Off grid backyard trailer solar

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Comments

  • tucsonjwt
    tucsonjwt Registered Users Posts: 36 ✭✭
    Everyone I know who has those heaters really likes them also. The one I had failed within a few months, so I returned it and got a replacement, which failed after another 6 months. The third one failed after a few months so I got a fourth replacement and the warranty was expired by that time, so I sold it on Ebay. I suspect that there was a manufacturing defect occurring during time period.

    I am not opposed to using propane. I have seen propane conversion kits for the Honda 2000i generator which allow for extended run time off a 5 gallon propane tank. That appeals to me, since modern gasoline with ethanol has ruined a lot of small engine equipment I have owned. The long term ideal system would be an auto generator start system connected to an electric start propane generator.  Propane does not degrade over time like gasoline and propane generators don't need to be choked prior to first start. Propane also burns clean so the carburetor does not gum up like a gasoline powered generator.

    The downside of propane is that it costs more than gasoline for a given energy output and the per gallon price is more than gasoline where I live. There are other places where propane is cheaper than gasoline, but in the desert southwest it usually costs more. I have read that propane prices follow gasoline prices, since propane is a byproduct of crude oil drilling.



  • Estragon
    Estragon Registered Users Posts: 4,496 ✭✭✭✭✭
    On the AGM vs flooded efficiency thing, I wouldn't extend this to imply you get 25% more useable amp-hours.  The efficiency difference is mainly in the latter part of charging, where flooded turn charging power into some heat and gassing which mixes electrolyte.  Charging an AGM to this point can damage it.

    If you cycle a smaller AGM bank more deeply, I'd expect cycle life to be impaired relative to a shallower cycled larger flooded.
    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
  • tucsonjwt
    tucsonjwt Registered Users Posts: 36 ✭✭
    Yes, I have read that FLA batteries are more tolerant of deep discharging than AGM. Considering my ambient temperature extremes (near or below freezing at times in the winter, and at or above 100 degrees in the summer, does this indicate that FLA batteries should be stored inside the trailer in a vented enclosure? I would plan for 80-85 degree temperature inside the trailer most, if not all of the time. If the batteries were located outside they would be under the trailer and shaded from direct sunlight by the trailer, but still subject to extreme temperatures. Outdoor temperatures are recorded in the shade, not in direct sunlight, so a battery in direct sunlight at 100 degrees will be much hotter than 100 degrees.

    During the winter, outdoor batteries would be subject to an average temperature range of 40 to 70 degrees. It is the summer when the temperature extremes are the greatest.

    If I go with FLA batteries I would probably but Interstate golf cart batteries from Costco with a no hassle one year free exchange. If I ruin those batteries I will get another new set in a year and with proper care I might get another year or two out of them. They are about $120 per battery with $15 core charge, tax, and fees.
  • tucsonjwt
    tucsonjwt Registered Users Posts: 36 ✭✭
    I can see cycling the batteries to 50% or below when I am away and no generator is running to supplement the solar power. The a/c would be at a high setting (maybe 85-90 degrees) when I was away, but cloudy days could drain the batteries lower than I would prefer.
  • tucsonjwt
    tucsonjwt Registered Users Posts: 36 ✭✭
    With regard to keeping a battery bank in an enclosure, is a passive venting system adequate or is a power vent necessary? I was thinking of one 4" screened dryer vent at the top of the enclosure. 

  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    AGM batteries do not need venting during normal operations.. But as they age and/or are overcharged by a bad charge controller, they will vent just like any other lead acid battery (venting AGMs will be very near the end of their useful life).

    Air circulation can also help with temperature control (keep a cold bank warm in the winter, keep a bank cooler in summer).

    Some sort of basement/ground contact battery box can moderate summer/winter temperatures a lot.

    AGMs are tolerant of freezing in that they won't crack the case like a flooded cell battery will (but should not be charged if "frozen").

    Running an off grid system on "automatic" when nobody is there is highly problematic. Now you are looking at safety gear to stop over discharging, charging when frozen, etc... And the more stuff you hang on a system, the more wild and wonderful things that can go wrong.

