Help Sizing Small Off Grid System

nch18
nch18 Registered Users Posts: 11
Hey everyone, I've gotten as far as I can alone and would appreciate some feedback. I took a short solar course several years ago but it's more complicated than I remember now that it's real money and I have real questions. Thanks in advance!

I'm moving completely off grid in a very rural area, no power poles and no option for grid power (very much by design).

I've calculated my needs at 1,500-2,000 wh/day, all AC with the exception of 2 small DC pumps. I also have a generator and am not afraid to use it (to either charge batteries here and there or connect direct to the house panel if the need for larger, short term loads were to arise).

I can break down the loads if helpful but basically it's just LED lighting, laptop/phone charging, two fans, stereo, and super efficient fridge all in a 1000 sq. ft. house. Heating with wood, cooking with gas, no AC, no freezer, no washer/dryer, etc.

I'm allowing 2 days of autonomy and get around 4 peak sun hours in the winter.

I have 6 Evergreen 215w, 24v panels. According to my calculations I only need 4 of them but it was a good deal.

I also had 3 AGM batteries (12v, 140ah) kind of fall in my lap. They are two years old and were used in a commercial security system, kept charged. Batteries are the only thing I don't like to buy used but they were $50 each.

I also have little money at the moment after buying the place.

So, my questions for the fine denizens of wind-sun:

I'm going to need a 4th battery to get to 24v I suppose. I know you shouldn't mix and match batteries but if I buy the same battery, can I mix in a new one with a few older ones of the same model? I can't really afford more than 1 additional AGM and have heard you shouldn't mix chemistry.

Secondly, that will only yield a battery capacity of 280ah @ 24v which, according to my math, will probably only do me for around 1,500wh/day, on the low end of my estimation. Before finding these batteries, I was shooting for at least a 320ah @ 24v battery bank.

Next, if I already have 6 panels, should I go ahead and hook them all up, even though 3 is probably enough for my little battery bank according to my charts? I only have my worksheets for ideal sizing to go by but what are the practical effects of having an oversized array relative to battery capacity?

Now I'd like to take a minute to tell yall how all of this would work in the fantasy land of my uninformed and inexperienced mind. Please be gentle as you crush my naive dreams...

I would hook up between 3 and 6 of my panels (pending feedback on above) to my 4 AGM batteries giving me enough power for the first few years, supplemented by a generator. When my batteries start to go, 3 of which are of mysterious vintage and could theoretically go out at any point, I would replace them with a 500+ah bank more suitably matched to my 6 panels. I would be really clever though and buy a charge controller and inverter that could handle both my initial 3 panel (645 watts @24v) array and small (280ah @24v) battery bank effectively and not mind when array (1290 watts @ 24v) and battery bank (500+ah @ 24v) double in a few years.

Feasible? Suggestions?

Thanks!

Comments

  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    Re: Help Sizing Small Off Grid System

    Not. A lot of time to reply in depth. A couple. Of quick rules about off grid. First, loads always grow with time, second people nearly always underestimate thier loads, and third, people almost always overestimated thier colar input.

    All calculations begin with the loads, so defining the loads carefull avoids (largely) the bigger financial pitfall of off grid,, ready, fire, aim.

    Finally, my rule of thumb is take the name plate rating of the PV, divide in 1/2 to account for all cumulative system losses, then take that number and multiply by four to account for the AVERAGE hours of good sun one can reasonably expect per day' on averge over the course of the year. So, for example 1000 watts of PV might look like this. 1000/2=500*4=200 WH/day. You might be able to fudge the number a bit, but not very far depending on circumstance.


    And finally ( again) to be truly successful, you need to well balance the loads, to the PV to the battery bank size.

    Welcome, and keep in touch,

    Tony

    PS. It might also be important to tell us if this is a full time residence, weekends, holiday weeks etc.

    T
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Help Sizing Small Off Grid System

    Welcome to the forum NCH.

