Size issues

the dudethe dude Registered Users Posts: 5
I have having a few sizing issues with my new system design. My brother and I searched through this and other locations on the internet and cannot find a definitive answer to some of the sizing concerns that we have. We are setting up a new system in Arkansas at my home and have already purchased a few solar panels. Found a great deal, and had to pick them up. That is how all this started. Now that we are in this to finish it, I find that we have several sizing questions.

We are looking to build a system that is connected to the grid, has battery backup and sends excess back to the electric company. Wind will eventually need to be added. The inverter we are looking at it the Sunny Boy 8000. Adding a Sunny Island 4248 for the battery control. I'm still not sure of these integrate for the two uses or if I will need to banks of panels/batteries to connect to each.

The panels that we have are Sharp ND-224UC1's. They are 20v panels and I plan to order more of them. Since they are 20v, do I want a 6v, 12v, 24v, or 48v battery setup? I don't understand if this relates completely to the panels, inverter, and charger except for each model in the Sunny line specifies charging sizes of 24v or 48v.

Guess my real question is why I need 48v over 24v and if I can configure 12v batteries in a 24v array why would I do that over buying 24v batteries? Since I don't own any, what is the best choice when I eventually plan to run my entire home off this system?

Thanks for any help.

Comments

  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Size issues

    i must say i'm a bit confused unless you are planning 2 separate systems here with 1 being straight gt and the other as a battery backed gt which can be done.

    the battery bank voltage can be whatever you like it to be in whatever type of batteries you would like, but higher voltages have the advantage of lower currents flowing for the same wattage. this allows lower losses and the use of smaller wires to carry that same wattage over battery banks that have smaller voltages. if you plan on the s i 4248 then you must use 48v as that's what it's made for. to use a different battery bank voltage the inverter must be made for it.
  • the dudethe dude Registered Users Posts: 5
    Re: Size issues

    Then what is the advantage of 24v or 12v? When people are purchasing new systems to power their homes, do most go with the higher voltage batteries? I run a lot of computers and an A/C unit, washer, etc. I understand how the 48v may be better for startup applications, but why are there so many 12v configurations from the higher-end battery makers?
  • dwhdwh Solar Expert Posts: 1,341 ✭✭✭
    Re: Size issues
    the dude wrote: »
    Then what is the advantage of 24v or 12v? When people are purchasing new systems to power their homes, do most go with the higher voltage batteries? I run a lot of computers and an A/C unit, washer, etc. I understand how the 48v may be better for startup applications, but why are there so many 12v configurations from the higher-end battery makers?

    Doesn't really matter. 12 or 24 is just a bunch of 2v cells wired in series to get whatever voltage. 12v batteries are the most common, so they are often cheaper.

    Either way, you either rig 4 12v batteries in series to get 48v or 2 24s. (Or 8 6v.)


    EDIT: Whichever way you go, it's going to end up being 24 2v cells.
  • BB.BB. Super Moderators, Administrators Posts: 29,801 admin
    Re: Size issues

    Remembering that Power=Voltage*Current --- And that there is a minimum voltage at which everything needs to operate.

    Basically, for a given distance, you can, roughly, send 4x as much power at 24 volts vs 12; and 8x as much power at 48 volts vs 12 volts for the same wire...

    For example, say you want to run a 1,200 watt inverter off of 12 volts:
    • 1,200 watts * 1/10.5 cut off * 1/0.80 eff * 1.25 NEC safety factor = 179 amp circuit rating
    • 1,200 watts * 1/42 cut off * 1/0.80 eff * 1.25 NEC safety factor = 45 amp circuit rating
    Get a 2,400 watt or 5kW 12 volt inverter on your system--and you are looking at some series copper to carry all of that current.

    Another issue--A 12 volt battery will run down to ~11.5 under load pretty easily--A 12 volt inverter will cutoff around 10.5 volts--Gives you 1.0 volt of drop across wiring/breaker/fuse to inverter.

    A 48 volt inverter will cutout at 42 volts, and the battery will run down to 46 volts under load--that gives you 4 volts of voltage drop to work with...

    Otherwise--the cost of XXXX lbs of battery at 12 volt vs configured at 48 volts is pretty much the same.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • the dudethe dude Registered Users Posts: 5
    Re: Size issues

    Thank you, I understand this a little better now. That gives me direction. Very helpful.
    BB. wrote: »
    Remembering that Power=Voltage*Current --- And that there is a minimum voltage at which everything needs to operate.

    Basically, for a given distance, you can, roughly, send 4x as much power at 24 volts vs 12; and 8x as much power at 48 volts vs 12 volts for the same wire...

    For example, say you want to run a 1,200 watt inverter off of 12 volts:
    • 1,200 watts * 1/10.5 cut off * 1/0.80 eff * 1.25 NEC safety factor = 179 amp circuit rating
    • 1,200 watts * 1/42 cut off * 1/0.80 eff * 1.25 NEC safety factor = 45 amp circuit rating
    Get a 2,400 watt or 5kW 12 volt inverter on your system--and you are looking at some series copper to carry all of that current.

