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Old 11-01-2012, 18:37   #31
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re: Best Solar Panels For The Buck

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Originally Posted by senormechanico View Post
prroots,
Here is a 210 watt panel 58" long that might fill the bill for you:

Solar Cell, Solar Panel, Solar PV, Solar Products, Charge Controllers, Solar Trackers
Thanks. I asked about those and they're all out
Pete
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Old 11-01-2012, 18:40   #32
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re: Best Solar Panels For The Buck

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The most common rule of thumb I've seen for horizontal panels on a sailboat in "the tropics" is to divide total watts by three to get approximate Ah per day you can expect.
Here's how I've run the number - but this is just me....

- Take nameplate wattage and divide by the charge voltage of 14V. The result is the amperage.

- Multiply by 5 hrs of full-sun-equivalent exposure for a sunny day. This number varies by location, and there are maps that show the correct value for different locations. I think the NREL web site has the details.

- At this point you are just about at the divide-by-three that you mention for "tropics".

- The only time you'll get the nameplate rating out of panels will be with an MPPT charger on a bright, clear, very cold day. I see it in January and February in VT where my solar house it. In moderate to hot weather output drops. The same is true when/where you have shorter days, and obviously when the weather is other than perfect. This is where you have to apply some judgement and basically pull a number out of the hat. Up here I de-rate by 50%. If you are closer to the equator and have more consistently long days, or have much more reliable weather, then less derating is likely OK.

One thing to keep in mind - I've never heard someone say they have an excess of solar. To the contrary, almost every case I know of people would happily welcome more.
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Old 11-01-2012, 19:25   #33
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re: Best Solar Panels For The Buck

Seahunter, I'm sure you understand what you are posting but it comes across as gibberish. "A 56V panel at 14+ volt float is over 13 amps," Volts is amps? No, it never is. You are hopelessly confusing, or misstating, amps ands watts and volts and that makes your statements into gibberish.

You're also refering to half price--but up front the question was not one of price, but one comparing two panels, same price, same wattage, and only asking about voltage making one or the other better.

So which is it? Same price? Different price? Same wattage? Different wattage? Absent an MPPT controller or other special device, it doesn't matter how much excess voltage you have, that gets thrown away by conventional regulators.

Twisted-
I'm not being optimistic, I'm using the most common rule of thumb for general panel power output. And I said, rule of thumb. Yes, there are specific curves available to show the average numbers of daylight, the average numbers of overcast days, the actual amount of sunlight and its inensity at any given location on any given day. That's all variables and the assumption used to simplify that by pretty much all the government and university sources is that on the average, when you don't have numbers for a specific location or a specific time of year, a solar panel will put out in one entire day the equivalent of four hours at maximum output.

Now, is the rated utput a lie? Perhaps. I've seen some panel makers claim 2-3% higher efficiencies than any independent source claims are possible with their technology. And every panel loses some output with heat and age.

It still comes down to about two hours of actual full output, one on each side of the local noon (not the time zone noon), and then somewhat less every hour further away from noon, but averaging a total of four hours of maximum output, and assuming the panels are also either located flat or orientated toward the sun. Orienting them to the sun makes a significant difference, but also requires shifting them every hour during the day. Again, that's why there is a rule of thumb, and why some folks beat it while others barely attain it. Flat or fixed, "four hours" is commonly attainable. If you're only getting three hours worth of power, no doubt there's a reason for it. Last time I tested panels I got snookered by my own multimeter. Turned out the one I was using had simply gone out of calibration. When I grabbed a second meter, they pointed at each other and snickered while disagreeing by 2-3 10/ths of a volt.
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Old 11-01-2012, 23:35   #34
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re: Best Solar Panels For The Buck

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Yikes. Do it your way then. I do this all day long. A 56V panel at 14+ volt float is over 13 amps, something a 28V panel can't do. You're right they are different but the whole idea is VALUE. At half the price they are the best deal. There was no discussion about MPPT charge controllers, it was about the best value in panels.
What the heck?