    I am not sure that AGM are better or worse at deeper cycles... AGMs just tend to cost something like 2x FLA pricing. And they tend to have a couple years of shorter life vs similar quality FLA batteries.

    The problem with lead acid batteries is that if they are deeply discharged, they really want to be fairly quickly recharged... Setting for days "near dead" is not good for them at all (especially as the batteries get a few years in them).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Estragon
    Estragon Registered Users Posts: 4,496 ✭✭✭✭✭
    Avoiding 100°+ temps is certainly a good thing.  Whether it's worth the extra cost for AGMs that could live in the trailer, I dunno.

    I wasn't suggesting flooded can be taken to lower states of charge than AGM generally without affecting cycle life.  Flooded may get better cycle life than AGM, but I think that's more related to the fact SG can be monitored and charging issues more easily caught early.  

    GC batteries are often a good choice for first go at batteries, as long as you can get the capacity you need with 1-2 strings.  More than 3 gets unwieldy for wiring, balance, and maintenance.

    Natural venting can work, but powered venting can be done pretty cheaply with a computer muffin fan on a relay that triggers on absorb stage charging.  My banks are in a plywood box in a crawlspace, and although I should probably get around to venting properly, natural seems to be ok so far.

    I wonder if the heat problem could be mitigated somewhat by covering with insulated panel in the heat of the day, and uncovering to cool at night?  In my insulated crawlspace, the batteries stay pretty cool even during hot spells - closer to average daily mean temp rather than getting to anything like daytime highs.  Just a thought.
    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
  • tucsonjwt
    tucsonjwt Registered Users Posts: 36 ✭✭
    If I were away from the system for days or weeks at a time the only thing on in the trailer would be the window a/c set to a very high thermostat setting, so the a/c would run in compressor mode only enough to keep the battery temperature at about 85-95 degrees. I don't know if that is possible with 1220 nominal watt hours of solar panels, but I suspect that it would be.

    I think that I could build a "mini insulated crawl space" under the trailer as an alternative, and then leave the converted refrigerator/freezer on to use 350 watt hours per day. My house has an uninsulated crawl space and it is much cooler than the outside temperature in the summer, but in the winter the temperature gets almost as cold as the outside.
  • tucsonjwt
    tucsonjwt Registered Users Posts: 36 ✭✭
    The golf cart batteries would likely be more than two strings. I think the Costco batteries are 210 amp hours, so two strings of 2 6 volt batteries would get me only 210 AH at 12 volts with 50% DOD. I would think 6 or 8 batteries at 6 volts would be more suitable, since the overnight issue of powering the a/c in the summer without continuous generator use is my goal. 



  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    tucsonjwt said:
    The golf cart batteries would likely be more than two strings. I think the Costco batteries are 210 amp hours, so two strings of 2 6 volt batteries would get me only 210 AH at 12 volts with 50% DOD. I would think 6 or 8 batteries at 6 volts would be more suitable, since the overnight issue of powering the a/c in the summer without continuous generator use is my goal. 
    Then you need to look at putting those 8 golf cart batteries in SERIES for a 48V system.   It will be more balanced, easier to wire, and batteries will stay in healthier shape than being in a massive parallel array
    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 ,

  • tucsonjwt
    tucsonjwt Registered Users Posts: 36 ✭✭
    I need some help in understanding the difference in having 8 golf cart batteries(6 volt) in 4 strings of 2 batteries, vs. having 8 golf cart batteries in one continuous series string of 48 volts.

    Illustrating my ignorance on this topic, I understand that the wire size can be smaller with a 48 volt system, but I don't understand how the batteries will be more balanced and stay healthier. 

    The current will flow from the charge controller (or from the generator) to the battery bank and connect to two terminals in a 48 volt system, or connect to the "daisy chains" of positive terminals and negative terminals which comprise the 4 groups of 2 golf cart batteries which are at 12 volts.  Do the multiple lengths of wire connecting the 12 volt batteries (pairs of 6 volt batteries) result in more resistance and reduce the flow of current to the batteries at the end of the chain.  

