Good price on local panles..but how and if can be used?

nyoffgrid Registered Users, Users Awaiting Email Confirmation Posts: 6 ✭✭

I'll be using this all in one inverter for my little off grid panel -

I'll have a single 200ah 24v lifepo4 battery.

I've found some local cheap panels. They are 250 watt with 37.2 open circuit. Please correct me if I am wrong but since this inverter is able to do up to 1600w of panels and 100v then I can connect two sets of 3x panels each for a 74.4v and 1500w array and be OK?

If that's true.. Then if I were to have these about 200ft away from this inverter.. What size wire I need? 


  • BB.
    BB. Super Moderators, Administrators Posts: 33,500 admin
    edited June 10 #2
    First Question: What is the Vmp-panel of your 250 Watt Panels?

    For a ~37.5 Volt Voc-panel, the Vmp-panel (std conditions) would probably be around 32 VDC Vmp-panel

    The Vmp (and Voc) of panel varies with temperature... And temperature fall below freezing, the Vmp/Voc rise... So knowing the max cold temperatures you expect is helpful.

    As a first guess with a 100 VDC max input voltage, very roughly 2x panels in series would cover you pretty much down to well below 0F... Three panels in series (37.2 volts * 3 =  111.6 volts Voc-array-STD conditions... Too high for the 100 VDC Max MPPT controller input voltage...

    And we need to know the Imp of the panels--Again roughly:

    250 Watts Pmp / 32 volts Vmp = 7.81 Amps rated Imp per panel (guess)
    1,400 Max array wattage

    Using the numbers we have, the maximum array would be 2 panels in series.

    60 amps PV input max / 7.81 amps per panel/2 panel string = 7.68 strings ~ 7 strings max PV input current
    1,400 Watt max PV input / (2*250 Watt panel strings) = 2.8 ~ 3 (3 OK?) strings max (using 1,400 Watt array limit)

    Note this is a 24 VDC input inverter--So, my first guess is a single panel with Vmp~32 volts is a bit low for a typical MPPT controller and a 24 volt battery bank (as panels get hot, Vmp and Voc fall). So you are pretty much limited to:

    2 series panels * 1 string (500 Watt array) (2 panels)
    2 series panels * 2 parallel strings (1,000 Watt array) (4 panels)
    2 series panels * 3 parallel strings (1,500 Watt array) (6 panels) (slightly over panels)
    2 series panels * 4 parallel strings (2,000 Watt array) (8 panels) (over paneled)
    2 series panels * 5 parallel strings (2,500 Watt array) (10 panels) (over paneled)

    We may have a bit of confusion over panels connected in series (increases voltage) and parallel (increases current)... 2 panels in series (64 VDC Vmp est) * 3 parallel strings (Imp-array amps)

    I have not read the manual, and just taking a few quick guesses... Not sure why both the 1,400 Watt array limit and a 60 amp input current limit)... Anyway, that is my first cut at the numbers and configuration (and why).

    I have to go now. Lets get the above numbers/requirements clear first, then we can talk about PV Array wiring length.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • nyoffgrid
    nyoffgrid Registered Users, Users Awaiting Email Confirmation Posts: 6 ✭✭
    Thanks Bill. 

    Here are the panel specs:

    I thought I can connect 3 of these panels in parallel so my V stays the same and my amps go up and that I can do that twice so I have 6 panels: two sets of 3 each connected in parallel. And then those two sets are connected in series to increase voltage. Is that not a possible thing to do? I thought it will keep me inside the max mppt of that unit.  
  • BB.
    BB. Super Moderators, Administrators Posts: 33,500 admin
    If I understand you correctly, yes you could connect a set of three in parallel (2x sets), then connect those two in series and get the same voltage/current results...

    However, we usually like to make the base sets as 2x panels in series, then connect those in 3x parallel strings. Each array that has 3 or more parallel strings usually needs a fuse or circuit breaker per string to prevent a shorted string from being fed too much current (overheating wiring, starting fire, etc.).

    With 2s x 3p you would need 3x fuses/breakers total. (one fuse per 2x panel set).

    With 3p x 2s, you would need 6x fuses/breakers total (one fuse per panel in this case).

    Also, we are trying for as high as practicable array voltage--And lower array current. This allows us to use smaller wiring and/or send power longer  distances with smaller AWG cable (and save on copper costs).

    For example, with your 2s * 3p array, that would be (note these are 235 Watt panels, not 250 Watts)

    3x 7.9 Amps Imp = 23.7 Amps Imp-array
    2x 29.8 volts Vmp = 59.6 volts Vmp-array

    Generally as a rough starting point, we want a maximum of 3% voltage drop in solar wiring for minimal energy loss. Using a voltage drop calculator:

    400 feet of 2 AWG cable is not cheap. A 500' spool is around $1,350 in my area. Plus you need to use conduit (for burial or find 2 AWG direct burial 2 pair cable).

    You could look at (roughly) 1/0 Aluminum cable (its own issues).

    Or you could accept higher voltage drop (6% or more?).