    Another issue--A 12 volt battery will run down to ~11.5 under load pretty easily--A 12 volt inverter will cutoff around 10.5 volts--Gives you 1.0 volt of drop across wiring/breaker/fuse to inverter.

    A 48 volt inverter will cutout at 42 volts, and the battery will run down to 46 volts under load--that gives you 4 volts of voltage drop to work with...

    Otherwise--the cost of XXXX lbs of battery at 12 volt vs configured at 48 volts is pretty much the same.

    -Bill
  • BB.BB. Super Moderators, Administrators Posts: 29,801 admin
    Re: Size issues

    Another recommendation a few of us toss around here is to limit paralleling of battery banks--For several reasons.

    The first is parallel strings of flooded cell batteries means that many more cells to monitor electrolyte levels.

    Another issue is cost of wiring... Each parallel string should have its own series fuse to protect the wiring against shorts... That adds up for hardware (heavier gauge wire, larger fuses/holders/etc.).

    And, paralleling batteries can run into issues of current sharing... Batteries and their wiring are very low resistance... And extra length of wire in one string, a slightly loose/dirty connection in another--and that string is no longer carrying 1/n current--is carrying very little of the load and the rest of the strings are carrying the bulk of the current.

    How to wire batteries for balanced current sharing

    Also, cells sometimes fail shorted (can discharge the whole bank) or open (parallel string not carrying it share of current). It can be more difficult to notice/debug a bunch of parallel battery strings. Some thing like a DC clamp on Amp Meter can help (here is a $100 DC camp meter that is probably good enough--You can also use a DMM set on 200 mV scale and measure the voltage drop across identical lengths of cable for equal voltage drop when the bank is under heavy load/charging).

    Towards that end--my own personal opinion is to limit parallel battery strings to about 3 maximum. Not to say that many people here have paralleled many more batteries and are very happy with the results.

    If you need more amp hours and have to move batteries by hand (one or two people, no crane/forklift access)--A couple vendors (here or there) are now offering 4 volt and 2 volt cells in standard sizes that are not too heavy (not like a huge 24 volt or 48 volt fork lift / traction battery).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • the dudethe dude Registered Users Posts: 5
    Re: Size issues
    BB. wrote: »
    Towards that end--my own personal opinion is to limit parallel battery strings to about 3 maximum. Not to say that many people here have paralleled many more batteries and are very happy with the results.

    Can the batteries be configured in series also? Or are you just saying that each parallel bank should not exceed 3 batteries? Expand on this a little for me if you can please. :D
  • BB.BB. Super Moderators, Administrators Posts: 29,801 admin
    Re: Size issues

    Series connected batteries is fine (up to 48 volts at least).

    Parallel connected batteries are difficult because of the current sharing issues.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • arkieoscararkieoscar Solar Expert Posts: 101 ✭✭✭✭
    Re: Size issues

    You should look at the XW units from Schneider/Xantrex. They will do what you want in one package. I have installed several and it's a good system.
    Where are you in AR? I'm in Marion County and could give you some support if it's not too far away. I'm working on two systems in the National Forest in Searcy County now.
  • the dudethe dude Registered Users Posts: 5
    Re: Size issues
    arkieoscar wrote: »
    You should look at the XW units from Schneider/Xantrex. They will do what you want in one package. I have installed several and it's a good system.
    Where are you in AR? I'm in Marion County and could give you some support if it's not too far away. I'm working on two systems in the National Forest in Searcy County now.

    We had decided on the Sunny Boy line, but I will look into these again before purchase. They have the charging integrated whereas the Sunny Boy needs another $3k "Island" unit. I was not sure about long term reliability of the Xantrex vs. the Sunny Boy line.

    I'm on the other side of Little Rock from you toward Mt. Ida. Love to hear more information on the Xantrex setups you have put together and how they last.
  • arkieoscararkieoscar Solar Expert Posts: 101 ✭✭✭✭
    Re: Size issues

    Some others may chime in but the XW line is most versatile inverter on the market that I know of. Originally Trace Eng., they were bought out by investors and became Xantrex several years ago and started marketing some pretty crappy chinese stuff but their SW and now XW lines were and are very high quality. They were recently bought by Schneider, a German company that also owns SquareD and have tried to distance them selves from the Xantrex consumer product line.
    Support has been spotty but that has been an issue with most companies, it seems. I'm hoping for improvement there.
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Size issues

    it would be good to hear from crewzer again as he is now working for them. it has been since the latter part of spring that we heard from him last so we don't know what the inside scoop is concerning the company and the products. i also tried to email him direct with no response.
  • mike95490mike95490 Solar Expert Posts: 8,490 ✭✭✭✭✭
    Re: Size issues

    Own an XW and no complaints so far (7 months old) Had a Xantrex GT inverter before, no complaints either. Being able to have 240V is handy for pumps, and high power devices.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

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