Best Bang for the Buck Build

SupraLanceSupraLance Posts: 21Registered Users ✭✭
edited February 8 in Solar Beginners Corner #1
This is to supplement a generator off-grid, searching for the greatest watts produced per dollar spent.  I'm aware this is not normally the way it is done...

I already have 2x SLIGC115 230ah 6v batteries that are in use @12v (charged via generator).  Since it's a bad idea to mix 2 new and 2 old batteries, I plan to run at 12v until these batteries quit me, and then upgrade to 24v.  If I understand correctly, I need to send 10-13% so 23-30a charging current to these batteries to charge them optimally.  So I need between [email protected]=331w and [email protected]=432w of actual solar production to charge these batteries without using the generator...  Is that correct?

After plenty of research, I believe the best bang for the buck is going to be with a 40a mppt controller.  The lower cost of panels you can use with mppt pays the difference over pwm, and the 40a models are only marginally more than 30a or 20a.  Larger 45a or 60a models are a huge jump in price.  A $139 EpEver Tracer 4210a MPPT controller is the best value I've found with decent reviews.  For another $50 I can get the 4215bn that allows higher Voc & better cooling, but I'm not sure it's needed?  I suppose that may depend on the next question... 

Going for the most value per dollar spent will mean maxing out or even over-dimensioning the controller.  It can output [email protected] and [email protected] and can handle 150% over-dimensioning.  But how do I figure out the point at which adding more pv is not worth it (due to clipping etc)?   Any tools out there to help optimize over-dimensioning and get the most out of the controller without killing it too soon?

Which panels to use is based mostly on lowest $/watt.  But in a case where that is equal, is it cheaper/better to install three(3x) 42lb 60cell panels or two(2) 61lb 72cell panels?  It would obviously be easier to get the smaller panels up on the roof, but am I making more work for myself in other ways and increasing wiring/racking costs too much?

I'm leaning towards $102 EcoSolargy 230w panels from SEP [I don't know if it's ok to link to there??]  because they seem to be the best value based on $/watt.  But I'm confused because SEP ranks them near the bottom of page 1 when they list panels by best value.  Am I missing something that makes the other panels a better deal?  Is there somewhere else I should be looking for small purchases of less than a pallet?



Comments

  • PhotowhitPhotowhit Posts: 4,662Solar Expert ✭✭✭✭
    Off grid system must be wasteful, they have to regularly fully charge the battery bank and shut down when full. To best optimize them(most watts) you must run loads when the charge controller reaches float.

    Best "Bang for the Buck" wattage wise will always be grid tied systems.
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Prosine 1800 and Exeltech 1100, ForkLift battery. Off grid for @13 of last 14 years. 1000 watts being added to current CC, @2700 watts to be added with an additional CC.
  • EstragonEstragon Posts: 2,608Registered Users ✭✭✭✭
    150% is about as high as you'd want to overpanel. Keep in mind that panels are rated based on STC (Standard Test Conditions) in a lab, not real life field conditions. A properly tilted panel will likely produce ~75% of STC in many locations. A cold, high altitude location could do better, hot hazy sea-level location less so.

    As well as making up for environmental factors, overpanelling also helps get at least some useful power on lightly overcast days.

    Offsetting these advantages is the likelihood that running a controller at near rated output (hot) all the time may reduce lifespan. It may also be possible that massive overpanelling could result in too much current hitting the controller faster than it can react and protect itself.

    Midnite solar string calculator warns at 120%.

    The big panels aren't that heavy, but definitely a two person job to move safely. In either case, it would be worth checking on what's available locally as freight can be prohibitive.
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • littleharbor2littleharbor2 Posts: 979Solar Expert ✭✭✭✭
    edited February 8 #4
    If you decide on the 4215 version you would be safe with 2, 72 cell or 3, 60 cell panels in series. If you go with the 4210 you wont be able to series wire any 72 cell panels and only 2 60 cell panels, safely. The exception here would be if you are in a tropical or subtropical area where you don't get very cold temps. Spend the extra money on the 4215 and  have the flexibility to go with up to 3, 60 cell panels in series for a 12 volt system or 3s, 2s2p, or 3s2p for a 24 volt system.

    2.1 Kw Suntech 175 mono, Classic 200, Trace SW 4024 ( 15 years old  but brand new out of sealed factory box Jan. 2015), Bogart Tri-metric, 700 ah @24 volt AGM battery bank. Plenty of Baja Sea of Cortez sunshine.