    I will take a stab at your questions... But, I really like to start from "scratch" and define your loads/location/seasonal needs and go from there.
    nch18 wrote: »
    I've calculated my needs at 1,500-2,000 wh/day, all AC with the exception of 2 small DC pumps. I also have a generator and am not afraid to use it (to either charge batteries here and there or connect direct to the house panel if the need for larger, short term loads were to arise).

    Sounds like a reasonable starting point--I suggest that 3.3 kWH per day (100 kWH per month) or less, is a good goal for off grid living.
    I can break down the loads if helpful but basically it's just LED lighting, laptop/phone charging, two fans, stereo, and super efficient fridge all in a 1000 sq. ft. house. Heating with wood, cooking with gas, no AC, no freezer, no washer/dryer, etc.

    The fridge is the one wild card--Do you have a WH/kWH per day load number in warm weather for the fridge (and how many cuft)?
    I'm allowing 2 days of autonomy and get around 4 peak sun hours in the winter.

    Where, roughly, will the location be? 4 hours per day of "noon time" eqivalent sun is still quite a bit to expect for many locations.
    I have 6 Evergreen 215w, 24v panels. According to my calculations I only need 4 of them but it was a good deal.

    Come back to that later...
    I also had 3 AGM batteries (12v, 140ah) kind of fall in my lap. They are two years old and were used in a commercial security system, kept charged. Batteries are the only thing I don't like to buy used but they were $50 each.

    It is tough... An "odd" number of batteries for an "unknown" size of system. Perhaps, use them for a smaller system (like on an RV), or even for heavy duty truck battery(ies).
    I also have little money at the moment after buying the place.

    Came to the right place--Most of us here were born cheap and went through the school of hard knocks. We want systems to perform to expectations and people to understand what their expectations will require design wise.
    I'm going to need a 4th battery to get to 24v I suppose. I know you shouldn't mix and match batteries but if I buy the same battery, can I mix in a new one with a few older ones of the same model? I can't really afford more than 1 additional AGM and have heard you shouldn't mix chemistry.

    That is a tough one--The conservative assumption would be--Buy the fourth battery and assume that you replace all batteries (most likely) when the other three start to fail. You can replace the other three--but then when will the fourth fail?
    Secondly, that will only yield a battery capacity of 280ah @ 24v which, according to my math, will probably only do me for around 1,500wh/day, on the low end of my estimation. Before finding these batteries, I was shooting for at least a 320ah @ 24v battery bank.

    If you have a (hopefully) good quality / less expensive bank--Then why not cycle it more deeply and see what happens. Your other choice is to buy more flooded cell or AGM batteries anyway and you still have 3-4 orphans to figure out what to do with.
    Next, if I already have 6 panels, should I go ahead and hook them all up, even though 3 is probably enough for my little battery bank according to my charts? I only have my worksheets for ideal sizing to go by but what are the practical effects of having an oversized array relative to battery capacity?

    Over a ~13% rate of charge--Battery bank may get hot (especially if deeply cycled and you have 13% to 25% rate of charge for flooded cell batteries). So, a remote battery temperature sensor is recommended for such a solar charge controller (hot batteries, depress charging voltage; low charging voltage, controller thinks batteries need more charging/charging current, batteries get hotter--Thermal runaway. RBTS helps stop that from happening).

    For AGM batteries, many can take very high charging and discharging currents (some brands/models up to C*4 rate -- 15 minute charging/discharging--requires lots of heavy copper cable to do that).

    The downside (that I have read here from other problems/experiences), when you get above ~C/2.55 rate of charge (40%) and MPPT charge controllers--MPPT controller "search" for Pmax=Vmp*Imp by pulling the maximum current from the solar array. If the battery is near full charge and the controller pulls maximum current, it appears that "too small" of AGM or flooded cell will allow the battery bus voltage to go way high (over 72 volts on a 48 volt battery bank has been reported)... So, I would use C/2.5 as my maximum rate of charge for all battery types. And C/2.5 as the max rate surge for flooded cell batteries.
    Now I'd like to take a minute to tell yall how all of this would work in the fantasy land of my uninformed and inexperienced mind. Please be gentle as you crush my naive dreams...

