Need help with electrical math and advice on choosing a solar controller

[Tenor]

It's been a while since I've posted, but I'm trying to get back on top of my bus.  Here is what I have for solar panels:

Quantity 6
Max Power 48W
Short Circuit 3.35A
Rated 3.02A
Max SVST Open Circuit V 600V (this one I don't understand)
Voltage Open Circuit 19.8
Voltage Rated 15.9

My plan:
Install 3 groups of 2 panels in series, then wire the 3 groups in parallel.  I should get 31.8V (based on rated voltage of 15.9) and 9A. 

If my math is correct, I need a 24V controller, preferably MPPT.  What size should I get?  Additionally, since I last was looking for one, there are many Chinese controllers on the market.  Has anyone had any experience with one?

Thanks!
Glenn

[Frank @ TX]

Hi Glenn,
Your math sounds right BUT it isn't.
When you tie the two panels together to make the higher voltage you get more voltage but the same amps as one panel.
The controller you would need would be 12 amps ( with a safety margin ) if the panels are tilted toward the sun.  But you may want to get the controller the size that you will need when even more panels are up there : maybe you will want to fill more roof area some day.

This volt / amp thing is the same as when you tie ( 4 ) 6 volt batteries together to make a 24 volt system.  Yes , you get 24 volts BUT the same amps as one battery.
Frank

[Tenor]

Thanks Frank!  Let me double check the math.  Each panel is 3A (+/-).  Combine 2 in series, you remain at 3A.  When you then combine the three groups of 2 in parallel, your voltage stays the same, but current should should up to 9A.  Am I missing something in translation?  I sure do appreciate it!

Glenn

[HB of CJ]

12 amps nominal at 24vdc, however I'm not quite clear as to what you want to do.  HB of CJ (old coot)

[Tenor]

HB,
I'm just using the system to keep batt's topped off.  Just shaves some time off of running the genset, and helps me plug in less freqently when the bus is not in use.  9A isn't anything to write home about! 

Glenn

[Bill B /bus]

Why not put 6 panels in series? Max voltage would be less than 120VDC. I would use an MPPT controller. Set the output at your desired output for the battery float and let the controller do the work. Since you are less than 5 amps wire size at #10AWG will be great and a lot easier to run. Use at least #6AWG from controller to battery. Don't forget fuses/circuit breakers on both legs.

Bill

[Frank @ TX]

Hi Glenn,
You got it right.
Electr can be a pain sometimes.
I have ( 2 ) 75 watt panels tied together and when we are out at QSite or the shore it is very rare to fire the gen.  But I can tilt the panels to get the most from them.  It all maters how you use the power.  If you run the inverter to frie some bacon in the electric pan you'll run the gen more that we do.  BUT with 9 amps you'll have lots of recharging power.
Frank

[Uglydog56]

http://www.solar-electric.com/ss-20l.html

This is my 2 cents.  I just picked an online store, you can use your favorite.  This one has some room for upgrading and isn't going to break the bank.  I would do the 2-3 setup you described in your original post.

[buswarrior]

One of my favorite learning and pricing sites:

http://www.altestore.com/learn/

Under the "Learn" tab, many evenings of stimulating educational reading.

They have been around for over a decade, so no johnny-come-lately-to-the-green-party here.

happy coaching!
buswarrior

[chev49]

i am just now working on my solar system, and was thinking i needed at least 1200w to run things during the day, maybe more.

[Tenor]

Thanks BW!  Yea, they do have a great site.  I found a nice Morningstar MPPT model that should do the job nicely, with a little room for expansion should I choose to.  Do you think you'll be attending the Back To The Bricks Rally again?

Glenn

[Uglydog56]

chev, I'm putting 1150 on top of mine when I get around to fabricating the supports.  I think your math checks out.

[Tenor]

An update:

I got a deal on a Blue Sky 3024i controller, IPN ProRemote cable and shunt for 300 on ebay!  So new question:

How should I wire the solar panels I have?

My panels are:

6 panels each are rated:
Short CKT 3.35A Rated 3.15A
Open CKT 19.8V Rated 15.9V

The controller has a maximum input voltage of 57V. 

I see 2 choices.   2 groups of 3 panels in series, parallel the 2 groups for approx 48V at 6.2A  OR 3 groups of 2 panels in series, parallel the 3 groups for approx 32V at 9.2A. 

Any advantage one way or another?  Thanks everyone!

Glenn

[bobofthenorth]

Mounting 6 panels to get 300 watts is a lot of work for not much return.  I'd save up for a couple of 200 watt panels - less work to mount them and more output.  Sell the 50 watt panels to someone who doesn't know any better.

[buswarrior]

I agree with bob, put those up on your favorite selling site.

Just as an example, go back to see what performance is available at what cost at my favorite alt energy site listed earlier in this thread.

You can easily match what you have and then plan for expansion to some serious silence by adding a few more as you see fit.

6 panels times 200 watts....

I hate the noise maker.

happy coaching!
buswarrior