We have a 56V panel and a 28V panel. They're both 200W. I have no idea why float voltage is mentioned since virtually no current will be accepted at float, so let's assume we're just about to kick from bulk mode to absorption mode. We're at high voltage and high current. Let's call it 14.2 V on the downstream side of the controller. Since neither one is a 12V nominal, and we're spending serious money to buy top of the line panels, we'll be using an MPPT controller, surely.

So, we'll say that it's local noon and the batteries can absorb all the current the panels can throw at them. We'll also assume no losses in the cables or in the controller (actual losses may be on the 5-10% level, perhaps).

So, with the 28V panel, the current upstream of the controller will be 200/28=7.1A. Downstream of the controller, going into the batteries will be 200/14.2=14A.

With the 56V panel, the current upstream of the controller will be 200/56=3.6A. Downstream of the controller, going into the batteries will be 200/14.2=14A.

So in a perfect world, the solar panel voltage doesn't matter at all! In reality there is lower loss in wires for higher voltages (important for long runs), and lower costs and efficiencies in higher voltage MPPT controllers, but nothing like the half you're talking about, more like a few percent.
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Old 12-01-2012, 04:21   #35
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re: Best Solar Panels For The Buck

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So in a perfect world, the solar panel voltage doesn't matter at all! In reality there is lower loss in wires for higher voltages (important for long runs), and lower costs and efficiencies in higher voltage MPPT controllers, but nothing like the half you're talking about, more like a few percent.
Also, when one gets around to sizing the controller, he may find that a more expensive controller is required for the higher Voltage panel. This will, in all likelihood, more than offset the additional wiring costs for the lower voltage panel assuming that one were increase the wire gauge to compensate for losses. This is what I discovered when comparing a nominal 12 Volt panel to one close to 18.
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Old 12-01-2012, 06:29   #36
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re: Best Solar Panels For The Buck

As for sizing, how much power output to expect, etc.....

If you want to use divide by 3 as a guideline, that's fine. I'm just trying to break it down further for anyone who's interested and/or cares. If nothing else, it can help people understand where the simpler rule of thumb is optimistic, and where it's pessimistic. Much also depends on how you plan to use the system, and what you expect it to do.

The 4-5 hrs of full sun per day is a fine assumption, and lets say that covers the daily power needs for the OP's boat. Now what happens when there's a rainy day and you get no useful output for the day, or you get two or three rainy days? If you are going to make up all your power deficit by running your generator or some other power source, then all is good.

But if you want your solar to make up the deficit, then they need to be larger. That's a big part of the weather-related de-rating. Whether this de-rating is big or small depends on whether you want your solar to make up the deficit, or you have other means.

As for panel rating, I'm not suggesting that panels are falsely rated. To the contrary, solar panels a very accurately rated with specific standards for doing so. However, those ratings are for a very specific set of operating conditions - bright, clear sunlight, 25 C, and operating at the panel's max power I and V. Then you have power loss in wiring and in your charge controller, and charge efficiency for your batteries. These things all add up.

We'll assume an MPPT charge controller which will operate the panel at it's max I-V power point. And we can even assume bright, clear sun. However a black panel sitting in the sun will heat up quickly - well above 25 C - and power output will drop. Most manufacturers publish the temp de-rating. Wiring is typically sized for 3% to 5% loss. And the DC-DC converter in the MPPT charger have some inefficiencies as well. Let's say that the combined loss is 10%. Last, batteries don't operate at perfect efficiency. You need to put in more than 100Ah in order to draw out 100Ah. 80-90% efficiency is typical (actually, most assume 80%).

So, between wire loss, DC-DC converter loss, and battery efficiency you are looking at 20% to 30% loss.

I can see how on boats, using a simple rule of thumb, even one that is optimistic, is OK. If the panels don't produce what you expect, most people won't notice, and the boat already has alternate sources of power to make up any deficit. The panels will still reduce gen run time which is all good.