    I would hook up between 3 and 6 of my panels (pending feedback on above) to my 4 AGM batteries giving me enough power for the first few years, supplemented by a generator. When my batteries start to go, 3 of which are of mysterious vintage and could theoretically go out at any point, I would replace them with a 500+ah bank more suitably matched to my 6 panels. I would be really clever though and buy a charge controller and inverter that could handle both my initial 3 panel (645 watts @24v) array and small (280ah @24v) battery bank effectively and not mind when array (1290 watts @ 24v) and battery bank (500+ah @ 24v) double in a few years.

    Feasible? Suggestions?

    Thanks!

    I like to design a "balanced system"... In your case, I will suggest a balanced design and then see if you can shoe-horn in your existing system and when battery bank replacement time comes, just drop in the new bank (in months or years--as you say, we have no clue).

    Call it 2,000 WH per day, 4 hours of sun minimum (for 9+ months a year, other 3 months use generator during bad weather) with flooded cell bank and AC inverter. Assume 2 days no sun (1-3 days suggested), and 5% to 13% rate of charge.
    • 2,000 WH * 1/0.85 inverter eff * 1/24 volt battery bank * 2 days no sun * 1/0.50 max discharge = 376 AH @ 24 volt battery bank

    Next, rate of charge from solar of 5% to 13% and 0.77 panel+charger derating:
    • 376 AH * 29 volts charging * 1/0.77 solar derating * 0.05 rate of charge = 708 Watt array "minimum"
    • 376 AH * 29 volts charging * 1/0.77 solar derating * 0.10 rate of charge = 1,416 Watt array "nominal"
    • 376 AH * 29 volts charging * 1/0.77 solar derating * 0.13 rate of charge = 1,841 Watt array "cost effective maximum"

    And your energy usage based on 4 hours of sun per day and 0.52 system derating:
    • 2,000 WH * 1/0.52 end to end losses * 1/4 hours of sun per day = 962 Watt Array minimum (4 hours of sun)

    So for a "balanced system, I would be suggesting around 962 to 1,841 watt array. Your 1,290 watt Evergreen array seems like a "good" fit.

    Now, if this was a 376 AH @ 24 volt flooded cell system--The size of inverter I would be suggestion would be around:
    • 376 AH * 24 volts * 0.85 inv eff * 1/8 rate of sustained discharge = 959 Watt Inverter (sort of max continuous rating)
    • 376 AH * 24 volts * 0.85 inv eff * 1/2.5 rate of sustained discharge = 3,068 Watt Inverter (max surge rating)

    Since inverters can have around 2x rated power for short surges, then the maximum cost effective inverter would be:
    • 3,086 Watts / 2 = 1,534 Watt maximum "useful" inverter...

    So, a 1,000 to 1,500 or 2,000 watt 24 volt inverter would seem to be a good fit.

    Your AGM bank, checking the maximum current based on C/2.5 would be:
    • 280 AH * 29 volts charging * 1/0.77 panel+controller derating * 1/2.5 max surge charging = 4,218 maximum solar array

    So, you do not appear to be in "any danger" with your ~1,290 watt solar array and a 280 AH @ 24 volt AGM battery bank.

    For picking the "optimum" charger for use with a small genset, here is a good thread to read:

    Question about battery charger selection with EU2000 generator.


    A non-optimum charger, assuming 0.67 power factor, 80% efficiency and 10% rate of charge (you can go from 5% to 25% rate of charge):
    • 376 AH * 29 volts charging * 1/0.80 charging efficiency * 1/0.67 PF * 0.10 rate of charge = 2,034 VA rated genset load
    • 376 AH * 0.10 rate of charge = 37.6 amps @ 24 volts...