    I have seen videos of solar battery banks in which the 12 volt batteries were connected to buss bars (made of flattened copper pipe, aluminum angle, or industrial purpose built heavy copper buss bars.)  The solar or generator input was then connected to the common buss bar.  Would that result in a more balanced system and promote battery health? 
  • tucsonjwt
    tucsonjwt Registered Users Posts: 36 ✭✭
    The buss bar concept appeals to me because bad batteries could be removed in groups of two and still maintain proper functioning of the 12 volt system.  Conversely, additional batteries can be added if the battery bank is proven to be too small for the loads drawn on it.  I have seen videos of one bad battery dragging down the performance of the entire battery bank.  Being able to remove one bad battery(or two 6 volt batteries) I think would be a beneficial characteristic of a 12 volt buss bar system.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    A 2s x 4p string (6 volt @ 220 AH "golf cart" batteries) = 12 volts @ ~880 AH

    An 8s x 1p string of GC batteries = 48 volts @ 220 AH

    They both store the same amount of energy, have the same number of cells, and if everything works correctly, they will work identically.

    The differences (note, both are 880 AH * 12 volts * 0.13  = 220 AH * 48 volts * 0.13 rate of charge = 1,373 Watts of charging power)  :
    • 880 AH * 0.13 rate of charge = 144 Amps charging current
    • 220 AH * 0.13 rate of charge = 29 amps charging current
    So, the first string would require at least 2x 70 Amp high end charge controllers. The second would require a 30 amp charge controller (much cheaper). You will save almost $500-800 in higher end MPPT charge controller costs.

    And if you want C/5 discharge rate (about the maximum AC inverter you can "reliably" connect to a flooded cell Lead Acid battery bank):
    • 880 AH * 1/5 Hour discharge rate * 1.25 NEC derated branch circuit/breaker continuous current = 220 Amp minimum rated branch circuit
    • 220 AH * 1/5 Hour discharge rate * 1.25 NEC derated branch circuit/breaker continuous current = 55 Amp minimum rated branch circuit
    • 880 AH * 12 volts * 1/5 hour discharge rate * 0.85 AC inverter derating = 1,795 Watt maximum rated AC inverter
    Look at an NEC wire chart and see the size of wire needed to carry 220 Amps vs 55 Amps:

    https://lugsdirect.com/WireCurrentAmpacitiesNEC-Table-301-16.htm

    Need 3/0 to 4/0 cable to carry 220 amps vs 8 or 6 AWG cable.

    And for a 12 volt battery bank, you want to wiring to have a maximum of ~0.5 volts drop from battery to inverter. For 48 volts, you can have ~2.0 volt drop:

    https://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=0.2028&voltage=12&phase=dc&noofconductor=1&distance=18&distanceunit=feet&amperes=220&x=55&y=34

    18 feet of 3/0 cable @ 0.5 volt drop at 220 Amps

    Vs:

    https://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=2.061&voltage=48&phase=dc&noofconductor=1&distance=29&distanceunit=feet&amperes=55&x=47&y=11

    29 feet of 8 AWG cable @ 2.0 volt drop at 55 Amps

    There are other issues too... You have to check the cables and connections (more of them for 12 volt bank), and you should have a high current fuse/breaker per parallel battery string (~110 Amps)--Those are not cheap, and not many people do that.

    And if you have a failed cell / string connection, the other three strings will do a pretty good job of 'hiding" the failing cell/connection (these days, DC Current Clamp Meters/DMMs, are pretty cheap and easy to get--So, you just need to double check that current is being properly shared between the 4 parallel strings during heavy charging/discharging current).

    And you should wire parallel banks properly to share current (equal resistance current paths for all batteries):

    http://www.smartgauge.co.uk/batt_con.html

    Now, I suggest that 1-3 strings are "optimum" for parallel battery banks. More than 3 strings is a bit of a pain to maintain and water. Some people prefer 2 parallel strings--If one battery fails, the second string will keep your minimum energy needs supplied until the problem is fixed/battery is replaced.

