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Old 18-10-2012, 14:16   #16
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A charge controller, regardless of which technology it uses, serves a couple of purposes. It monitors the state of battery charge and regulate the flow of current/voltage to those batteries.

Due to varying efficiencies with panel manufacturers, the most reliable method I have found to compare them is the delivered amperage.

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Old 18-10-2012, 14:36   #17
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Re: Solar - best prices?

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A charge controller, regardless of which technology it uses, serves a couple of purposes. It monitors the state of battery charge and regulate the flow of current/voltage to those batteries.
A PWM controller is basically a fast acting switch. It connects and disconnects the panels from the batteries. As such, the panels operate at battery voltage. In that case you can measure the current going in and all is equal.

An MPPT controller has a voltage converter built in. If your panels will deliver more power at 17V (which it typical) it runs the panels at 17V. It then converts that power to battery charging voltage (13.8/14.4). In the process current goes up. If your panel is delivering 10A @ 17V and it goes through an MPPT controller you will get 17/14.4 * 10 = 11.8A into the batteries. MPPT controllers are not 100% efficient in the conversion, more like 95% (or sometimes even worse), so what you really get is 11.8 * .95 = 11.2A. That's still about a 10% improvement.

If you look at that in Watts it is actually much simpler, in the PWM case you have 144W into your batteries, in the MPPT case you have 161W into your batteries.

With an MPPT controller you can also use higher voltage panel. Take a look at this thread: Simple Solar For A Simpleton. Vic has a panel that delivers 5.8A at 55VDC. With the MPPT controller he can get ~22A into the batteries at 14VDC. That's why panels get rated in Watts and that's why comparison in Watts makes sense. If you were to compare that based on Amps it would only show up as a 5.8A panel.

Amperage is simpler in a direct connected or PWM connected system, but not in an MPPT system.
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Old 18-10-2012, 15:34   #18
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I think we are splitting hairs here with the two technologies and that the formula shown for the mppt controller is not quite accurate, though anytime you lower voltage, available amperage increases.

The big reason to compare amperage instead of watts on panels (please note we are talking panels here - they are in front of the charge controllers) is that panels vary in efficiency AND that manufacurers are listing them by wattage because the aggregate total of the individual cells offer the biggest number that can be argued as correct. The math for calculating wattage is AMPS * VOLTAGE = WATTS.

No matter what small boost a charge controller can give, it must work with what is delivered to it. It would be awesome if you could put one in that erased the inefficiencies ahead of it and, if you find one please let me know.

I think that the next charge controller I put in will be mppt technology, though. No sense in not getting the most from your system.
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Old 18-10-2012, 16:21   #19
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Re: Solar - best prices?

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Originally Posted by Muscongus View Post
The big reason to compare amperage instead of watts on panels (please note we are talking panels here - they are in front of the charge controllers) is that panels vary in efficiency AND that manufacurers are listing them by wattage because the aggregate total of the individual cells offer the biggest number that can be argued as correct. The math for calculating wattage is AMPS * VOLTAGE = WATTS.
I really don't get what you are trying to say here. Some solar cells are 10% efficient, some are 15% efficient, and some are >20% efficient. That efficiency is already taken into account in the module rating.

That leaves us with Amps * Volts = Watts (or P=EI). Are you saying that some panels have different voltages? Because that is the only way for two panels with the same current to have different Watt ratings. And that is the case, some panels do operate at slightly higher or lower voltages than others. And that shows up in the Watt rating of the panel. If you use PWM or direct connection then a 5A 18V panel and a 5A 17V panel will put the same amount into the batteries (even though one will be rated 5W higher than the other) because the panels, even though rated at 17 or 18V, only operate at whatever voltage the battery is at. That has nothing to do with the panel manufacturer trying to maximize their module rating, and everything to do with poorly matching the module to the system, which is the designer/purchaser's fault.

With an MPPT controller however, an 18V panel will convert that voltage down to 14V just as well (well almost as well if we quibble) as it converts down 17V, and so you you will get more power from the 18V panel than from the 17V panel, and that means more juice into your batteries. The example I gave above is pretty typical, but if your batteries are at a relatively low SoC then MPPT gives you more, as does a higher Wattage panel. An 18V panel operating at 5A produces 90W. Convert that to 13V (at 95% efficiency) and you have 6.6A into the batteries. Through a PWM controller you'd only have 5A (actually, given panel I-V curves probably about 5.1A) so you get an extra 1.5A into the batteries, almost a 30% increase in the power delivered from the same panel. If it were a 17V panel (so rated 85W instead of 90) you'd get 6.2A. If you compared them based on current you'd say they were the same, but if you compared them based on Watt rating the 18V panel wins when using an MPPT controller.
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Old 18-10-2012, 16:34   #20
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The higher voltage is spot on. I have to disagree about the wattage ratings taking efficiency into account, though. It is my understanding that the wattage rating of a panel is the accumulation of the rated wattage of the individual cells (as arranged to achieve panel voltage). The added inefficiencies are from the connections and voltage drop. That is why the nameplates of various panels of the same wattage (but different manufacturers) have different available amperages listed. amperage is the measurement of current flow (power traveling feom point A to B).
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Old 18-10-2012, 17:03   #21
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Re: Solar - best prices?

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Originally Posted by Dsanduril View Post
....That leaves us with Amps * Volts = Watts (or P=EI). Are you saying that some panels have different voltages? Because that is the only way for two panels with the same current to have different Watt ratings. And that is the case, some panels do operate at slightly higher or lower voltages than others. And that shows up in the Watt rating of the panel..
I agree with the above. For instance where we buy panels:

They have one 12 volt panel with a Vmp of 17.0 and a Imp of 5.80 so 17.0 X 5.80 = 98.6 watts and they have the panel listed as a 100 watt panel.

