend to use more electric as you add more comfort to the boat. My personal advice is to fit the biggest practical array you can with a good capacity battery bank and an Adverc alternator controller. Why be uncomfortable ? We're here for a good life, not an uncomfortable one.

Hey!! A lot enjoy camping on board.

[s/v Jedi]

Originally Posted by sailorchic34

Actually the voltage is temperature depended and is ~0.6V at standard temperature of 20 degrees C or 68 degrees F. Above that it drops as the temperature rises. Very roughly it drops 1V for 20 degrees F, more or less depending on type of cell. This is why a 12V panel puts out 17 volts, to allow for the voltage drop when the cells are baking in the sun

On my little boat, I like and installed two smaller panels, one off the back and one off the side. With two it means that most of the time at least one panel is in full sun. Because I can pivot the panels in one plane and alsomove the boom out the other side using the vang as preventer to hold the boom away from the boat (and panels), I can get both panels in full sun for a great part of a day.

Really using large panels or small panels will work it just depends on how you do it. Though I think having the panels located as far from the mast as possible is a great idea

All the panels are pretty much made exactly the same way so why spend more for "Marine" panels.

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:

[noelex 77]

Originally Posted by s/v Jedi

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:

The temperature coefficients do have a practical impact on solar panel performance.
STC conditions are quote for a temperature of 25 degrees Celsius. That sounds reasonable untill you realise they are talking about cell temperature not air temperature.
A cell temperature of 45C is more commonly found near peak production, even with good ventilation under the panels (which is often not the case in yacht installations)

This will tpically reduce the Vmp of a 36 cell panel by more than 1.8v from the published specifications. This is about a 10% loss of power (if using a MPPT controller).

[ErikFinn]

Originally Posted by silviris

My last question is to Erik, with regard to your solar panel bimini. Are the panels sturdy enough to walk on? The searunner has the cockpit in the center of the boat and I am planning on making a plywood cover sturdy enough to walk on so it will make messing around with the mainsail a bit easier. If I end up having to do two panels, I may just use them as the bimini cover if they're safe to walk on. Then again, I imagine the panels get pretty hot.....

Well to be honest I don't know if they are sturdy enough to walk on, but I would not try that in any case.
For a center cockpit design I would imagine there would be way too much shading from the boom (and mainsail) if the bimini had solar panels? My panels are just aft of the boom so no issue there, only the backstay sometimes shades the panels a little (but not much here in the tropics).
Also there should be enough room for air circulation under the panels as they do get hot, and the hotter they get the less efficient they become.