    If you go with 1 string, then you look for larger capacity (Amp*Hour) batteries... For example 2x 6 volt x 820 AH batteries in series for a 12 volt @ 820 AH 

    https://www.solar-electric.com/6cs-25ps.html (~$1,112 per battery)

    12 volt battery bank--A 2s x 4p battery bank has 24 cells to water. A 2s x 1p battery bank only has 6 cells to water.

    A GC battery costs around $100 each (very roughly). 8x $100 GC batteries = $800

    2x $1,112 industrial batteries cost ~$2,224 for the bank

    Now, the $2k battery bank will probably last ~2-3x longer (6-8 years) vs the $800 GC battery bank (3-5 years)...

    Now, the issue of finding a 12 volt vs 48 volt inverter... You can find ~120 Watt to 3,500 Watt Pure Sine Wave inverters (I would suggest that you go no larger than ~1,800 Watts on a 12 volt battery bank). Lots of selections in smaller inverters.

    https://www.solar-electric.com/residential/inverters/off-grid-inverters.html?nav_inv_input_voltage=436&sine_wave_type=549

    48 volt inverter... 

    https://www.solar-electric.com/residential/inverters/off-grid-inverters.html?nav_inv_input_voltage=439&product_list_limit=all&sine_wave_type=549

    Roughly 400 Watt minimum PSW inverters--Not a great selection of smaller inverters (although, there are more smaller 48 vdc input inverters now than they had a few years ago at NAWS, our host). Up to >8,000 Watt inverters.

    With an 880/12 or 220/48 volt battery bank, same amount of stored energy--More or less, suggest around a maximum of 2,200 Watts of AC inverter / maximum solar array sizing... (These are rough maximum suggested ratings--There are methods and reasons to "violate" these rules of thumbs, but they are "good enough" to keep you on the road while penciling in a basic paper system design).

    Anyway--Lots of stuff here. And there are more issues and options--But this is sort of the "big stuff" why we suggest what we do. Going off the mainstream suggestions generally is a bit more complex and more costly. Can be done, but you want to have reasons for doing this.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • tucsonjwt
    tucsonjwt Registered Users Posts: 36 ✭✭
    I see 48 volt inverters in the $660-$800 range for 2000-2500 watts.  Inverter/chargers in that size wattage are $1500 and up.  Based on previous discussion here, does that mean I need to spend $1500+ and buy an inverter/charger for a 48 volt system?  A 2 year warranty on a $1500+ purchase does not inspire me with confidence.  I am suspicious of complicated and delicate electronics lasting the test of time, based on my experience with other consumer electronics.  Are these 48 volt inverter/charges robust enough to be used in a moving RV?
  • Estragon
    Estragon Registered Users Posts: 4,496 ✭✭✭✭✭
    FWIW, the life expectancy of a decent (Outback/Schneider/etc) inverter is ~10years (less in hot ambient temps, more in cool).  A two year warranty gets you past the "infant mortality" period in which many/most manufacturing defects tend to show up in PCBs etc.

    Personally, I think you mostly get what you pay for with inverters.  Some of the cheap stuff downright scares me (like high output 12v ones), but to each his own.
    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
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    In the "olden days" (30+ years ago) an (a?) US or European middle of the road quality AC to DC computer power supply was around $0.50 per Watt wholesale price... And retail was probably closer to $1 per Watt (we actually marked up our "hardware costs" by a factor of 10x for multiuser computer systems--Sort if covered our manufacturing/warranty/software/development costs). And they lasted around 5-10+ years (frequently, a 10+ year old power supply would work fine--Until the 24x7 system was shutdown for service, then when turned back on, the power supply croaked).

    And, frankly, it gets difficult for a manufacturer/service center to repair electronics over ~5 years of age (complex parts have short manufacturing cycles, lose the "recipe" for manufacturing/programming/testing complex items, few people with detailed knowledge, Power Electronics when they fail--take out multiple components/boards with all the "energy" going the wrong places, etc.).

