Determing Size of Solar Panel and Battery

System
System Posts: 2,511 admin
in Off Grid Solar & Battery Systems #1
I have a 28' sailboat that has no engine, so I'm planning to generate electricity using a solar panel and storing the electricity using a deep-cycle battery. I've been reading and researching and believe I'm close to figuring out a few things, but I'd appreciate help from those with experience and knowledge on this subject.

I've calculated my daily amp hours at 9.75. It's very low because I have LED cabin lights and LED navigation lights and not much else.

I will be sailing all over the world, primarily between 45 degrees north latitude and 45 degrees south latitude.

I'm thinking a 55 watt solar panel should be sufficient. It outputs 3.1 amps. So, if I take out 9.75 amps from the battery each day, this panel would be able to put back in about 15 amps in a day (provided I get about 5 hours of good sun). The panel will be mounted so that it can be angled to receive the sun's rays at 90 degrees.

I'm having more trouble sizing the battery. I know that it's important not to discharge the battery too deeply (i.e. more than about 75%), but that you shouldn't have really shallow discharges either (i.e. less than about 10%). Therefore, I think it's important to get the right size -- too large and it won't be discharged deeply enough and its life expectancy will be reduced; too small and either the discharges will be too deep or it won't have enough capacity at all.

I also realize I'll need a charge controller -- for my setup, do I need a three-phase charge controller, or would something like a Morningstar SunSaver 6 be okay?

Thanks in advance for any help, advice, and suggestions you can offer regarding the solar panel size, battery, and charge controller!

Jay
S/V Dove
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Comments

  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
    Re: Determing Size of Solar Panel and Battery

    Welcome to the forum, Jay.

    And congratulations on keeping your energy needs down; that's Step #1 accomplished! :D

    Step #2: Round that 9.75 Amp hours figure up a bit. Let's call it 10. I'm going to presume this is a 12 Volt system. So the most you want to discharge a deep cycle is 50%. On a boat, you probably want a little more margin for error because there isn't going to be any back-up generator, is there?

    Let's say 25% DOD. That would be 40 Amp hour 12 Volt battery. Minimum charge rate of 5% which is 2 Amps @ 14.4 Volts. You can probably get away with that as there most likely won't be loads running during daylight charging, since it's primarily for lights, right?

    Now about that panel. You're traveling mostly in good sun areas? The 45 degrees starts to eat in to the optimum sun angle, as you'll likely want the panels mounted flat instead of having to try to tilt them and point them at the sun all the time. This will result in a loss of output. High temps will drop it even more so. On a good day you might see 75% of the panels rating, or about 40 Watts true output. That will still be in the neighbourhood of 2 - 3 Amps charge current.

    A couple of suggestions: get an AGM battery, as it will be safer against accidental spills and off-gassing (you can have it in living quarters - no problem). You might need to upsize the panel a bit depending on what battery you can get. Universal makes a 35 Amp hour AGM and then ups to 50 Amp hour, for example. The Sunsaver 6 is probably a good choice, with the Low Voltage Disconnect; wire the lights to the LOAD terminals and they will shut down before the battery goes too low.

    Think carefully before buying anything. The above view is counting on a certainty of good sun, which may not always be the case. If you feel you need more battery capacity, you have to up the panels to maintain good charging.
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  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    #3
    Re: Determing Size of Solar Panel and Battery

    I'd consider at least 2 panels, on opposite sides of the boat, on in shade, one in sun. Ecah panel has its own charge controller that can handle marine weather.

    With no generator at all ? maybe consider one of the harbor freight $90 800W 2 cycle gensets and a battery charger, just in case. A line can break, and smash a panel, or a wave takes it out....

    I'd also have a 12 V stack of D batteries to run the radio / beacon in an emgerency, in case something bad happens to the rest of the system.

    Don't forget lines, stays, masts, and all the stuff that casts shadow, will disable a PV panel. Any shadow, on a small panel= no output.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #4
    Re: Determing Size of Solar Panel and Battery

    Assuming that your power measurements are correct (10 AH @ 12 volts sounds reasonable for a LED equipped navigation/cockpit lighting setup for smaller vessel)... I would still opt for more solar panels.

    First, it is a good start to estimate that your panels+charger+battery bank will only be ~50-60% efficient. So--doubling your power needs. And on a sail boat (orientation, shading, etc.)... I would use another 50% derating...

    So, would take your load, multiply it by 4x then divide by hours of sun per day you plan on using.

    -Bill

    PS: I have a few free minutes:
    • 9.75 AH * 12 volts * 1/0.25 overall derating * 1/3 hours of sun = 156 watts of panels
    So--I would suggest 2-3x your planned array at a minimum.
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #5
    Re: Determing Size of Solar Panel and Battery

    am i to understand that you don't need power for radios? if it were me given a choice between them i'd forget the lights as you'd never find me thousands of miles in the ocean with no motor and no radio. if you have to be out there without a motor i'd certainly have reliable power for contacting to get help if need be. that's nuts!:cry: are you worried that a ship might not see you and hit you in the middle of the ocean? am i missing something here?
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  • System2
    System2 Posts: 6,290 admin
    #6
    Re: Determing Size of Solar Panel and Battery
    niel wrote: »
    am i to understand that you don't need power for radios? if it were me given a choice between them i'd forget the lights as you'd never find me thousands of miles in the ocean with no motor and no radio. if you have to be out there without a motor i'd certainly have reliable power for contacting to get help if need be. that's nuts!:cry: are you worried that a ship might not see you and hit you in the middle of the ocean? am i missing something here?

    I'll have a 12-volt VHF radio, but it won't get much use -- only when entering port. Navigation lights at night are an international, legal requirement. I'll have an EPIRB should I have to "abandon ship," but that has its own battery.
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  • Photowhit
    Photowhit Solar Expert Posts: 6,003 ✭✭✭✭✭
    #7
    Re: Determing Size of Solar Panel and Battery

    While I have had a sailboat and planned a trip around FL, I figured a 12 volt 100 Amp Hour AGM and a 100 Watt panel which I could hang from which ever's side hand rail got the most exposure, Almost 50 degrees on my little "mud puppie." In reality I only changed it when ancored out for a couple days. Though I never took the Florida trip, My first skip was the longest of the trip, Shell point to Cedar Key and I left in Small craft warnings but the wind never swung around and I wanted to run into Stienhatchie but my motor blew craps and I spent the night in 6-8' seas. I cut loose in the morning (another reason to have a motor) as I couldn't get over my ancor which was well set at that point and sailed home. pretty short trip...

    Don't you have to have a motor at over 26'? I thought the coast guard required 2 modes of locomtion, oars were fine on my little 19' sail boat (though entirely silly) Also there are a lot of channels that are not practical to navigate with out a motor. Are you fixed keel or a swing keel (really like the twin keel mud puppie I had for sitting out in shallows) Though I had tried I never made it to my dad's dock with out a motor, to many wind shadows and swirls between the buildings.


    Are you not planning on checking the weather regularly? Listen to music, Ocean going w/o a fridge?

    "Fair winds and beware of the spring tide!"
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
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