I've read that 12v panels actually make closer to 18v (no load) so the charge controller would be the critical piece
There are two relevant voltages in PV world.
Voc (volts open circuit) is usually around 22-25 for a "12v" panel. It is an important number for charge controllers if you want them to live. Think of it like the RPM limit of an engine while not connected to anything. It isn't really useful outside of picking a charge controller and panel pairing, i'm not sure what an engine parellel would be. The panel (engine) does NOT go poof at redline, the controller may though...
The Vmp (voltage at a maximum power) is the voltage the panel will be at while doing the most work it can possibly do. Think of it like the RPM at max HP. It could go higher or lower, but it is not doing as much TOTAL work at that point... You usually NEED this to be AT LEAST max charging voltage plus 20-30 percent depending on how you are going about things. There are some caveats in regards to temperature and such.
Ok I am happy to admit to not being the sharpest crayon in the box...
First off no 24v anything on my rig...I would simply pick a MPPT controler that can handle the higher input of a 24v panel...after that all 12v down the line.
I have about a 48in by 48in space to place my panels..I will post up exact numbers.
So if I read things correctly you said 12v panels will be cheaper to get the same power....BUT there less voltage loss & more voltage to lose makes a 24v panel better.
So if I want to spend LESS money buy 12v, if I want MORE performance buy 24v....right?
That becomes a bigger factor in big systems and high current applications with long wire runs. higher voltages equal lower currents for the same power. Less amps equals cheaper panels, cheaper wire, less loss, higher efficiency, etc etc...
20 volts at 50 amps is 1000 watts, so is 100 volts at 10 amps. It takes a much bigger wire to safely carry 50 amps 100 feet than it does to carry 10.
Let's start with the "12v" system. We will choose a wire that will allow a < 4% drop over a 100 foot run (2 gauge for those keeping score) it is expensive and heavy, and expensive. When all is said and done you only have 19.2 volt at the end (960 watts or a 4% loss). Also this has to be before the regulator charge controler, because all things being equal the 14.8v output of the controller would drop to 14 at the battery if the run is on that side, which would be 700 watts or a 30% loss wouldn't charge the battery very efficiently... it would however run 50 amps worth of accessories at that distance I guess.
At 100 volts you could have that same 1000 watts of power with 10 amps, run it over some 12 gauge speaker wire (not that I would choose that), you'd drop 1.6 volts for the resistance of the wire leaving you wil 98.4v and 10 amps (984 Watts or a 1.6% loss), 14 ga would be 2.6volts or 2.6% loss... 16ga would actually give you the same 4% as 2ga at 20v.
There are also some efficiency gains turning DC back into AC when the DC is higher voltage as well.
So I think this controller will work with either 12v or 24v panels (understanding they actually make 18-36v) for $230 and I think I need some remote display??
http://www.solarblvd.com/Charge-Con...-MPPT-15L-Charge-Controller/product_info.html
Looking at panels there are 12v 75w panels that measure 39in x 24in so I can fit two in a space of 39in x 48 in and get 150 watts at 30lbs for $200 ($94 ea)
or I can get a Sharp 24v 224w panel that measures 65in x 39in (I might be able to squeeze that into my space) and get 224watts for 44lbs for $190
So it looks like the cost is about the same but I get 224 watts out of 24v vs 150 watts out of 12v IF I can squeeze in a 69x39 panel.....
As an aside I was looking at solar blvd's website and all the 24v panels are roughly 69x39 so I will have to search around for other sizes it they are out there.
Any other sources that people like? I am in Phoenix and I know there are a number of local shops so happy to go local or web....I am thinking of waiting till the overland expo to see if there are any deals but I would also like the system to USE there
You don't need fancy remote monitoring a few minutes with a volt meter will tell you what you need to know for the most part.
I personally like more panels better than bigger panels. More of a pain but more redundency. With one big panel you are one spun rock away from having no power generation...
I like Amazon for bang over buck ratio.
Big question......how bad is it if a roof rack cross bar cast a shadow over part of the panel????
So I think this is the system I want and would be happy to hear what everyone thinks before I order it all up...Also if there are any well trusted sources please let me know:
MADE IN THE USA...very nice 24v 220watt panel for $160...just got to check if it will fit..
http://www.solarblvd.com/Solar-Pane...0W-24V-Solar-Panel-MX60-220/product_info.html
Morningstar controller that can handle 24v panels $230
http://www.solarblvd.com/Charge-Con...-MPPT-15L-Charge-Controller/product_info.html
Remote display so I know what is going on with the system $75
http://www.solarblvd.com/Charge-Con...SunSaver-Duo,-SunSaver-MPPT/product_info.html
I think I will also need some cable and misc mounting hardware but it looks like around $500 to get a 220watt system that is 39x65in
All thoughts welcome
On the shading it depends on the type of crystal the panel is made of. Mono vs poly vs amorphous. Additionally it depends on panel arrangement, half of a 4 panel system blacked out Worst Case Scenario is only a 50% reduction in power generation, half of a single panel arrangement (depending on panel type) could be a 95% reduction in power generation. I doubt it would be that extreme but
The Max open circuit voltage of that controller is 78 volts when doing "12v", the VOC of those "24v" panels are 36.5 so you
should be able to run 2 in series but you
can't do it year round because once the cold kicks in the VOC goes up and then it will be over 75v. The Impp is only 7.61 so you
can't run 2 in parallel because it already exceeds the 15a
output rating of the controller. If you were running a 24v battery system you could run two in parallel and get 440 watts. Sorry there is no cheap or easy shortcut, or at least I continue to look for one.
If it were me go with the "12v" panels and this controller:
http://www.solarblvd.com/Charge-Con...saver-Duo-With-Remote-Meter/product_info.html $145.86 monitor included.
It isn't MPPT but you don't really need it at those voltages, the gain wouldn't be staggering...
That's going to give you ~8.58 amps @ whatever your charging voltage is which is pretty dang good for the square footage you are playing with. The net net will be pretty close. A little wider and you could have run these:
http://www.amazon.com/145w-Polycrystalline-Solar-Panel-Pack/dp/B007VPQP5Q and double your current.
I love those.