    Today, you can purchase power supplies for around $0.50 per Watt retail. Yes, the are built who knows where (high volume, variable quality, questionable warranty service)... And The tend to last 10+ years, with some 5+ year failures (and a few infant mortality).

    Buy one very expensive supply or 1 + a spare cheap supply... Kind of your choice.

    Keep the stuff dry, free of "bugs" (real bugs, lizards, snakes, dust, animal fur, etc.). Keep the room temperature cool (75F/25C or less). Limit thermal cycling (cold/hot cycling rooms, good air flow to prevent electronics from thermal cycling/extremes).

    Again, I would suggest before you "go down the hardware road", that you get an idea of your actual energy needs and limitations (i.e., roof top only array, flat or tilted or year round adjust racking, etc.). Then once you have the paper requirements, you can now go through and price a "cheap" and "expensive/all the bells and whistles" systems and see what works best for you.

    Things like a remote shutdown for the AC inverter (battery gets discharged, remote login and you just system down due to weather forecast, etc.). Charger + Inverter vs Charger-inverter. etc.

    You tending towards 2.0 to 2.5 kWatt suggests a minimum of 24 volt battery bus would be a good fit. There are a fair amount of 24 VDC appliances out there (heavy truck, marine, computer fans, etc.) that may be of interest if useful (save running an inverter for small loads).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • tucsonjwt
    tucsonjwt Registered Users Posts: 36 ✭✭
    I just got my hands on and checked a used 10.3 cu. ft. Haier frostless refrigerator/freezer.  I ran it for 24 hours (admittedly at average 60 degree temperature) and it consumed exactly .4 killowatt hours.  That is less than 1/3 the power consumption of my current 10 cu. ft. Whirlpool frostless refrigerator consumes.  This Haier refrigerator has the worst reviews on the internet, but my Whirlpool has gone through 3 defrost timers in 6 years, so it is no peach either. 

    So, that brings me back to a critical winter energy consumption of ~ .5 killowatt hours for the refrigerator.  Add 350 watts for 15 minutes use of the 1400 watt 2.5 gallon water heater and we are at less than 1 killowatt hour of consumption for a constant energy demand, regardless of weather.  I know there are other considerations, but I think that is in the ballpark. A few LED lights could be added to the mix, but I don't see that as a big power drain.

    That leaves me with the summer seasonal load of the 420 watt window a/c.  I think that could be a roughly 6 month demand (May-September.)  The water heater will run for 15 minutes during daylight hours.  Is it reasonable to expect that 1220 watts of nominal solar panels will power a refrigerator for .2 killowatt hours consumption between 6 am and 6 pm, as well as a 420 watt window a/c for about 4 hours (1/3 duty cycle) during peak daylight hours?  Would that leave any "residual energy" coming from the solar panels to contribute to charging the battery bank?

    I am beginning to think that I should focus on an inverter or invetter/charger for a system dedicated to just the refrigerator and water heater and address the window a/c with a separate system, since it won't be running in the winter and I can turn off the a/c (and associated inverter/charger) when I am away for days or weeks in the summer.

    Am I on the wrong track again?
  • tucsonjwt
    tucsonjwt Registered Users Posts: 36 ✭✭
    edited November 2018 #79
    305 nominal watt panel x 4 panels = 1220 watts x 6 hours of peak sunlight = 7320 watts total daily solar panel input x .52 (sum of inefficiencies) = 3806 watts x .50 FLA DOD = 1903 watts per day available for 120 volt household outlet use. 

    Do I have this correct?
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    edited November 2018 #80
    Not quite... The "extra 0.50 FLA DOD" is not really part of the equation...
    1. 305 Watt panel * 4 panels = 1,220 Watt array
    2. 1,220 Watt array * 0.77 panel+controller deratings * 6 hours (summer min avg sun) = 5,636 Watt*Hours of harvested energy
    3. 1,220 Watt array * 0.52 end to end off grid solar AC power efficiency * 6 hours of sun = 3,806 WH of "available" energy
    4. 1,220 Watt array * 0.77 panel+controller derating * 0.85 AC inverter eff * 6 hours of sun = 4,791 wH of "available" daytime solar=>AC output of "available" energy
    OK... The details... #1 is obvious. The size of the array (assuming 6 hours of sun is that the panels are free of shadows and tilted 73 degrees from vertical (17 degrees from horizontal) for optimum summer harvest in Tucson AZ.