    So, you can go from 1/2 that to 2.5x that amount (VA/Amp Rating) for your genset.

    If you go looking for a PF corrected charger (or inverter/charger), the 1/0.67 (1.49) number "goes away" from the above equation... (i.e., the generator has to be 1.49x larger because of the "typically" poor power factor of many standard AC battery chargers).

    It does not cost you more fuel to run "poor power factor", but you have to have a larger genset and you cannot run other AC loads if the total VA of battery charger plus other AC loads exceed genset rating.

    Questions?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Photowhit
    Photowhit Solar Expert Posts: 6,002 ✭✭✭✭✭
    Re: Help Sizing Small Off Grid System

    Very likely your solar panels are NOT 24 volt panels, to be made to charge 24 volt battery bank they need a VMP of at least 33 volts and 35 is a better reality. If they are in that mid range of panels with a VMP of 25-32 volts, you'll need a MPPT charge controller to get close to the panel's rated wattage.

    What is your largest load, including the type of load, likely one of the pumps? Since the other problem is that you don't mix old and new batteries, you might be able to get away with a 12 volt system, but most likely you'll want to sell of the batteries and buy some flooded lead acid golf cart batteries, likely the best bang for the buck...
    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.
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Help Sizing Small Off Grid System

    This can be done. It is not unreasonable.

    Couple of things. First, the 215 Watt Evergreens I found while looking for specs are 18.4 Vmp. Usable on a 12 or 24 Volta system with a PWM controller or an MPPT controller. Check the specifications on your panels.

    Second, like Photowhit asked: what is your biggest load (peak Watts)? If it really is small (1 kW or less) you can do this on a 12 Volt system without difficulty.

    Configuration:

    2000 Watt hours per day on 12 VDC is approximately 167 Amp hours, or a 334 Amp hour battery bank at 50% DOD. You've got 3 * 140 or 420 Amp hours. You'd be hitting about 40% DOD which is not too unreasonable (batteries will need to be replaced sooner). Some of your daily load is bound to come from the panels directly.

    Those 18.4 Vmp panels are about 11 Imp (if they are the ones I found). Four of them in parallel on a 45 Amp Morningstar TriStar would work for that battery bank. That would be an 860 Watt array and it should yield about 1788 Watt hours AC with 4 hours sun. This is not too far off the mark for estimated loads.

    Other configurations are possible, but much depends on the exact panel specs, how precise the load calculations are, your site's insolation, et cetera.

    For what it's worth I can squeeze 2800 Watt hours out of 700 Watts of panel in Summer. Winter? Forget it; we drop 10 hours of daylight between the two. I use 232 Amp hours @ 24 Volts and much load shifting to make use of "extra" power the panels are capable of after the batteries are charged. I also make use of a Honda EU2000i when the weather does not co-operate! I would like to increase both the battery and panel capacity by about 2X, just so you know.
  • nch18
    nch18 Registered Users Posts: 11
    Re: Help Sizing Small Off Grid System

    Thank you all so much!! That's an incredible amount of detailed guidance and I really appreciate it from the bottom of my heart. I hope that 'what goes around comes around' and you all encounter the same generousity the next time you need a hand with something in life.

    It will probably take me until tomorrow to fully sift through everything and reseach the parts I don't understand but all in all I'm encouraged by what I'm hearing. I'm usually very thorough but my first post was so long and I was afraid if I included too much detail, no one would read it. But I'll go through and answer everyone's questions about loads and regional insolation, and all the nitty gritty details here in a bit.

    Thanks!
  • nch18
    nch18 Registered Users Posts: 11
    Re: Help Sizing Small Off Grid System

    Ok, just to flesh out my situation more...

