System sizing question

couchsachragacouchsachraga Solar Expert Posts: 84 ✭✭
Thank you all for your help getting me sorted battery venting wise - we appreciate it!

One of the concerns brought up was the amount of panels vs. the storage we had, and I've been reading other posts and want to plan for the future, but also monitor how the system works for now. Usage right now is generally at most 2 nights / days a week... and sometimes more like a weekend or two a month (unfortunately...). Monitoring the system so far (trimetric meter with battery specs input) seems to use at most 15% per day. I had planned on two days (25% per day, about 75AH), but it appears I may be able to get away with three. With my limited use (so far) the system is able to recharge during the week to 100%. Is this acceptable, or would it be better to recharge it faster?

System wise I have 6 panels, but they are 30 year old SX100 (SolarEx), which I have been led to believe are 30w each. I'm assuming on a good day I might be pulling in 150w from them.... Batteries consist of 2 Trojan 1275s (150AH each). Wired together with BIG cables to keep loss to a minimum. Though I initially planned to go to 4 batteries, if 2 keep me going for 3 days I'll likely stick with 2. The cabin is wired 12v, but I do have a small (MorningStar SureSine 300) inverter to run a Rheem propane instant hot water heater and the odd 110 item I might want to use. I've tried to keep the loads as small and efficient as possible. My "budget" worked out to 75AH a day, but so far as mentioned above it has been significantly less (water pump uses less than anticipated, and runs less (so far) than anticipated, and the freezer fridge also uses less than anticipated so far).

My thought currently would be to get two panels that produce 200+ w and replace my current system with them. I'd likely get a new controller as well though (one with a Low Voltage Disconnect, as my current system is without that), mostly to take advantage of the higher voltages in the newer panels (my controller is a SunSelector NDR30 (30 amp, similar to a unit now made by MorningStar but without the LV disconnect).

Thank you for any further thoughts and advice! The other route for me to consider is microhydro, as I do have the drop and water (100' drop or so, 60 gal a minute minimum flow (though I wouldn't want to take all the water). The line would be 1/4 mile though...and I'd likely bury some or all of it as we do get a bit cold in winter here.

Comments

  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: System sizing question

    Two main things you have to look at when sizing an array: enough peak current to charge the batteries in a reasonable amount of time and enough Watt hour capacity to cover usage.

    What you've got is 300 Amp hours @ 12 Volts. You say your DOD is only 15%, so that is approximately 540 Watt hours.

    Working backwards, 540 Watt hours accumulated in 4 hours would be 135 Watt hours per hour, and with typical efficiency (77%) that would mean 175 Watt array. So far your 200 Watt array idea is working fine.

    Assuming the middle-of-the-road 10% peak charge rate of 30 Amps @ 12 Volts and again typical efficiency you'd want a 467 Watt array. The charge rate you'd expect from 200 Watts would be about 12 Amps or 4%. That will be too low for satisfactory charging in my opinion (especially with loads on during charge time).

    However if you could up your panel allotment to something like a pair of KD 140's you'd have about 15 Amps peak current which would be the 5% recommended minimum. This is still pretty borderline. One more such panel would move you into the comfort zone.

    Usual disclaimer: rough calculations, just my opinion, anyone is free to disagree.
  • couchsachragacouchsachraga Solar Expert Posts: 84 ✭✭
    Re: System sizing question

    Thank you for your input - I'm waiting until next year so I can build up funds to move forward with what makes the most sense and not have to worry about cost excessively. That said, I don't want to overbuild the system either - I'm well aware that as it gets larger I'd be better off with micro-hydro (there is something to be said about producing power 24/7 after all!).

    One question - when you say "charge the batteries in a reasonable amount of time" - what is your definition? The ability to charge from 50% to 100% in one (hopefully sunny) day? Or less than a day?

    Thank you again!
  • BB.BB. Super Moderators, Administrators Posts: 32,656 admin
    Re: System sizing question

    Yes, recharge the batteries in a day or two (remember, you have the recharge + the loads for those one or two days too).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: System sizing question

    Well I like to think a reasonable amount of time is getting from 50% DOD to Float on a good, sunny day. How long and sunny your sunny day is will vary from place to place. Your usage may vary also; if no loads are drawn during charging it doesn't take as much to do that charging.

    An "unreasonable amount of time" is the old school method of "it will recharge eventually" which costs a lot in battery life. :roll:
  • couchsachragacouchsachraga Solar Expert Posts: 84 ✭✭
    Re: System sizing question

    I'm finishing up a solar hot water system here at the house (tied in to both the domestic hot water and one circuit of our heating system) and the folks doing the work may be able to give me a deal on some panels. Not sure what brand yet.

    So I may move up my upgrade plans. Though the system has been working great, it admittedly takes a few days to recharge (from 70% usually two days, 30% DOD, so staying well above 50% so far) and I would like to reduce that time.

    So, again, I have 2 batteries at this time, Trojan 1275's (150 AH each), so I have 150 "useable" AH is the way I look at it.

    I would replace my current (borrowed) panels completely. If possible sticking with my same 30 amp charge controller.

    The panels are apparently 150 w. I'm thinking of purchasing 3, perhaps 4, which if I understand the helpful information a few of you shared with me last time brings me up to an acceptable level (450 or 600 watts).

    Questions 1) Does this sound good, and 2) Anything I should be considering when wiring it all up (gauge of wire, etc...). Panels are roughly 10' (+-) from the charge controller. I'm running a 12v system and for now (especially at this small size) expect to continue to do so.