    #2 is the average summer energy harvest (solar panels=>charge controller=>DC battery bus). If you were using DC battery bus voltage (no storage of energy in battery bank or voltage change like 24 VDC to 120 VAC).

    #3 is assuming your entire daily harvest is sent to a flooded cell lead acid battery bank under average conditions/usage (charging to >~90% state of charge). And taking the energy from the battery bank and using it at night (or night+heavy clouds the next day--So solar energy available).

    #4 is assuming that every Watt of solar harvest is sent directly to an AC inverter (such as using most of your harvest for daytime A/C, running computer/printer/daytime equipment)... No losses to battery storage/generating power for night/cloudy days.

    More or less, predictions are "accurate" to about +/-10%...

    As you can see, how you choose to use your energy can affect your harvest... For the conservative "worst case" design rules of thumbs, I assume that you harvest 100% during the day, and use 100% of your energy at night/during the next day or two of "no sun" (heavy/storm clouds can reduce harvest to 10% or even 1% of "sunny day" harvest.

    If your energy needs are such that you can run different loads under different conditions (A/C during the day / sunny weather), and not run the A/C during cloudy/cool weather, you do get around (4,791-3,896=) 895 WH of "extra power" (daytime only) usage.

    If that "extra" energy was make or break--Then simply adding (895 WH extra * 1/0.52 off grid AC system eff * 1/6 hours=) 287 Watt "extra" to your array... And you don't have to "micromanage" your daily energy usage... Just a personal set of energy choices. Some people like to have a hands on approach to energy manage--Others are less interested.

    The 0.50 "battery depth of charge" number... Just another design choice about how deep you want to take your battery bank State of Charge/Depth of Discharge. A "handy rule of thumb" that results in roughly 2x longer battery cycle life (note that taking a FLA battery below 20% SoC can actually "kill" a cell/battery).

    I also suggest 2 days of stored energy--This can reduce genset runtime (two days of bad weather vs 1 day... And a 3 day battery bank is quite large and expensive--Plus other issues). But it also better matches an FLA battery charging cycle to hours of sun per day... It is very difficult to take a FLA battery from 50% to >90% SoC in one "solar day" (you have ~6-8 hours of usable sunlight)--This works better for utility charging where you have 12+ hours of available charging time.

    Taking a FLA battery from 75% to >90% SoC works better with the 6-8 hours of active solar charging (tracking solar array, or "virtual" tracking can be very nice--It does give you more "active hours" of solar charging per day).

    The equation for sizing a battery bank... The total energy use per day (assuming 120/240 VAC) where the battery bank is assumed fully charged and the next day(s) zero solar charging available (bad weather). 2 days and 50% usually works out about "optimum" for FLA and AGM battery banks. Using the 3.8 kWH from the above 1,220 Watt and 6 hours of usage (again, I am after a "balanced" system design):
    • 3,806 WH per day * 1/0.85 AC inverter eff * 1/24 volt battery bank * 2 days storage * 1/0.50 max discharge = 746 AH @ 24 volt battery bank
    If your actual energy usage is 2,000 WH A/C (daytime summer) and 1,806 WH per day overnight/bad weather (computer, lighting, fridge)... Then you might want a smaller battery bank.