They have another 12 volt panel with a Vmp of 17.0 and a Imp of 8.00 so 17.0 X 8.00 = 136 watts and they have the panel listed as a 140 watt panel.

They have a 24 volt panel with a Vmp of 34.8 and a Imp of 4.90 so 34.8 X 4.90 = 170.52 watts and they have the panel listed as a 170 watt panel.

You should be able to find the specs for the voltage and amperage for panels one is looking at and they should match, pretty closely, the advertise wattage.

It is all pretty easy and straight forward. I'll also add that why not maximize your effort with a MPPT controller even if it only increases output 10%. If you figure the difference in cost between a PWM and a MPPT and divide that by the useful life you will get out of the array it won't cost much to go for the other 10% and if you ever have to make up that 10% by running a gas or diesel engine to do it you will pay for it very quickly with the price of fuel being what it is,

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Old 18-10-2012, 17:04   #22
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Re: Solar - best prices?

I've never seen a panel/module rated that way, but can't say it doesn't exist.

Look for bin-sorted, "flash" tested modules. These are tested and rated as modules, not as a collection of cells. For instance, take a look at the Kyocera KD215GX-LFBS and KD220GX-LFBS. These are the exact same module, made on the exact same assembly line, and possibly even at the same time. One is rated 215W, the other is rated 220W. The difference is that they are flash tested after assembly. One tested at 215W so got labeled that way, the other tested at 220 and got labeled that way. This is very common in the larger modules, it allows the manufacturer to say their panels will always be X Watt + Y % rather than X +/- Y. And then they charge a little more for the one rated 220 even though it used all the same materials and came off the same line at the same time. You'll frequently see a series of modules from one manufacturer with the same basic specs and ratings 5W apart. These are all flash tested and so you end up with identical 230W, 235W, 240W, 245W, and 250W modules. Any module that was rated using flash testing was rated as a module. Any module that wasn't rated by flash testing, who knows, buyer beware.
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Old 18-10-2012, 18:54   #23
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Ok, ok.

Perhaps my looking at amperage is a hold over from years ago - dunno.

You are definately correct that you should maximize your system capabilities with the mppt style controller.

The thing we should be keeping in mind here, though, is that the OP is looking for some advice on an inexpensive solar charging system rather than a couple blokes discussing differing views of manufacturer statements on his thread.

You guys have a great night and thanks for the conversation and the insights to the mppt / pwm style controllers and the food for thought on the panel specs.
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Old 18-10-2012, 19:22   #24
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Re: Solar - best prices?

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the OP is looking for some advice on an inexpensive solar charging system rather than a couple blokes discussing differing views of manufacturer statements on his thread.
Ain't that the truth. In that light, I agree with something posted way up there, if you have the room, you're almost always better off spending the $ on more panels than you are on controllers. Controllers come in to play only when you've maxed out the panels you can fit and you still don't reach fully charged at the end of the day.
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Old 18-10-2012, 21:39   #25
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Re: Solar - best prices?

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Bruce,

I don't know if this will help with your interest in finding the cheapest sources for solar, but you might want to talk with Bob Williams at SALT Service in Marathon. He's been in the business for a long time and can answer any question you have. He can certainly help you understand costs and trade-offs.

I just purchased a wind/solar system from him. It wasn't the absolute lowest cost system, but it was very competitive and I got everything needed to do the install down to the wire ties. Very staisfied customer.

Scott
Yeah I'm not sure but I think I spoke to him at one of the those flea market / festivals near mile marker 50 (just south of Vons) back in 2011. I can't remember the details but I got the impression the $$$ would be high so I didn't follow up with him. Perhaps I should get in touch with these folks and check again. Thanks for the tip.

Bruce
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Old 21-10-2012, 11:26   #26
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Re: Solar - best prices?

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I am looking at size and weight as well.
At the moment the plan is to get 705W by installing 3x 235W panels type
Sanyo/Panasonic HIT N235SE10.
(size 1580 x 798 mm)
The best price I have found in Europe up to now is 359 EUR each (=466 US$). (+tax)
Where is this ?
shop ?
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Old 21-10-2012, 13:15   #27
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Re: Solar - best prices?

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Where is this ?
shop ?
Photovoltaik Solarstrom || Photovoltaik-Anlagen - NwComp Solar GmbH
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Old 21-10-2012, 14:07   #28
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Re: Solar - best prices?

Near as I have been able to tell the best pricing by far is if you get larger panels (240W +). But to make it affordable you have to be able to go pick them then up or the shipping just kills it.

http://www.altestore.com/store/Solar...age/1/sort/5a/
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Old 21-10-2012, 14:30   #29
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Re: Solar - best prices?

Thanks Myocean
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Old 21-10-2012, 14:56   #30
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Re: Solar - best prices?

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Ok, ok.

Perhaps my looking at amperage is a hold over from years ago - dunno.

You are definately correct that you should maximize your system capabilities with the mppt style controller.

The thing we should be keeping in mind here, though, is that the OP is looking for some advice on an inexpensive solar charging system rather than a couple blokes discussing differing views of manufacturer statements on his thread.

You guys have a great night and thanks for the conversation and the insights to the mppt / pwm style controllers and the food for thought on the panel specs.
Yeah but I do very much appreciate everyone who responded here as along with trying to get the lowest priced system am also interested in all the other things I need to learn/apply to the system I will be putting in place. Thanks folks!
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