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Old 25-08-2008, 08:59   #1
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Point of diminishing returns on Solar?

I'm trying to sort out how much solar I want to add to the boat, and I'm wondering if there is a point of diminishing returns where adding more solar doesn't improve output much. The reason I'm thinking this is because I know that during the day the battery voltage will be coming up due to the solar output, and as the voltage comes up the panels output will drop.

So if I'm pulling out 100 amp hours a day, then a 300 watt solar setup would theoretically make me break even. But I'm wondering if by 2pm the batteries would be topped up, and the solar panels would be putting out less than their rated output? If this is true, does it make financial sense to keep the solar bank at about 80% of what I'm pulling out, so that they are always putting out closer to their capacity? Space and initial capital outlay are a concern, which is why I don't want to add panels that will be less than fully effective. I don't mind running the engine every 4 or 5 days, I just don't want to run it every other day.

So does running an amp-hour deficit everyday make more sense than trying to add more solar to break even? Is there a point of diminishing returns? Am I over-thinking this?
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Old 25-08-2008, 09:16   #2
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IMO, there is no such thing as over-doing it.

Regardless of your planned usage, you may find cloudy weeks or weeks with fog, or times when you need to do a lot of work with power tools or stay online a lot.

The extra panels, although they may not contribute their $$ share each day, you will be happy to have the extra power.


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Originally Posted by Fishspearit View Post
I'm trying to sort out how much solar I want to add to the boat, and I'm wondering if there is a point of diminishing returns where adding more solar doesn't improve output much. The reason I'm thinking this is because I know that during the day the battery voltage will be coming up due to the solar output, and as the voltage comes up the panels output will drop.

So if I'm pulling out 100 amp hours a day, then a 300 watt solar setup would theoretically make me break even. But I'm wondering if by 2pm the batteries would be topped up, and the solar panels would be putting out less than their rated output? If this is true, does it make financial sense to keep the solar bank at about 80% of what I'm pulling out, so that they are always putting out closer to their capacity? Space and initial capital outlay are a concern, which is why I don't want to add panels that will be less than fully effective. I don't mind running the engine every 4 or 5 days, I just don't want to run it every other day.

So does running an amp-hour deficit everyday make more sense than trying to add more solar to break even? Is there a point of diminishing returns? Am I over-thinking this?
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Old 25-08-2008, 18:31   #3
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I would suggest 4x daily amp hours required in wattage along with an Mppt controller gives you best bang for the buck and will allow you to average your daily needs easily over time. Battery bank should be at least 4x daily needs or more to allow at least 2 days of no sunshine without the need to run the engine or get good output from the panels.
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Old 25-08-2008, 18:37   #4
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That is just a great problem to have! You are generating more free energy than you need!

If anything, try to have just over what you need.
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Old 26-08-2008, 00:36   #5
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Technically it is not free energy. There is initial cost and then there is replacement cost when they eventually breakdown.

So balancing the number is the right thing to do. It also depends on your alternative charging methods if any and your storage capacity. I think there is an optimum to be had and once you add all the bits (solar, wind, genset, alternator), understand their life cycle costs, understand how much power you need over a 5 year period, how frequently you can tolerate using alternate generation methods - genset, engine, wind - and blend all that into a cost milkshake.

You may have enough solar to never fire up the genset, but you insist on having the genset as backup. Eventually it simply rots away and so there is still a life cycle cost for the genset.
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Old 26-08-2008, 02:52   #6
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In theory the limiting factor is the rate at which the battery bank can accept a charge. This depends on size, number, and type of batteries that make up the bank. Assuming that there is enough space to mount the necessary solar panels - then an output of 1-1/4 times the maximum acceptance rate of the battery bank for the solar array seems reasonable. Above this amount there would seem to be little advantage to adding more capability.

However, in the real world, i suspect that the limiting factors are really the cost and the available space to mount the solar panels.

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Old 26-08-2008, 06:42   #7
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I have 4 golf cart batteries of undetermined age, but they seem to be in good shape. I have room to add 2 more, but don't want to add 2 new batteries to an older bank, I was thinking I'd try to get by with just the 4 until time to replace all.
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Assuming that there is enough space to mount the necessary solar panels - then an output of 1-1/4 times the maximum acceptance rate of the battery bank for the solar array seems reasonable.
I'm not sure what the maximum acceptance rate is for a 440 Amp-hour bank. The only realistic place I have for panels is on the davits, unless I put the lightweight flexible ones on the bimini, but they don't seem to put out much for the cost. I thought about a wind generator, but I couldn't see a way of mounting it on the back of the boat without shading the panels on the davits. So my only backup charging method is a high-output balmar alternator.
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I would suggest 4x daily amp hours required in wattage along with an Mppt controller gives you best bang for the buck
I don't think I'll be able to put 400W back there, so it sounds like I should just put what I can fit.