    It's a permanent residence but I was originally thinking I might go without electricity for the first year or two anyway so it's not everyone's idea of permanent res. I've spent a decent amount of time in places with little to no electricity and generally know what I'm getting myself into.

    Insolation

    The nearest solar data I've found is from Raleigh, NC (at latitude):

    Avg Sun Hours: 5
    Jan:3.8 Feb:4.5 Mar:5.2 Apr:5.7 May:5.7 Jun:5.7 Jul:5.6 Aug:5.5 Sep:5.2 Oct:4.9 Nov:4.1 Dec:3.6

    Panels

    The solar panels may or may not be true 24v but they're not the ones Cariboocoot found. Here's a link to a brochure

    Evergreen ES-E-215-fc3
    Pmp: 215
    Vmp: 29
    Imp: 7.43
    Voc: 35.6
    Isc: 8.12

    Loads

    As for my load calcs, the largest connected item was a small vacuum rated at 1000W run for 1 hour a week (could be spread out over a couple days). Is that doable? I know it probably surges like crazy. I would also love to be able to use a blender maybe once a week for 5 minutes (upwards of 500W).

    The second potential problem area is the fridge. I want to do an energy efficient chest freezer with a thermostat to run it at fridge temps per Tom Chalko's designs. I haven't rigged one up yet so I can't verify his claims but everyone I've seen that's done it is pretty happy with it. The idea is just to have an efficient chest freezer rigged with a high temp cut off so that it only runs for around 90 seconds an hour and keeps things at fridge temps. I have no real way to factor this in accurately except that it supposedly will only pull around 150-250 watt hours a day (100 if you use a super efficient freezer like a sunfrost) and I have around 4 independent sources claiming this. Not sure how to tackle this one but I wasn't planning on having refrigeration originally so... anything's better than nothing and I'll figure something out.

    Other than that, I had daily calcs of: 175WH - lights (all 3.5W LEDs), 350WH - ceiling fan, 100WH - table fan, 300WH - laptop, 100WH - stereo 100WH - DC pump. And then just a bunch of little odds and ends that are either tiny draws like a cell phone charger, or rare-use items that can be load shifted like a sewing machine.

    It also seems that my loads are very summer dominant, fans won't run in winter much and the fridge should have an easier time as well so that might help.

    I have my total connected watts around 3000-3500W and my average daily load around 1500 WH.

    "Now" Setup

    I like the idea of just cycling my cheap batteries a little deeper, I should have thought of that. They claim a 10 year life span and were only $50 so even if I just get a couple years out of them I think that would be fine.

    But then the question is should I just wire the 3 batteries as a 12v system, use a really deep discharge, and have a super cheap and easy solution until they give out or buy one more battery, cycle just a little past .50 and have a 24v system?

    So with Bill's formulas, which are a little more conservative than the ones in my book, to get 1,500AH/Day with the 3 batteries in a 12V system, I'd have to cycle them to .70 DOD:

    1,500 WH * 1/0.85 inverter eff * 1/12 volt battery bank * 2 days no sun * 1/0.70 max discharge = 420 AH @ 12v battery bank which is exactly what I'd have.

    I really like the simplicity of going this route but my feeling is that I want a 24v system for long term scalability. I know a good charge controller will step down my 24v panels to a 12v bank but I think I want to get a 24v inverter so I only have to buy it once. I'd also be pushing the 3 batteries pretty hard (.70 DOD) so I'm feeling that one more battery would put both those concerns to rest - keep everything 24v and lowering the DOD to a more reasonable level (maybe .60 or so). It would mean having a single AGM orphan when the 3 older ones give out but I can find something to do with it.

    "Later" Setup

    I'd like my longer term system to be 2,500WH/Day:
    2,500 WH * 1/0.85 inverter eff * 1/24 volt battery bank * 2 days no sun * 1/0.50 max discharge = 490 AH @ 24v

    Which puts my array at:
    490 AH * 29 volts charging * 1/0.77 solar derating * 0.05 rate of charge = 923 Watt array "minimum"
    Which would put my 1,290W array between 'minimum' (923W) and 'nominal' (1,845W). I can always add a couple panels though.