    Thank you all again for your help - I think I'm on the right track from reading a bunch of other threads but don't want to do anything stupid.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: System sizing question

    On a 12 Volt system a 30 Amp PWM charge controller will be maxed out around 3 to 4 of those panels (assuming Imp of about 8 to 8.5). That's the number you have to check on the panels with that type of controller. So probably 3 panels, 450 Watts. It's about right for that much battery too.
  • couchsachragacouchsachraga Solar Expert Posts: 84 ✭✭
    Re: System sizing question

    Cheaper too:)

    I realized that after I typed it... 600 watts needs closer to a 45 or 50 amp controller (if I understand things correctly). Good to know 450 should be adequate - I was thinking 480-570 was my sweet spot. I'll post up info on the panels when I have it. And until then check out some charge controllers from our host:)
  • couchsachragacouchsachraga Solar Expert Posts: 84 ✭✭
    Re: System sizing question

    My one concern with these panels is I'm assuming they may be too high voltage for my older controller, and I'd may need to get a newer style (likely with MPPT). If I go that route I may go back up to 4 panels and feel better about the ability to add batteries (and more panels) if needed.

    Slippery slope it is!
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: System sizing question

    A PWM type controller usually does not care what the input Voltage is. Usually. They can take "24 Volt" panels and connect to a 12 Volt system; the power available from the higehr Voltage is simply lost. The PWM can't make use of it; it only passes the current.

    A 150 Watt panel is likely to be a "12 Volt" with a Vmp of around 17.5 and therefor should work with any controller on a 12 Volt system.

    That said, it's always a darn good idea to have all the specs for panels and controller just so there aren't any surprises. :roll:
  • couchsachragacouchsachraga Solar Expert Posts: 84 ✭✭
    Re: System sizing question

    ... especially as i just found out they are 255w....!
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: System sizing question
    ... especially as i just found out they are 255w....!

    That is a very big difference indeed!
    Now you're looking at a Vmp around 30, I'd bet. That means you'd need the MPPT function to utilize them properly on a battery system.
  • SkippySkippy Solar Expert Posts: 308 ✭✭
    Re: System sizing question
    ... especially as i just found out they are 255w....!

    I was going to order a couple of large output panels from a dealer, then I noticed a small disclaimer at the bottom of the screen : "For grid interie use only" . . . apparently the panels I was looking at only came in really high output voltage for intertie use. . .

    Just something to keep an eye on. . .
    2 - 255W + 4 - 285W PV - Tristar 45 MPPT CC / 3 - 110W PV -wired for 36V- 24V Sunsaver MPPT CC / midnite bat. monitor.
    1 KW PSW inverter 24V / 2.5 KW MSW inverter-24V ~ 105 AHR battery.
    3 ton GSHP.- 100 gallon warm water storage / house heat - radiant floor / rad
    9 -220W PV - net meter - Enphase inverters and internet reporting system.
    420 Gallon rain water system for laundry.***  6" Rocket Mass Heater with 10' bed for workshop heat.
    Current project is drawing up plans for a below grade Hobbit / underground home.
  • couchsachragacouchsachraga Solar Expert Posts: 84 ✭✭
    Re: System sizing question

    Skippy - I suspect the primary use of these panels is indeed for intertie use.... which is not my intention obviously. Will a MPPT charge controller handle them, or should I give up on them? I know, specifics of what the panels are is needed... but in general.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: System sizing question

    In general an MPPT type controller can down-convert higher Voltage to proper system charging Voltage. Most "top end" MPPT controllers can handle 150 Volts max. MidNite Classics are available in models that will handle 250 Volts. Xantrex XW 600 MPPT 80 can take 600 Volts.

    You can see the why of these panels: 30 Vmp * 12 = 360 Volt 3kW string, which is fine for most central GT inverters.
    For battery systems you'd use strings of one for 12 Volt, two for 24 Volt, or three for 48. The further from battery Voltage the array is, the less efficient the controller runs.
  • couchsachragacouchsachraga Solar Expert Posts: 84 ✭✭
    Re: System sizing question

    I think I follow all that - here's what I get out of it:
    1) MPPT controller is needed to make these work, and stout ones at that.
    2) Each panel set up independently (wired individually back to the controller, not in parallel)

    I'll have to do some calculations once I find out the full specs of the panels in terms of how many I can use for a given controller. That is a lot of voltage.

    Distance is short - 10 to 15' max, thankfully
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: System sizing question
    I think I follow all that - here's what I get out of it:
    1) MPPT controller is needed to make these work, and stout ones at that.
    2) Each panel set up independently (wired individually back to the controller, not in parallel)

    I'll have to do some calculations once I find out the full specs of the panels in terms of how many I can use for a given controller. That is a lot of voltage.

    Distance is short - 10 to 15' max, thankfully

    You can use these panels with any MPPT controller that can handle the Voltage and current. If the Vmp is 30, then the Imp will be 8.5.
    One of these panels would "max out" a Morningstar 15 Amp MPPT controller on a 12 Volt system.

    But for the bigger controllers they could be wired in any combination of series and/or parallel so long as the controller's maximum input Voltage is not exceeded, the Voltage is above battery Voltage, and the output current doesn't work out to too much above the controller's maximum.

    So for example a Morningstar 45 Amp MPPT controller on a 12 Volt system could take three of these panels, either all in parallel or all in series.

    Here's a piece about MPPT controllers and panel configurations: http://forum.solar-electric.com/showthread.php?16241-Different-Panel-Configurations-on-an-MPPT-Controller
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