However, in situations where you need to depend on solar for all but extreme weather situations (1-2 weeks of overcast skies), these calculations can be the difference between a system that works and one that doesn't.
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Old 12-01-2012, 17:55   #37
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re: Best Solar Panels For The Buck

My knowledge about solar installations increased dramatically after I read this article:
The RV Battery Charging Puzzle HandyBob's Blog
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Old 13-01-2012, 05:37   #38
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re: Best Solar Panels For The Buck

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My knowledge about solar installations increased dramatically after I read this article:
The RV Battery Charging Puzzle HandyBob's Blog
Good read. The moral of the story was don't skimp on wire size, put controller very close to batteries, and be sure that the charge controller voltages are set properly for full charge (eg, to 14.8 acceptance on golf cart batteries).
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Old 13-01-2012, 07:58   #39
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re: Best Solar Panels For The Buck

I'd be interested in recommendations for MPPT charge controllers. At the moment, I'm comparing Blue Sky SB3024iL with Morningstar TS-MPPT-45. The Morningstar has a standard RS232 interface to control and monitor operation via a PC. The Morningstar claims a 99% efficiency vs 97% claimed for the Blue Sky. I'm sure there are many more significant differences, but overall the Morningstar seems to have a greater range of Voltages and current with more flexibility. For example, the Morningstar works with nominal PV voltages of 12, 24, and 48 with maximum current output of 45 Amps. The Blue Sky works with nominal PV voltages of 12 and 24 with max currents of 32 and 12 Amps respectively.
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Old 13-01-2012, 10:03   #40
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re: Best Solar Panels For The Buck

We have two 24v 235watt panels wired in series to 48v to an MPPT controller that drops the charge voltage to 24 (Nom) and we get about 70 Ah a day @ 24v with our normal consumption of about 45 per day we have full batteries nearly always. Cost, $500 per panel, and 220 for the 20A MPPT controller.
I suggest you look at 24v panels. They are cheaper expressed in $/w.
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Old 13-01-2012, 12:40   #41
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re: Best Solar Panels For The Buck

"The Morningstar has a standard RS232 interface to control and monitor operation via a PC."
I mourn the passing of RS-232 but since it probably has not shipped on any mass-market PC in five years...Does the BS have a front-panel integral programming option instead? USB-to-RS-232 converters aren't expensive but they are 'more stuff' and sometimes problematic.
Do the two offer the same warranty in marine use?
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Old 13-01-2012, 12:54   #42
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re: Best Solar Panels For The Buck

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We have two 24v 235watt panels wired in series to 48v to an MPPT controller that drops the charge voltage to 24 (Nom) and we get about 70 Ah a day @ 24v with our normal consumption of about 45 per day we have full batteries nearly always. Cost, $500 per panel, and 220 for the 20A MPPT controller.
I suggest you look at 24v panels. They are cheaper expressed in $/w.
Coincidentally, we just order two 24 Volt / 200 Watt panels from SolarBlvd to supply our four golf cart batteries. They are "B grade" panels which should have met a spec of 210 Watts, but didn't. The cost was $200 each ie, $1.00/watt. We also purchased the Morningstar TS-MPPT-45 controller for $370. We splurged on the controller, but really like it's flexibility which will come in handy if we ever have to change out the panels for something else down the road; they handle nominal 12-48 volt panels.
Pete
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Old 13-01-2012, 12:59   #43
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re: Best Solar Panels For The Buck

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"The Morningstar has a standard RS232 interface to control and monitor operation via a PC."
I mourn the passing of RS-232 but since it probably has not shipped on any mass-market PC in five years...Does the BS have a front-panel integral programming option instead? USB-to-RS-232 converters aren't expensive but they are 'more stuff' and sometimes problematic.
Do the two offer the same warranty in marine use?
The Blue Sky is very limited in programming unless you also buy their expensive pro-version remote panel. The Morningstar can be programmed via DIP switches or custom via the RS-232. I liked the flexibility of the Morningstar. We already have a few USB/serial adapters laying around so that's not much bother.
Pete
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Old 13-01-2012, 13:26   #44
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re: Best Solar Panels For The Buck

Hey i just wanted to ask is there any are most panels ok in a marine environment? Cheers i'm learning heaps from your postings! Frank
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Old 13-01-2012, 14:11   #45
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re: Best Solar Panels For The Buck

Almost none of the solar panel makers warranty them in the marine environment.
They are built for storms and hail on a roof, but they do not test them on boats
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