    Now for inverters
    490 AH * 24 volts * 0.85 inv eff * 1/8 rate of sustained discharge = 1,250 Watt Inverter (sort of max continuous rating)
    490 AH * 24 volts * 0.85 inv eff * 1/2.5 rate of sustained discharge = 3,998 Watt Inverter (max surge rating)

    Which I guess would call for...
    3,998 Watts / 2 = 2,000 Watt maximum "useful" inverter...


    I don't really understand how the generator fits into the picture at the moment, currently researching. I have mutliple AC generators (gas, diesel, military, civilian, you name it) but nothing DC. My breaker panel has the ability to hook up the generator to run the house and I'm comfortable with that. But if I want to use an AC generator to charge my batteries, I would need an inverter/charger which is a special type of inverter, is that correct?


    So would an inverter like this meet my needs (or maybe not because it's modified sine, should I shell out for pure?)

    Xantrex Off Grid Inverter Charger TR2424


    Thanks to everone! just let me know if I overlooked anything...
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Help Sizing Small Off Grid System

    Okay Photowhit; you were right. :D

    Those panels are not "true" 24 Volt panels in that their Vmp is 29; below the level needed to properly charge a 24 Volt system. You will have to have an MPPT charge controller to use them, no matter what system Voltage.

    It will still work for a 12 Volt system as I described.
    I can understand your desire to go 24 Volt, especially if you plan on running 3000+ Watts all at once. Do not try that with a 12 Volt system! The DC current is high (250+ Amps).

    Beware of refrigerators. They can eat up power. Mine takes 1200 Watt hours a day on its own. It also has a autodefrost that takes 500 Watts (ordinary running is <130, with a start surge over 1000). Check the thread under energy use about small refrigerators; lots of real world data in there.

    Several of the items you want to run have AC induction motors. They will appreciate true sine wave. On MSW they will use more power, run hotter, and not last as long. No immediate catastrophic failure, but better on sine wave.
  • Photowhit
    Photowhit Solar Expert Posts: 6,002 ✭✭✭✭✭
    Re: Help Sizing Small Off Grid System

    Generators charge back through your inverter, the inverter you selected has a charger capability. MOdified sine wave inverters typically shorten the life of motors, fridge compressors would be a motor, they also run a bit less effeicently with motor producing heat, one of the things shortening the life of motors...

    If you've lived with poor/little electric, I think you'll adapt to solar very well.

    The panels aren't designed to work with the cheaper PWM charge controllers, You'll need a MPPT type controller to use them effeicently.

    Sorry, I started this reply before Coot had posted, so, "What Coot said..."lol.
    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.
  • nch18
    nch18 Registered Users Posts: 11
    Re: Help Sizing Small Off Grid System

    Ok cool, thanks for the clarification. Sorry for the competitor's link, wasn't thinking. Good to know about the panels, I just assumed all panels were nominal 12/24/48v etc but now I know better. I was planning on going MTTP anyway though so not too nasty a surprise.

    I just found a stupid good deal on a new Xantrax Prosine 1800/24 so I went ahead and got it. It doesn't have a charging feature but I'll figure that out later, I could buy a brand new DC gen for the money I saved. So I'm locked into 24v now but I feel good about that.