    Or you could do a battery/inverter shed at the array, and send 120 VAC to your home instead (higher voltage, lower current, smaller cable potentially).

    There are now solar charge controllers that take upwards of 600 VDC input voltage. You would put all of your panels in series to use a "high voltage" VDC cable run and save money there. (higher voltage solar charge controllers are usually not cheap).

    In any case, before you purchase any hardware, do some different paper designs and see what works best for you.

    Probably more questions than answers right now. Have you worked out actual load needs (Watt*Hours per day, amount of sun in your area by season, possible backup Genset, etc.)? Have you looked at surge/lightning protection (if needed)?

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • littleharbor2
    littleharbor2 Solar Expert Posts: 2,064 ✭✭✭✭✭
    BB. said:
    There are now solar charge controllers that take upwards of 600 VDC input voltage. You would put all of your panels in series to use a "high voltage" VDC cable run and save money there. (higher voltage solar charge controllers are usually not cheap).
    There are some relatively inexpensive AIO (All In One) inverter chargers that take up to 550 vdc into their MPPT chargers. Another option.

    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,  460 Ah. 24 volt LiFePo4 battery bank. Plenty of Baja Sea of Cortez sunshine.

  • nyoffgrid
    nyoffgrid Registered Users, Users Awaiting Email Confirmation Posts: 6 ✭✭
    Thank you little harbor. I just found one that has just that. Basically identical to the one I found before but for $60 more it has 500vdc and 4000w pv so quite a lot more. 

    Bill - I am thinking to abandon the solar until next year. I only spend about 30 days a year in the cabin and the way I had it so far where my cabin runs off battery during the day and then at night the generator runs cabin + charges it - is simple for me and doesn't cost much. About $160 a year to run (and I buy expensive ethanol free premium gas). 

    I am hoping that you can help me verify a few other things. This is the invertor I found (thanks Harbor!).

    Similar specs to the other one. My battery is

    I know I use about 2500wh between 8am and 8pm. So if I start my day with this battery fully charged: 

    When I switch on the generator (honda generator 2200w) at 8pm it only needs to replenish 2500wh of the battery. 

    so two questions:

    1. How long will it take the generator to replenish that? 

    2. I have the option to limit the charging current (ac input) on this inverter. Not sure if I need to... The honda is an inverter generator so it adjust its RPM when in eco mode to provide as much power as you need... so when you use a miter saw, for example, and you click it so it will cut - the generator will ramp up to supply enough power for it and then will ramp down when you finish cutting.. But if I try to connect let's say some device that needs 25amp - it obviously can't supply that and will trip the generator. I am not sure if this can happen here as well? in other words - if the inverters is set to allow up to 40amp charging current - will it "request" that from generator and then trip it? If so then I guess I should limit the current? If you agree please tell me to what I should limit it to. 

    And on a different topic. I will be relocating my generator and will run new wire from it directly to the inverter. It is 100ft. I plugged into Southwire calculator 110ft (to be safe), 110v, 19amps (it can't even give that but just in case) and it showed that I am under 3% voltage drop with 8awg. Can you confirm that I will be fine with that? I will run a pvc conduit with 8/2 romex inside. I plan on just running it on the ground since my soil is so rocky it will be impossible to dig a trench of 100ft. 

  • nyoffgrid
    nyoffgrid Registered Users, Users Awaiting Email Confirmation Posts: 6 ✭✭
    I think I just figured out that I should be fine with generator? max charging current is 40amp so at 24v it will only pull 960 watt? that is less than half my generator rating and easily be supported by my generator. 

    I hope that is correct. 

    And then if I need to replenish 2500wh = 104ah about. So at 40ah it will take 2.5 hours. Not really since it can only charge that in the bulk so probably 3.5 to 100%? 

  • BB.
    BB. Super Moderators, Administrators Posts: 33,500 admin
    edited June 14 #8
    With a 1 month per year cabin, it is a bit difficult to justify the solar hardware vs just running a (small/inverter type Genset)  when you need the power... Add a "smallish" battery+small inverter (for lighting, charging phone/laptop, radio, etc.) can work well.

    And you do not leave a bunch of expensive equipment unattended for 11 months of the year (hate 5 finger discount, but is is out there). Just less to pack up and take home (Genset, battery, etc.) when the stay is over.

    Your planned system is certainly large enough that cyou can run the Genset whenever you wish--However, you might want to run it in the AM/Daytime when you loads are greatest (guessing)--You cycle the battery less and keep the battery topped off for those days you did not get more gasoline (bad weather, etc.)/Genset failed to start/etc.

    In any case, with LiFePO4 batteries--They are pretty much near perfect. I suggest that you cycle them between 20% and 90% full for best life. That would be an ideal discharge (between charging) of:

    100AH * 24 VDC * (0.90 - 0.20 SoC) = 1,600 Watt*Hours per "ideal cycle"

    Trying for 2,500 WH per cycle on a (100AH*24VDC) 2,400 WH battery is expecting "too much" (let along the aging/loss of capacity of upwards of 20% over time)... So running the genset twice a day/during periods of heavy loads (water pumping, cooking, etc.) to stay within battery capacity would help.

