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Old 03-12-2012, 05:25   #1
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Solar panels - Large or Small?

One of my main winter projects is to do solar panels. For the location I want to use a large panel at higher voltage fits best. But I'm sure I could use smaller panels and increase the number. I know I remember a thread about this but can't find it.

So for a given power rating is really a difference between using a large panel or multiple smaller panels? In my case the choices I'm considering are:

1 large panel, 290W, 24V, 77"x39.1" - $1.27/w, $367

2 smaller panels, 140W ea, 12V, 59.1"x26.3", $2.05/w, $570

Pro's for the larger panel:
- cost (shipping isn't an issue as I can go pick-up)
- smaller total footprint
- higher voltage

The only pro I can think of for using the smaller panels are:
-less weight, but this is only an item where installing and removing and the 20 lbs isn't that big a deal
- wired in parallel there is less of a shading issue for output

What am I not considering?
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Old 03-12-2012, 05:36   #2
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Re: solar panels - large or small?

I am about to do this very same thing very shortly. I havent had much time to research everything. From what I gather, when part of the panel is in shade, the entire panel shuts down. If this is correct, the multiple panels are the way to go - I think. With a sailboat, your boom can shut down a panel depending on the angle of the sun. I would think that if you had 4 panels, the boom might only shut down two. I have a crane on my aft cabin roof and this could shut down the panels so the more panels, the more likely that at least 1 panel at a time will be in a good position.....Maybe.
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Old 03-12-2012, 05:50   #3
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Re: solar panels - large or small?

How big is your battery bank 24 volts or 12 volts? I chose to use 12 volt
panel(s) so if the charge controller died I had the option to connect one
panel directly to my house bank which is 12 volt. Just an option I wanted
as I used the money, that a spare controller would cost, to buy another
panel.
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Old 03-12-2012, 05:59   #4
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Re: solar panels - large or small?

I wouldn't go with anything over 100 watts unless you have a large hard top. too hard to keep them secure. also if you have a book fall on A panel if you have large ones you're much more screwed than if say 1 out of 4 gets damaged.
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Old 03-12-2012, 06:09   #5
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Re: solar panels - large or small?

I have 12V battery bank and plan on a MPPT controller so the larger panel's voltage isn't a problem. I know an advantage of the larger voltage is the wiring size, but that is a small consideration in the total project.

Yes if I used the smaller 12V panels in theory I could get away from using a MPPT controller. This would make the project costs about the same if I substituted the standard controller. But the physical size of the panels would remain.

I understand that using the multiple smaller panels is going to be more reliable in theory, but for the most part from my reading the panels are pretty reliable (the same with the controllers).
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Old 03-12-2012, 06:27   #6
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Re: solar panels - large or small?

We went with two smaller panels partly for the shading effect that Tony mentioned - it's real, and significant, as I've seen with the Xantrex. The other advantage was that by mounting the two panels with a small gap between them, we could peek through the gap to check on the shape of the mainsail.
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Old 03-12-2012, 06:33   #7
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Re: Solar panels - Large or Small?

I would go for the larger panel for the reasons you have outlined.

Multiple smaller panels do work better in the shaded conditions that are inevitable on a boat, but the advantage is modest. The larger panel has an extra 10w and this will go a long way to making up the difference.

The panels themselves are very reliable and physical damage from a collision etc is the greatest problem. The controllers are less reliable, but the 24v panel can still be connected directly to the 12v batteries as long as you are careful with switching them off as the batteries become charged. Without the MPPT voltage conversion half the power will be lost, so it would only work like a 145w panel, but you would get some usable power.
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Old 03-12-2012, 06:45   #8
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Re: solar panels - large or small?

One or two panels will not make two much difference in power output.
One panel would make it and easyer install.
They make a 327 watt panels that may be perfect. Its light and small.

The pictures are from my friends Island Packet 35 with a frame built above the bimini and two SunPower 240 watt panels installed. He used a Blue Ski MPPT controller.
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Old 03-12-2012, 06:54   #9
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Re: Solar panels - Large or Small?

I'm using Motec 240's and the Midnite Solar MPPT. The controller is head,shoulders, and even a couple more inches over the others ie Morningstar, Xantrex, etc.
I can get Midnites for 625+ shipping. Can also get pv panels for around 1.12 a watt.
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Old 03-12-2012, 08:34   #10
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Re: Solar panels - Large or Small?

Large panels are expensive to ship because they have to go motor freight. But at least the OP said that is not an issue.

But with the smaller panels, which have a nominal voltage of about 17 volts, you can get by with a much cheaper, non MPPT controller. An MPPT controller at best adds about 10% to the amps going to the batteries. That factor just about equalizes the cost between the big and small panels for less than 300 watts. And if you have to pay shipping, then the cost favors the small panels.

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Old 03-12-2012, 10:39   #11
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Re: Solar panels - Large or Small?

I feel that by the time I factor in mounting hardware etc the change from the larger lower cost/watt panel to the smaller higher cost/watt panel would be about the same IF I didn't use a MPPT controller.

But the cost isn't my driving issue because in the long run saving $100-200 on the project isn't my goal. I would prefer to have a clean installation that fits into my space and that meets my needs.

Using the traveler arch on my boat there is a nice place to mount the panel. But I don't want to block the viewing window in the bimini because I want to able to see the mainsail still. And I don't want the width of the panels being so much that they stick way back aft of the boat.

I could mount 2 of the larger panels in the spot and be OK, so if I even decided I needed more that the 290W I could expand (the same MPPT controller I was looking at would still work) with another for 580W. But if I went with the smaller panels I couldn't do more than 3 panels for 420W.

So I'm really just considering/questioning whether there is a useful power advantage to the 2 smaller panels over the 1 larger panel.
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Old 03-12-2012, 10:51   #12
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Re: Solar panels - Large or Small?

Too much?
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Old 03-12-2012, 11:18   #13
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Re: Solar panels - Large or Small?

In the early panels, I think I am right in saying that a shadow across part would stop all output of the panel, but I thought I read somewhere that this was no longer so on modern panels, ie shadow reduces effectiveness but doesnt "kill" that panel. Is that so?
On my Sabre 38 I had two flexible ones on the Bimini, and a big one across the davitts, on my Little Harbor I have one on each modified guard rail aft, obviously, even with adjustable tilts, one is less effective than the other, but overall they run fridge and freezer, cold beer is assured in the Med!
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Old 03-12-2012, 11:39   #14
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Re: Solar panels - Large or Small?

If I remember right newer solar panels(depending on type), are much more forgiving about shade.

But, with one big panel, if that certain amount of shade covers the panel, the entire panel's performance is compromised.

With smaller panels, shade on one(even if completely shaded), will have no affect on the remaining panels, which will, each, be subject to the maximum amount of shading before they're left ineffective. A broken "large" panel becomes useless . . . a broken smaller panel also becomes useless, but you still have a couple more panels to break before you lose power.

I would say go with smaller panels and forget the extra 10 watts you might get with a single large panel.

I would also consider other smaller panels(or flexible panels), to be placed on other parts of the boats where they'd be safe from being stepped on.

I'm even considering very small panels(5 watts or so), to be dedicated to recharging d, c, AA, AAA and small(6-7ah), batteries for inside LED lighting.
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Old 03-12-2012, 13:04   #15
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Didn't I pay extra several years ago for a "shade tolerant" panel? As I recall it was a decision to be made when buying them. In the literature of the time, (2005). The shade tolerant type were built differently and more expensive. About a third more as I recall.
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