Solar Panel demands - Page 2 - SailNet Community
Old 02-22-2012
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Basically it looks that no one knows the answer! Typical "do an audit" response, which of course is worth doing. But you would think there are people here on SN that have similar boats with solar and could just answer the answer. Audits and theory are fine, but real life experience would sure be better.

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so if I employ 4 kyocera panels above my bimini each 135KW how do you back into what that would supply to the batteries for power on a daily basis. I would guess you would apply the amount of KW in the panels against your most demanding 24 hour period of need. I am trying to measure what the boat can handle conservatively with panels vs. worst and then average amp hour needs.
Old 02-22-2012
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Quote:
Originally Posted by jkzdad View Post
so if I employ 4 kyocera panels above my bimini each 135KW how do you back into what that would supply to the batteries for power on a daily basis.
The typical math is wattage times 5 hours up north or 6 hours if you're in a sunny part of the world.

So 4 x 135w x 6 hours = 3240 watts of power gained over the day. At 12v that's 270 amps of power.

Now having said that it gets a lot more complex. Batteries don't charge in a "1 amp in = 1 amp out" nice flat curve. The physics behind battery charging is complex. Also where you have the panels mounted can impact things a lot. The tinyest bit of shade can drastically reduce output.

So no one can really say "you need to install X" because there's too many variables that are completely custom to every boat. The total battery bank available, how much energy you use, where you mount the panels, will they tilt, the type of charger used, etc etc.

You're going to have to track down all the energy usage you'll need for your boat. Figure out all the wattage ratings, think about how many hours you'll be using each item, and based on that you'll know your daily budget. Then you can decide on how much battery bank you'll need for all of that. If you use 100 amps in a day, you might want a 400 amp house bank.

Then you can get an idea of how much solar wattage you'll want along with some idea of what you'll do if the sun doesn't shine. Maybe you use a larger house bank and put more panels up. Maybe you'll put up a wind generator instead or maybe you'll buy a genset. Or maybe you'll just shut down the fridge on cloudy days instead or otherwise try to use less power.

All of those decisions are a personal thing because some people hate wind gennies, some people don't like to run a gas genny, while other people love having that 2kw Honda they can fire up and have as much power on tap as they need a couple times a week.
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Old 02-22-2012
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I performed an assessment of my wattage requirements by merely connecting an amp-meter between the positive post of my primary battery and the positive battery cable. Then it's just a matter of turning on the equipment you would normally be using. No guesswork involved. I then purchased the solar system I needed to keep that battery charged with the typical current draw. The most difficult part of the job was determining how to mount the panels so they would be out of the way, easily adjustable and open to direct sunlight through most of the daylight hours.

Good Luck,

Gary
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Old 02-22-2012
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jkz, there are two ways to look at it.

The "proper" way is to do your energy budget. Look at every label plate on every device on your boat, check the wattage on every light bulb, figure out how many hours you use the DVD player and espresso machine, total up the actual number of watts you consume every 24 hours.

Then you divide that by four or five or six depending on how conservative you are, to get the number of watts of solar panel you'll need to provide that much power on any given sunny day. (A solar panel will not produce the "rated" wattage all day, but soething more like 4-6 hours worth of power at that rating, from the entire day's production.)

So if you burn 1000 watt-hours in 24 hours, a 250-watt panel would roughly feed that back into your batteries. You'd want somewhat more to account for cloudy days, etc.

But what usually happens is that you will find out you don't have good places to mount panels (unshaded, out of the way, won't be hit or stepped on) or the panels cost more than your budget.In which case you can save the math by simply buying as much panel as you can reasonably afford and fit, very few people ever say they bought too much panel.

And consider an MPPT controller for them all, you can find plenty of discussion on them but basically an MPPT controller squeezes 20-30% more power out of the panels and into the battery than any other type.
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Old 02-22-2012
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Quote:
Originally Posted by jkzdad View Post
so if I employ 4 kyocera panels above my bimini each 135KW how do you back into what that would supply to the batteries for power on a daily basis. I would guess you would apply the amount of KW in the panels against your most demanding 24 hour period of need. I am trying to measure what the boat can handle conservatively with panels vs. worst and then average amp hour needs.
That would give you 135W x 4 = 540W. Figure about 5 hours/day x 540W for fixed panels (probably less when you're actually sailing, since the boat is swinging around), and you get 2700 Wh(watthours). If you divide that by 15 (remember, charging voltages are higher than 12V; and you have to account for inefficiencies in the charge controller -- even an MPPT controller ain't perfect), you get 180Ah/day. I would round that down to 150 Ah/day, since there are all sorts of other inefficiencies involved in such a system, and that gives you a pretty good estimate of the maximum charging capability of your panels. If you are going to go with that sized PV system, I would try to keep your daily usage somewhat below that (maybe 25%, or 112.5 Ah) to give you enough charging capacity to bring you batteries back up after a few cloudy days in a row. Your battery bank should be large enough to last at least 4 or 5 days w/o any charging (unless you want to run the engine, or have a wind turbine), so you'll have to have four or five pairs of golfcart batteries (or the equivalent capacity). If you want to go with your original 500 Ah estimate (which I still think is way too high) you need to scale everything (panels, batteries, controller, et cetera) up by a factor of four or five.

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Old 02-22-2012
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Quote:
Originally Posted by hellosailor View Post
But what usually happens is that you will find out you don't have good places to mount panels
+1 to this. It's the finding a good place to mount them that's the real PITA.

There's never enough room for all the toys you want to mount.
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Old 02-22-2012
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That seems very high

Quote:
Originally Posted by jkzdad View Post
With my 46 Beneteau I am guessing most demand on electrical system is a full day of sailing using auto pilot, fridge, nav system, running lights etc. coud produce a 24 hour day that could potentially require 500+ amp hours. Does that audit sound way off?
Not sure how you could be using that many amp-hours. What does your auto pilot draw? If it averaged 6 amps X 24 hours, that is 144 and I can't imagine it would use 6 amps. Fridge should be no more than 150. So if take those two at 300 a/h there is a lot more to be found.

Changing your nav lights to LEDs is well worth it, especially if you are using lights (3) at the deck rather than a trilight. Even with a trilight you save much a lot. We also changed our anchor light to an LED since that is getting used so much.

After the refit we have decided to sell Ainia. We want something smaller that would be could for the light summer winds of Lake Ontario, although we plan to spend at least a couple of winters in the Caribbean before heading north.
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Thanks for the good ideas regarding the led lighting. Another big amp draw while cruising would be the nav system usage. I am Trying to plan for the days with the most energy draws. I think the need for some energy austerity on these big draw days is a big part of the answer. Thanks again

Bill
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Old 02-23-2012
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Quote:
Originally Posted by jkzdad View Post
Thanks for the good ideas regarding the led lighting. Another big amp draw while cruising would be the nav system usage. I am Trying to plan for the days with the most energy draws. I think the need for some energy austerity on these big draw days is a big part of the answer. Thanks again

Bill
Wavelength
Does your boat currently have an amp meter on the panel (would show total amp boat draw)? If not, you should add one- gives a good indication of power use- check west marine for a selection. Once your panels are added, you can add an amp meter to the panels to indicate how much amps the panels are producing at any instant. This information will give a good indication of your energy balance.
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