    A 40 Amp charger is a bit large (100 AH battery is rated for 30 amps nominal charging, and probably 50 amp max charge/continuous loads).

    100 AH / 40 amp charging = 2.5 hours from 0-100% SoC (lithium batteries are very charge efficient--And time of charge too--Not like Lead Acid that require around 2-6 hours of "absorb charging" to reach 100% SoC).

    Another question... Larger inverters usually require more Tare Energy (inverter power wasted when turned on and no loads)... The two all in one inverters you suggested take 50 Watts Tare. While not a big deal for a larger system, it still adds up over time:

    50 Watts * 24 hours per day = 1,200 WH per day inverter "just on"
    1,200 WH / 24 VDC battery bus = 50 AH or 1/2 of your batteries storage capacity of 100 AH

    Note: Not sure if you have 100 AH or 200 AH battery... I thought the first time I clicked the link it was 200 AH, but now I see 100 AH--So I may be wrong here.

    If you only run the inverter "when needed", that is a possible solution. Another, if you want 24 hour per day AC power and usually run smaller devices (small fridge, laptop, cell charging, some AC LED lightning)--Having a (possible) second smaller AC inverter (such as 300 Watts) can cut your 24 Hour Tare losses by a bunch. This Morning Star brand has 3.9 Watts of Tare/Self Consumption losses or almost 1/10th that of your larger inverters:

    I like the Honda Inverter Generators (I have the older eu2000i version). Your Genset, I would like to suggest a maximum continuous loading of 80% (if you are in the mountains, you also have max power vs altitude deratings):

    1,800 Watts max continuous * 0.80 derating = 1,440 Watts (or max VA--another discussion)

    Your AC charger loaded (guesses, need mfg data or Kill-a-Watt type meter readings):

    40 Amps * 29 VDC * 1/0.85 typical charger eff = 1,365 Watt (or VA) max estimated charger draw

    1,800 Watts * 0.80 derating * 1/120 VAC output = 12 Amps suggested max continuous Genset load current (best life)

    So still a good match with charger+eu2200i Genset. Limiting charging current to 30 amps would be a bit "nicer" to your 100 AH battery. Unload the Genset a bit (if high altitude/hot weather). And give you some extra power for 120 VAC loads (or just run the inverter for your 120 VAC and save the hassles of plugging loads into Genset AC power, etc.). You will not waste very much power running from the inverter while charging.

    Not sure what to do with your 100 feet of 8 AWG cable... Putting in conduit really means you cannot run and pull cable when you are there. And copper thieves would love that much copper to sell or use themselves.

    Getting outdoor/UV Rated cable (suggest black to be less obvious vs Orange) and just put it how when you are there 1 month of year and roll up/lock when gone. (sorry--I am getting paranoid in my old age--Years ago somebody stole everything from well pump to whole cabin for one poor sole). Just temporary bury for under paths and such. 

    8 AWG should be fine for your use... Note that "over sizing" and "derating" for loads/time/temperature/altitude--All those "derating factors" quickly add up (I.e., 80% max suggested Genset loading, 85% efficient battery charger, 3% cable losses, 85-95% Efficient AC inverter, Tare losses, battery temperature/cycle life/capacity degradation/large AC inverter + small AC inverter for 24 hour operation, etc.).

    It is nice to have a "well or over designed" system... But it can get costly). The numbers/ideas above are just starting/sizing suggestions. And help you to ensure that you have the most efficient loads you can find. It is almost always cheaper (in the long run) to have smaller/high efficiency loads and smaller power solar/Genset power system.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • nyoffgrid
    nyoffgrid Registered Users, Users Awaiting Email Confirmation Posts: 6 ✭✭
    Apologies for confusion..... The battery I bought is a 24v (25.6v to be exact) 200AH and not a 100ah

    And I am there 30 days a year but those are multiple weekends. I don't expect to NOT visit at least once a month throughout the year. 

    I have added a gps tracker to my genset and I have a wonderful off grid camera that has a cellular sim in it.. I get notifications from the cabin live. Really excellent camera for the price. 

    You think it is worth to set some kind of limit on inverter/charger to not let genset fully charge battery to 100% ? 

    I intend on experimenting with the genset run times since it is currently fed by a 5 galloon tank (extended run time) which means it easily runs non stop for 12 hours 7pm-7am. But if I really only will have the battery at a 60% SOC at 7pm, when genset is started, then it should hit that 90% or 100% SOC by 10-11 pm correct? If that is the case then I really don't need to let it keep running so I might just let it run on its 1 gallon tank and just die when fuel is done. Which can be anywhere from 5-7 hours. 

    For battery to inverter cables I got a 1/0 even though I was told 2 awg will be fine. It was a few dollar more so I figured why not. I got a 5ft black and a 1ft red and a 4 ft red - while the red will have the circuit breaker in between.