Another question, whenever I read about the mppt controllers they always talk about putting the panels in series and then the mppt matches the voltage for optimal charging. Does the mppt still serve it's purpose with only one panel, if for example I went with one 200W panel?
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Old 26-08-2008, 06:53   #8
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Don't worry about having too much solar--your wife will easily adapt her electrical usage to consume any extra power!

MPPT controllers are also designed to work with single panels or panels wired in parallel--the series wiring does cut down the power losses in the wiring, but its not that significant.

We have about 300W, and it provides enough energy to run lights and fridge on most days. Half our panels are adjustable, so we can get more output.
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Old 26-08-2008, 07:00   #9
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A single 200 watt panel would be pretty large at about 5 or 6 feet by 3 feet. Do you have a spot on a Westerly that can mount a panel this size without interfering with the normal stuff we all carry?
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Old 26-08-2008, 07:45   #10
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A single 200 watt panel would be pretty large at about 5 or 6 feet by 3 feet. Do you have a spot on a Westerly that can mount a panel this size without interfering with the normal stuff we all carry?
The only place I have to mount panels is on top of the davits. I had to mount the davits pretty wide on the stern and angle them in a little. So my davit arms are 70" apart. About 40" out. So yes, I'm pretty sure I can mount a 6' x 3' panel, and it might even be easier than trying to mount two 4' x 2.5' panels.
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Old 26-08-2008, 09:18   #11
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However, in the real world, i suspect that the limiting factors are really the cost and the available space to mount the solar panels.


The limiting factor is actually rainy/cloudy weeks in real life. More is definitely better.
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Old 26-08-2008, 15:04   #12
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I have two 135 watt panels mounted on the davits. These panels were selected to have a maximum power-point voltage (23 volts) just below the maximum input of the MPPT controller. These are 18 volt panels. The MPPT controller was selected to handle the 270 watts available from the solar panels. It turns out that each of these panels are actually two panels wired in parallel. Each of the panels has its own set of wires running to the controller where they are conected in parallel. This setup does not come anywhere near matching the maximum acceptance current of my battery bank. However, over a normal day it wil recharge the bank.

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Old 26-08-2008, 17:18   #13
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So does running an amp-hour deficit everyday make more sense than trying to add more solar to break even? Is there a point of diminishing returns? Am I over-thinking this?
I'm not the sharpest pencil in the box, but I'm not sure how you're casting the question. Are you looking for an answer in terms of dollars, amps, efficiency (amps/$) or lifecycle cost?
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Old 26-08-2008, 19:31   #14
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Fishspearit, the rule of thumb (which you'll find detailed here and there all over the web) is that solar panels will put out their maximum rated power for a short time around mid-day, and that the entire daily output will be equal to some 4-6 hours of the max output. So for a 100-W rated panel, you can expect some 500 watt-hours of power during a fully sunny day.

Without knowing the battery state, you can roughly convert that into amp-hours by dividing by 14.4 (nominal charging voltage) and that comes out to around 35AH per day from each 100W rated panel. With an MPPT controller you'll gain maybe 5-15% more power, again depending on things like charge state of the batteries, but the 35AH would still be a rule of thumb to design by, with a maximum charing RATE of some 7amps coming from each 100W panel.


Your 400AH wet battery set should be able to charge at roughly 1/5th "C" rate, that would be an 80A charging rate. That's again the typical rate for wet cells, and again MPPT controllers can actually beat that a little, and your charge state, etc. will affect reality too. But for a rule of thumb--your batteries can accept an 80A charge rate. If you plan to go to 600AH, that would match a 120A charge rate. And if you change to AGM batteries rated at 600AH, they can accept charging at 1/4th "C", which would be 150A rate.

So, with each 100W panel giving you a maximum of 7 amps, your existing battery bank could absorb the power from about 1200W in solar panels, more or less. And if you went to six AGMs, you could make use of some 2200W in panels.

That's all figuring sunny days at noontime, and a fat wallet. Obviously when the sunlight is dimmer, you could use even MORE panels because they won't be running at full output. And the MPPT controller is absolutely wizard at making charging more efficient--no matter which way you go.

Given the size and price of panels, "as much as you can afford" probably becomes the real limiting factor.[g]
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Old 26-08-2008, 21:16   #15
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From what I read if the panels are in series (assuming you get more than one panel) and a small amount of shade hits one portion then all your output drops to "very little".
If in parallel one bit of shade only kills that one panels output.

Don't know if you need to consider it or not though.
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