    One thing about charge controllers is kind of confusing me. My book has a footnote that MPPT feature will only work well if the "input amperage to the controller is lower than the rated output amperage to the battery." I've calculated my Array short circuit amps at 60.9 which I guess is the max "input amperage to the controller" bit. So does that mean that a 60 amp charge controller would be too small? I kind of had my eye on the 60 amp TriStar MPPT. Would that really be undersized? It looks like people routinely use it for larger arrays than mine :confused:

    I also tracked down one of the teachers from a solar course I took a while ago and he's going to help me out with wiring everything up. The fuses and breakers and battery monitors and wiring is still fuzzy to me but everything is slowly coming into focus.
  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: Help Sizing Small Off Grid System
    nch18 wrote: »
    Ok cool, thanks for the clarification. Sorry for the competitor's link, wasn't thinking. Good to know about the panels, I just assumed all panels were nominal 12/24/48v etc but now I know better. I was planning on going MTTP anyway though so not too nasty a surprise.

    I just found a stupid good deal on a new Xantrax Prosine 1800/24 so I went ahead and got it. It doesn't have a charging feature but I'll figure that out later, I could buy a brand new DC gen for the money I saved. So I'm locked into 24v now but I feel good about that.

    One thing about charge controllers is kind of confusing me. My book has a footnote that MPPT feature will only work well if the "input amperage to the controller is lower than the rated output amperage to the battery." I've calculated my Array short circuit amps at 60.9 which I guess is the max "input amperage to the controller" bit. So does that mean that a 60 amp charge controller would be too small? I kind of had my eye on the 60 amp TriStar MPPT. Would that really be undersized? It looks like people routinely use it for larger arrays than mine :confused:

    I also tracked down one of the teachers from a solar course I took a while ago and he's going to help me out with wiring everything up. The fuses and breakers and battery monitors and wiring is still fuzzy to me but everything is slowly coming into focus.

    If your panel output at a reasonable voltage (which is not too much less than Voc) adds up to more than 60Amps, then the extra power the MPPT CC will give you to the batteries (by transforming current A at high voltage to a larger current B at lower voltage) will not be usable since the output current of the CC is limited to 60Amp.
    But during part of the times of day/year when your panels are producing less than 60Amps, you will still be getting 60Amps of output to the batteries. Just don't go whole hog and put in a panel system which produces a lot more power than the CC can transfer to the batteries when they are discharged.

    People who use it for larger arrays than yours may be trying to get all of the power they can from limited sun hours. Or they may be using a higher voltage battery pack which can take more power from the arrays at the same maximum current (60 Amps.)
    SMA SB 3000, old BP panels.
  • Photowhit
    Photowhit Solar Expert Posts: 6,002 ✭✭✭✭✭
    Re: Help Sizing Small Off Grid System
    nch18 wrote: »
    One thing about charge controllers is kind of confusing me. My book has a footnote that MPPT feature will only work well if the "input amperage to the controller is lower than the rated output amperage to the battery." I've calculated my Array short circuit amps at 60.9 which I guess is the max "input amperage to the controller" bit. So does that mean that a 60 amp charge controller would be too small? I kind of had my eye on the 60 amp TriStar MPPT. Would that really be undersized?

    I like the prosine units and I think you'll like them too!

    The Charge Controller(CC) should be fine. They work on the amperage and for the most part you need only worry about presenting (inputing) too much voltage, Voltage Open Curuit (VOC) rather than Voltage Maximum Power (VMP). Likely you will need to run the panels in strings of 2 or 3 for the Tri Star MPPT Charge controller (be careful Morning star has similarly named PWM CC's)

    Since watts = Amps x Volts and your running a 24 Volt system... your looking at inputing 2 strings at 7.43A at 3x29V=87V or @15Amps max and outputing 215 watts x 6 for an array of 1290 watts / 24V = 53.75 Amps (your batteries may actually drop lower in voltage and your panels can produce more current on cold clear days, but you have a safe margin)

    You can check the CC to see what the max input voltage is, this will be the VOC X the number of panels in the string. 35.6V x 3 in this case might be too much.

    Other factors before you buy a CC will be how far your CC will be from the batteries, since higher voltages allow lower voltage drops on the same size wires.
    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.
  • nch18
    nch18 Registered Users Posts: 11
    Re: Help Sizing Small Off Grid System

    Thanks inetdog, that make sense. I think I've got it figured out now.

