Too much panel a problem? (off grid)

Dan_
Dan_ Solar Expert Posts: 61 ✭✭✭✭
in Solar Beginners Corner #1
I found a deal on 24 Kyocera 120W panels. I had planned only 1500-2000W as suitable for my system though. Have 208Ah at 48v FLA battery bank and Outback FM60 CC

Do you think I should go with 1440W, 2160W or full 2880W. Comes with two pole mount racking that hold up to 12 panels. I could sell off any unused panels.
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  • Photowhit
    Photowhit Solar Expert Posts: 6,006 ✭✭✭✭✭
    #2
    Re: Too much panel a problem? (off grid)

    There was a big array taken down recently, I think they were being sold at 40 - 50 cents a watt in pallet loads. Though they are premium panels, I'm not sure how good a deal that is. I think these are 10-15+ year old panels, closer to 15+ year old if they just have junction boxes on them with out leads. Since they are used you wouldn't be able to claim a tax credit, so an 80 cent a watt new panel with a 30% tax credit might be a better deal with a final cost of 56 cents a watt... but if it's local and your allowed to test them. no shipping might make a difference.
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,642 admin
    #3
    Re: Too much panel a problem? (off grid)

    The rule of thumb for "cost effective" maximum array I use is:

    208 AH * 59 volts charging * 1/0.77 panel+controller deratings * 0.13 max cost effective solar battery charge rate = 2,072 Watt array

    More or less, that is about the maximum current you want to pump into a flooded cell battery bank (~13% rate of charge) for long term charging.

    Lead acid batteries.

    You can go as high as 20-25% rate of charge--I would suggest you have a remote battery temperature sensor attached (so battery does not get into thermal runaway). Also, if you have a Midnite with a Whizbang jr. or Outback that you can program maximum battery charging current--That would help too.

    Depending on the brand/model of charge controller, there have been cases where too small of battery bank (vs solar array size), that the battery bank voltage has exceeded ~72 VDC during parts of the charging cycle...

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Dan_
    Dan_ Solar Expert Posts: 61 ✭✭✭✭
    #4
    Re: Too much panel a problem? (off grid)
    Photowhit wrote: »
    There was a big array taken down recently, I think they were being sold at 40 - 50 cents a watt in pallet loads. Though they are premium panels, I'm not sure how good a deal that is. I think these are 10-15+ year old panels, closer to 15+ year old if they just have junction boxes on them with out leads. Since they are used you wouldn't be able to claim a tax credit, so an 80 cent a watt new panel with a 30% tax credit might be a better deal with a final cost of 56 cents a watt... but if it's local and your allowed to test them. no shipping might make a difference.

    These are local panels from Craigslist no shipping. Manufacture date is 2012. Getting them for 50c/W
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  • Dan_
    Dan_ Solar Expert Posts: 61 ✭✭✭✭
    #5
    Re: Too much panel a problem? (off grid)
    BB. wrote: »
    The rule of thumb for "cost effective" maximum array I use is:

    208 AH * 59 volts charging * 1/0.77 panel+controller deratings * 0.13 max cost effective solar battery charge rate = 2,072 Watt array

    More or less, that is about the maximum current you want to pump into a flooded cell battery bank (~13% rate of charge) for long term charging.

    Lead acid batteries.

    You can go as high as 20-25% rate of charge--I would suggest you have a remote battery temperature sensor attached (so battery does not get into thermal runaway). Also, if you have a Midnite with a Whizbang jr. or Outback that you can program maximum battery charging current--That would help too.

    Depending on the brand/model of charge controller, there have been cases where too small of battery bank (vs solar array size), that the battery bank voltage has exceeded ~72 VDC during parts of the charging cycle...

    -Bill

    Thanks Bill. I do have an Outback FM60 with RTS

    If you were in my shoes would you keep the 2800watts or stick with the more reasonable 2160 and sell the excess?
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  • Photowhit
    Photowhit Solar Expert Posts: 6,006 ✭✭✭✭✭
    #6
    Re: Too much panel a problem? (off grid)
    Dan_ wrote: »
    These are local panels from Craigslist no shipping. Manufacture date is 2012. Getting them for 50c/W

    Nice! I didn't realize they still made 120 watt panels, hadn't seen any new in quite a while. Sounds like a heck of a deal! Kyocera makes a quality panel, used 12 volt nominal panels, from a good manufacturer, still command near $1 a watt around here. Nice find!
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
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  • CDN_VT
    CDN_VT Solar Expert Posts: 492 ✭✭✭
    #7
    Re: Too much panel a problem? (off grid)
    Dan_ wrote: »
    Thanks Bill. I do have an Outback FM60 with RTS

    If you were in my shoes would you keep the 2800watts or stick with the more reasonable 2160 and sell the excess?

    I purchased my panels plus a bunch of extras for the long run. i have the same as spares if one gets damaged , or if another project on the property. I bought mine for 65c watt in twin pallets stacked , cheapest way for shipping & moving. Shipping was 350.00 for the weight/space . With all the reading of trying to match , I figured a good deal will keep me in light when a tree / bird drops something. We do have storms here.

    VT
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  • solar_dave
    solar_dave Solar Expert Posts: 2,397 ✭✭✭✭
    #8
    Re: Too much panel a problem? (off grid)
    BB. wrote: »
    The rule of thumb for "cost effective" maximum array I use is:

    208 AH * 59 volts charging * 1/0.77 panel+controller deratings * 0.13 max cost effective solar battery charge rate = 2,072 Watt array

    More or less, that is about the maximum current you want to pump into a flooded cell battery bank (~13% rate of charge) for long term charging.

    Lead acid batteries.

    You can go as high as 20-25% rate of charge--I would suggest you have a remote battery temperature sensor attached (so battery does not get into thermal runaway). Also, if you have a Midnite with a Whizbang jr. or Outback that you can program maximum battery charging current--That would help too.

    Depending on the brand/model of charge controller, there have been cases where too small of battery bank (vs solar array size), that the battery bank voltage has exceeded ~72 VDC during parts of the charging cycle...

    -Bill

    As Bill states 13% is a nice rate, FLA can take more. But if you have opportunity loads in the peak of the day the extra is quite consumable.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #9
    Re: Too much panel a problem? (off grid)

    2880 Watts on a 48 Volt system = about 46 Amps. For 208 Amp hour battery bank that would be 22% peak charge rate. That is too much for a flooded battery and well about practical charge acceptance. If you go with that much panel you should set a current limit on the FM60 of about 30 Amps. Then the extra panels will help keep charge rate up on less than perfect days.

    Or you could sell off a few and limit the current that way. Or keep them on hand for spares or the next project or system expansion. Or up the battery bank to double its present capacity.
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