  • SupraLanceSupraLance Posts: 21Registered Users ✭✭
    edited February 8 #5
    Good point Littleharbor, but would running the 60cell's 3s2p vs 2s3p (when I upgrade to 24v) reduce wiring/installation costs and electrical losses enough to be worth the $50 extra over the life of the controller (hopefully 10years)?  Or perhaps the value is in extending the life of the controller by spending the extra $50?

    That's exactly what I plan to do, Photowit: The batteries will only accept so much charge and then it's time to "use or lose" the electricity being produced in the afternoon.  Tieing to the grid makes sense once the electrical use exceeds a certain amount (there is a monthly connect fee, and significant cost to connect).  But below a certain level of electrical usage, depending on connection cost, solar is cheaper (at least fences/signs, I think it extends to other minimal use).  Part of what I'm doing here is finding that threshold.

    Estragon, I will likely be picking the panels up myself from the warehouse.  I assume 2 72cell panels are cheaper to rack/wire than 3 60cell, but about how much cheaper?  I just don't have the experience to know...

    edit** I'm making a thread on over-dimensioning in the technical area for math discussions, since it's probably not really a newbie discussion, although I still am, lol.

    I appreciate any and all thoughts and opinions...

  • PhotowhitPhotowhit Posts: 4,662Solar Expert ✭✭✭✭
    Look at where you want to end up, and try not to spend money on something like a 40 amp charge controller, when you will end up with a 60-90amp need. 

    If you never intend to use a greater than 24 volt system, you should consider PWM charge controllers and 72 cell panels. Y-solar makes pretty good units cheap, that are sold under many other company names.

    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Prosine 1800 and Exeltech 1100, ForkLift battery. Off grid for @13 of last 14 years. 1000 watts being added to current CC, @2700 watts to be added with an additional CC.
  • littleharbor2littleharbor2 Posts: 979Solar Expert ✭✭✭✭
    Good point Littleharbor, but would running the 60cell's 3s2p vs 2s3p (when I upgrade to 24v) reduce wiring/installation costs and electrical losses enough to be worth the $50 extra over the life of the controller (hopefully 10years)?  Or perhaps the value is in extending the life of the controller by spending the extra $50?

    I believe 3s2p would save you money in that 1, you can use smaller wire to carry the higher voltage/ lower amperage from your array to your CC. And 2, you don't need any fusing or combiner with only 2 strings, a simple Y branch parallel connector would be all you need, or you could parallel in a simple transition box if you want to change from PV wire to regular stranded wire in conduit.


    2.1 Kw Suntech 175 mono, Classic 200, Trace SW 4024 ( 15 years old  but brand new out of sealed factory box Jan. 2015), Bogart Tri-metric, 700 ah @24 volt AGM battery bank. Plenty of Baja Sea of Cortez sunshine.

  • SupraLanceSupraLance Posts: 21Registered Users ✭✭
    I think your right Littleharbor, I'll go with the 4215bn instead.  It has better cooling as well and I think heat will be an issue with over-dimensioning.  I think if I end up needing 80a later, it will still be cheaper to use 2 of these rather than a single 80a mppt.  Pwm might offer some savings at 24v, but it doesn't at 12v due to more expensive per watt panels. 

    So, as for which panels to buy and how many for the 40a 520w mppt.... 
    Any of you familiar with the SEP website?  Any idea why they don't consider the $102 EcoSolargy 230w panel to be the best value?  I calculate it to be the lowest dollar/watt, but I must be missing something because they sort a bunch of stuff ahead of it by best value...  What am I not considering that they are?
  • littleharbor2littleharbor2 Posts: 979Solar Expert ✭✭✭✭
    This panel could be the best price per watt because it is old stock. 230 watt 60 cell panels were manufactured probably 6 - 8 years ago. Efficiency levels have gone up since then. There are 60 cell panels these days from 285 watts to over 300 watts.

    2.1 Kw Suntech 175 mono, Classic 200, Trace SW 4024 ( 15 years old  but brand new out of sealed factory box Jan. 2015), Bogart Tri-metric, 700 ah @24 volt AGM battery bank. Plenty of Baja Sea of Cortez sunshine.

  • EstragonEstragon Posts: 2,608Registered Users ✭✭✭✭
    Could also be new stock using inferior silicon? I think cells are tested and sorted by output in manufacturing. The better ones go into 275+w panels, the lesser ones in lesser wattage panels?

    If space, wiring, and racking costs aren't factors, lower output panels could well be a good buy. In most cases, these are factors, so I'm not surprised the less efficient panels would be a lower $/watt.
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
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