    Photowit, I actually saw that you had the same Prosine after I bought it and was pretty pleased with myself, that's gotta be a good sign.

    This is probably a stupid question but what determines how many strings I have. What made you choose 2 strings of 3 instead of using 3 strings of 2 to decrease the voltage somewhat? I get what you're saying in general though and it's made me realize that I was thinking of things wrong in terms of the input side of things. I didn't realize the freedom I had to play around with different voltages coming in.

    If anybody has any suggestions for a good CC for me I'm all ears as well. I just picked the morningstar out of a hat more or less. Anything cheaper that would do the job well?
  • Photowhit
    Photowhit Solar Expert Posts: 6,002 ✭✭✭✭✭
    Re: Help Sizing Small Off Grid System
    nch18 wrote: »
    This is probably a stupid question but what determines how many strings I have. What made you choose 2 strings of 3 instead of using 3 strings of 2 to decrease the voltage somewhat? I get what you're saying in general though and it's made me realize that I was thinking of things wrong in terms of the input side of things. I didn't realize the freedom I had to play around with different voltages coming in.

    Always nice to have 1 or 2 strings, this makes a combiner box(a fuse box for the panels) not required as one set can't overload the other in case of a short. It also cuts down on the wiring needed. In truth though MPPT CC generally(always?) work most efficiently with about double the voltage, but the savings in wire and fusing for a small gain wouldn't be worth it for most.

    I recently bought 2 midnite classic 150 lites, likely the best bang for the buck right now if your are computer literate, they have all the features of the regular Midnite classic's accept for arc fault protection (contact switches?) but with out the display. for standard setups you can use dip switches, you can display whats happening or do custom settings with a network link to your computer. At $500 it would also give you room to expand, as it will handle up to 94Amps.
    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.
  • nch18
    nch18 Registered Users Posts: 11
    Re: Help Sizing Small Off Grid System

    I think I've got it narrowed down to either the 150 or 200 MidNite Classic Lite. I've got my max volts at 126v (OC with all the safety and max temperature factors included). Is the 150 a good bet or should I go with the 200 just to be on the super safe side given that either one puts out more than enough current for me. Thanks for the tip, these look really good, like that they offer a stripped down version and like that they're made in usa. Thanks!
  • ywhic
    ywhic Solar Expert Posts: 621 ✭✭
    Re: Help Sizing Small Off Grid System

    I went to the Midnite Classic 150 (not lite).. the difference from NAWS is like $120.. and you'll have the graphic display on the front in-case you can't/don't have a computer near by to monitor with the Midnite 'Local App'.

    And it looks nice and neat..

    ControlPanelSmall.jpg

    Granted at $500 flat (for the Lite) the performance and ability to download firmware and monitor locally with the 'Local App' or via the internet cinches the deal.. it has dip switches to set it all up.. so really no computer is needed.. just the Lite and a Battery Tester.. LoL.. or you can later by the display if you want..

    For me I went from a Morningstar TS45 (PWM) to the Classic 150 for the GROWTH possibility and the meter front that showed in/out V/A at the same time.. and the MPPT to try and get more from the panels then the PWM was allowing..

    The Morningstar TriStar PWM and TriStar MPPT requires you to dig into the diagnostics menus to get the PV input voltage.. and its doesn't refresh unless you go in/out of that screen.. (so it stinks IMHO)..
  • nch18
    nch18 Registered Users Posts: 11
    Re: Help Sizing Small Off Grid System

    That's a nice clean install there, hope mine will look as good. I'm on the computer all the time so I think I'd probably do 99% of my interfacing via comp anyway whether I had the nice display or not. So I think the lite will be fine but I do like that I can add on the display later if my computer addiction wanes in the future.

    I guess I'll just go ahead and get the 150, seems to be the popular choice and I guess the extra amps can't hurt, maybe I'll add some strings in the future.