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post #1 of 16 Old 04-02-2015 Thread Starter
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solar panel questions

Hello,

I am thinking about getting a solar panel on my 26 ft sailboat to maintain the battery. I am going to be on it a few nights a week throughout the summer and for short voyages and daysails. Basically I just need to keep the cabin/running lights on, cell charged, maybe a laptop, and if i ever get the auto helm it came with back up and running. I guess my question is how many watt solar panel on an average summer day in NJ, would charge a 12v deep cycle battery from almost dead and in what time duration would it take. 100w would be too big for the boat i think. Is a 50w enough? Does anyone have a single panel, how long does it usually take to get a good battery charge up?

Thank You very much,

Jeff
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post #2 of 16 Old 04-03-2015
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Re: solar panel questions

you need to audit your needs, list the expected Ah you will use from each item on the boat then add a fudge factor call it 5%. this PVWatts Calculator will help you find out how much power you may get from the panels, other can give you better data then I but this should get you started. and welcome to SN

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post #3 of 16 Old 04-03-2015
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Re: solar panel questions

Actually a 100 watt panel would not be to big just rail mount on the push pit. If you don't have a refer then a hundred would be more than enough and if you start looking at complete kits with panel controller wires and all mounting hardware it will make sense to go for a 100 watt kit verses the slightly cheaper 40 watt kits ( never heard of any complaints of to much solar just not enough ) Complete Kit 100 w Watt 100W Photovoltaic PV Solar Panel 12V RV Boat Off Grid | eBay this is an example. We have the 200 watt kit on our 42 ft Spencer and are very happy with it

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post #4 of 16 Old 04-03-2015
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Re: solar panel questions

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Originally Posted by cappinstabbin View Post
how many watt solar panel on an average summer day ing NJ, would charge a 12v deep cycle battery from almost dead and in what time duration would it take.
Assuming a 100 ah battery depleted to 50%, on a sunny day a 100 watt panel would probable get it back to 80% charge. This seems like a good size for the requirements you listed.

If you get the autopilot working and use it a lot, or if you install refrigeration, you will probably want a second battery and a second 100w panel.


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post #5 of 16 Old 04-03-2015
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Re: solar panel questions

That deal on a 100 watt panel looks very good. You certainly dont need 100 watts but as the deal comes with a controller u shouldnt cook your batteries when off the boat.

Solar is really a dream come true for a boat owner... And the panels really do last 'forever' so you can take it with u onto the next boat, or home.

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post #6 of 16 Old 04-03-2015
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Re: solar panel questions

Do some research on charge controllers they are NOT all the same. Find the best controller for your application.
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post #7 of 16 Old 04-03-2015 Thread Starter
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Re: solar panel questions

Thank you all for your help. I was just curious to kind of get my head around the average output of these things, i think it would be helpful if anyone has an estimate from experience how long it would take to charge up a battery in ideal light with a 100w.
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post #8 of 16 Old 04-03-2015
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Re: solar panel questions

If you have LED lighting in your cabin, with all the lights on from dark until bed time, you'll probably not burn more than one amp. If they're 12v lights, lets say 3-4 amps. I'm not sure how much power your auto pilot uses, but for a 26ft boat with a tiller, I would bet maybe 2-3 amps. For wheel steering, a couple more. (That is a complete guess on my part) If you're jamming out on the radio all day during your sail, a 25w head unit (typical aftermarket) uses 2 amps.

Basically, you need to add up all your load (how many amps from each device you're using), multiply by how many hours that device will be operating, give yourself several amps of wiggle room, and that's your total amp hours needed for a day. For your stereo, 2amps times 8 hours = 16amps. You'll probably not use more than 4 amps all night for your lights if they bulbs are 12v. So that's 20amps. So a 100ah battery might come down to only 80 amps. Maybe 70 or so with an auto pilot.

The average 60w panel will probably produce a max of 3.5amps. When looking at panels, pay close attention to how many amps the panel will produce. This number (let's say 3.5amps) x 8 hours of sun a day will charge 28 amps. So if your total usage was 20-30 amps and your 60w panel produces 28 amps, then that's all you need. Any excess power produced will be released your charge controller in the form of heat. It is your charge controller's job to regulate the power your PV array (panels) deliver to the battery. It allows the battery to not be over charged.

One 50-60w panel, which shouldn't cost more than about $70-75, one charge controller (Maybe a 7amp controller... it is good for your controller to be able to handle twice the amount of power as your PV array puts out so it doesn't get over worked), and you're set. To allow for bad weather, just add another battery or two. Your reserve power (the amount of amp hours in your batteries) has more to do with your needs than any other part of the system. Even a very small solar panel can keep your batteries topped off for part timers and weekenders if the battery bank is large. With three 100ah batteries (300ah), if you had cloudy/rainy weather for four days straight and still used 25 amps a day (4 days x 25amps = 100 amps), you would not deplete your batteries below 66%. A good battery bank allows for bad weather. Even just adding one doubles the amount of power you can use before needing to be topped off. Of course, the batteries don't all have to be the same size. One 100ah battery and one 150ah battery obviously gives you 250ah.

In short, for your current needs, a 100w panel would be fine, but you really don't need one that big, even if you decide to add a fridge. A 50-60w panel, a 7 or 8 amp charge controller, and one more battery is all you should need I think. Most panels under 100w can be had for $1.30 a watt or less. Larger panels can be bought for under $1 a watt. Check out solarblvd.com... Good prices, lots of choices, educational articles... I love that site. You can always grab a calculator and do some simple math to see how your needs are changing. Watts = Volts x Amps. 36W = 12V x 3A. 3A = 36W / 12V, etc.

Good luck mate!
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post #9 of 16 Old 04-03-2015
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Re: solar panel questions

Oh, and as far as mounting locations, someone suggested mounting the panel on the pushpit. I agree, that would be the best place to mount them. If you're not keen on that idea and you struggle to find the appropriate space (I just looked at a 60w panel that was about 27" x 27" square, two 20-30w rectangular panels gives you the same power output. You can always use two smaller panels and get the same amount of power. You may find it easier to find a place for two 20"x14" panels, rather than a place for one 27" square panel. Just remember, if they're anywhere that could get foot traffic, even on the coach roof near the companionway, the possibility of damage goes way up. The stern rail is probably your safest bet, or on top of the bimini, if you have one.
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post #10 of 16 Old 04-03-2015
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Re: solar panel questions

cap-
I would rashly suggest that you just buy the biggest panel you can find a place to install, remembering they don't like to be in shadow.
Then you install it with a good MPPT controller, which typically gains you about 15% more power from the panel. You don't need MPPT, but since your panel will probably be "too small" it will help. Avoid unbranded junk from nameless vendors that often performs like nameless junk.

Once you've got that, try a Turnigy Watt Meter or other in-line device that can show totalized watt-hours you are getting from the panel. There's no math to do that way, for $30 it will tell you what you're getting on any particular day.

Then all you have to do is figure out "OK, how far can I stretch that?" which may mean more battery, or converting to LEDs, or turning the stereo in for an iPod instead.

Yeah, that's backwards from the conventional logic of "What am I using, what will I need..." but unless you've got an aircraft carrier, you usually run out of space and budget before you can install all the solar panel you can really use. So, max out the panel, then deal with the reality of the rest.
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