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Old 10-07-2013, 16:55   #1
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Need Help With Solar Controller Size

Hi all,

I am sorry for asking this question, as I read a bunch of threads and even some info on controller websites. I just don't get which measurements for the panels are being used for sizing the controller.
I have three BP 175w 24v panels, and may add two more.
Stats are here: http://www.bp.com/liveassets/bp_inte...heet_10_06.pdf

Could someone please tell me which values I need to pay attention to for sizing the MPPT controller? Total watts (right now) is 525, and as I said earlier, they are 24v.

Thanks in advance, Bill
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Old 10-07-2013, 17:11   #2
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Re: Need Help With Solar Controller Size

It really is as simple as you think.

You need a controller that can handle 525 watts. Or, 875 watts if you want to be able to add two more panels. Then you should add some margin for safety and longevity, say, make it 600W now or 1000W to allow for the expansion.

And, the controller will have to work with 24 volts if you put all the panels in parallel. Most will. But you may also find that wiring considerations make it more effective to run four panels, two in series at 48 volts, with two sets of those in parallel. In that case, you need a controller that can handle 48 volts.

And make sure that's "nominal" voltage rating on the controller, or else check the "maximum" voltage rating of the controller and panels and match those up. Even a 24 volt panel may be putting out over 30 volts at some times, so a controller rated for a maximum 24-volt supply, could easily burn out with "nominal" 24 volt panels.
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Old 10-07-2013, 18:03   #3
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Re: Need Help With Solar Controller Size

Thanks for the reply HelloSailor,

I understand the total watts of the panels must be less than what the controller can handle, however I still have a few questions.

1) Is the nominal voltage of the controller for the battery bank (12v) or incoming voltage (24v+)?
2) Max voltage for these panels is 35,8v, but open circuit voltage rises to 44.2v. Is this value cumulative, that is, do I add 44.2v for each panel, or just use 44.2v as the max voltage?
3) Same question for the amps - the short circuit amps for each panel is 5.4a. Do I add these for each panel, or use just the 5.4a value?

Thanks for your patience, as you can see, I have a lot to learn about electricity.

Cheers, Bill
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Old 10-07-2013, 18:49   #4
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Re: Need Help With Solar Controller Size

>>1) Is the nominal voltage of the controller for the battery bank (12v) or incoming voltage (24v+)?
Both. You need to see that the input voltage is greater than what the panels can produce, and the output voltage is matched to your batteries. If you have a nominal 12-volt battery bank, your controller would be sold as a 12-volt controller. Many have options and can be set to 12/24.

>>2) Max voltage for these panels is 35,8v, but open circuit voltage rises to 44.2v. Is this value cumulative, that is, do I add 44.2v for each panel, or just use 44.2v as the max voltage?
If the panels are in parallel, the max voltage is 44.2v. But if they are wired up in series, you add that for each panel, i.e. two panels adds up to 88.4v.

>>3) Same question for the amps - the short circuit amps for each panel is 5.4a. Do I add these for each panel, or use just the 5.4a value?
For the amps, the amperage from each panel gets ADDED when they are in series, but not when they are in parallel. The opposite of how/when you add up volts.
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Old 10-07-2013, 19:23   #5
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Re: Need Help With Solar Controller Size

Quote:
Originally Posted by hellosailor View Post
>>1) Is the nominal voltage of the controller for the battery bank (12v) or incoming voltage (24v+)?
Both. You need to see that the input voltage is greater than what the panels can produce, and the output voltage is matched to your batteries. If you have a nominal 12-volt battery bank, your controller would be sold as a 12-volt controller. Many have options and can be set to 12/24.

>>2) Max voltage for these panels is 35,8v, but open circuit voltage rises to 44.2v. Is this value cumulative, that is, do I add 44.2v for each panel, or just use 44.2v as the max voltage?
If the panels are in parallel, the max voltage is 44.2v. But if they are wired up in series, you add that for each panel, i.e. two panels adds up to 88.4v.

>>3) Same question for the amps - the short circuit amps for each panel is 5.4a. Do I add these for each panel, or use just the 5.4a value?
For the amps, the amperage from each panel gets ADDED when they are in series, but not when they are in parallel. The opposite of how/when you add up volts.
I think you were thinking correctly on number 3 but you wrote it backwards. Amperage is constant in series and additive in parallel. You are correct it is the opposite of how you add up voltage.
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Old 10-07-2013, 20:03   #6
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Re: Need Help With Solar Controller Size

Damn, that's the decaf. Thanks, Captain Bill.
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Old 10-07-2013, 20:14   #7
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Re: Need Help With Solar Controller Size

Go to Solar Electric Power Discussion Forum by Northern Arizona Wind & Sun. Its like CruisersForum but for solar. Super helpful folks over there. You have it about right but need to decide if you will wire them in series or parallel. Lots of discussion on that here. I went with 3 240W panels in series. Smaller wires for the run from the panels to the CC. Need a large MPPT controller for that. Sourcing appropriately sized breakers with a decent enclosure was a b*tch. Look at Altech.
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Old 11-07-2013, 14:18   #8
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Re: Need Help With Solar Controller Size

"Sourcing appropriately sized breakers with"
There's a reason you see those cigar-sized fuses in so many equipment panels. At a certain point, breakers become the hard way to do things.
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Old 11-07-2013, 15:13   #9
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Re: Need Help With Solar Controller Size

Hi Bill
The 4. 9A (Imp) input from each of the panels (or 14.7A for 3) @35.8v will be converted to 13.5A (or 40.5A for 3) @13v.

It is this latter figure that the controller needs to rated for.

So you need a controller rated for a minimum of 40-45A no matter how the 3 panels are wired (with 12v batteries)

If the panels are wired in parallel the controller will need to be able cope with Voc. (44.2v) usually the controller is instantly damaged if this is exceeded and panels can occasionally exceed Voc so a min voltage rating of about 50v is needed.
For series connection it needs to be 150v.

To allow for expansion of 2 more panels the current rating needs to be 67-75A. In parallel the voltage stays the same the same at 50v

The Outback 80 would be a good choice, but also look at the range from Midnite.
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