    Sizing a battery bank for charging... 5% to 13% rate of charge with 10%+ rate of charge suggested for "full time" off grid charging:
    • 746 AH * 29.0 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 1,405 Watt array minimum
    • 746 AH * 29.0 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 2,810 Watt array nominal
    • 746 AH * 29.0 volts charging * 1/0.77 panel+controller derating * 0.13 rate of charge = 3,652 Watt array "cost effective" maximum
    You are in Tucson and have a lot of sun (relatively)... So your minimum array is does not even give you a 5% rate of charge... To figure out the "optimum" size battery bank for your 1,220 Watt array:
    • 1,220 Watt array * 0.77 panel+controller derating * 1/0.13 rate of charge * 1/24 volt battery bank = 301 AH @ 24 volt minimum bank
    • 1,220 Watt array * 0.77 panel+controller derating * 1/0.10 rate of charge * 1/24 volt battery bank = 391 AH @ 24 volt nominal bank
    • 1,220 Watt array * 0.77 panel+controller derating * 1/0.05 rate of charge * 1/24 volt battery bank = 783 AH @ 24 volt maximum bank
    If you have a 391 AH (call it 400 AH) @ 24 volt FLA battery bank (optimum for your array)... The 2 day/50% formula works out to:
    • 400 AH * 24 volts * 0.85 AC inverter eff * 1/2 days * 0.50 max discharge = 2,040 WH of stored energy (overnight, next cloudy day)
    And the rest of the harvest is available for daytime usage (almost ~1,800 WH for A/C for example).

    And limitations... FLA battery banks can supply ~C/20 hour average energy (say 5 hours per night, two nights to 50% discharge):
    • 400 AH * 24 volts * 0.85 AC inverter eff * 1/20 nominal discharge = 408 Watt AC load for 5 hours (per night)
    • 400 AH * 24 volts * 0.85 AC inverter eff * 1/5 maximum discharge rage = 1,632 Watt maximum AC load (for a few hours)
    Etc.... Lots of tradeoffs when figuring things out. If your limitations is the solar array, and full time off grid, a 400 AH @ 24 volt battery bank AC inverter support (practically speaking) is around 400-1,700 Watts. Using a 2,000+ Watt inverter (to support heavy surge/short term loads) will be iffy (as battery bank gets discharged/older, it will not be able to support such heavy current draw).

    -Bill

    Nov 22nd... Add missing *6 hours of sun to equation #4
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • tucsonjwt
    tucsonjwt Registered Users Posts: 36 ✭✭
    Is wire size required based only on amp draw or is voltage also a consideration? My only electrical experience is in wiring 3 houses with 200 amp services, and I have no DC and low voltage experience. I would think that Ohm's law would dictate that voltage would be part of the calculation. As I recall, I used #2/0 copper for the 200 amp service entrance panels (which were 2 legs of 120 volts to provide a service of 240 volts.) I would think that a 12 volt, 24 volt, or 48 volt system would require a wire size smaller than #2/0 copper wire for 200 amps. 
  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    wire gauge is driven by the Amps it carries.  Also by how much resistance (loss) your plan can tolerate.

    Voltage determines the thickness of the insulation on the wire. 
    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 ,

  • Horsefly
    Horsefly Registered Users Posts: 470 ✭✭✭✭
    edited November 2018 #83
    tucsonjwt said:
    Is wire size required based only on amp draw or is voltage also a consideration? My only electrical experience is in wiring 3 houses with 200 amp services, and I have no DC and low voltage experience. I would think that Ohm's law would dictate that voltage would be part of the calculation. As I recall, I used #2/0 copper for the 200 amp service entrance panels (which were 2 legs of 120 volts to provide a service of 240 volts.) I would think that a 12 volt, 24 volt, or 48 volt system would require a wire size smaller than #2/0 copper wire for 200 amps. 

    Like @mike95490 says, it is the amps on the wire (and the size of the wire) that drives what the voltage drop is on the line. The voltage of the line factors in too, because if you have a higher voltage the loss on the line will be a smaller % of that voltage. For that reason, your statement is kind of backwards. If #2/0 is good for 200A at 120V, at a lower voltage you need a larger wire for 200A in order to have a similar % loss. What really matters is the resistance of the wire, which goes down the larger the wire. The resistance is expressed in the NEC table as ohms per 1000 ft. The voltage loss is (per ohms law) the current times the total resistance on the wire. The % loss is then the ratio of that voltage loss to the nominal voltage on the line.
    Off-grid cabin: 6 x Canadian Solar CSK-280M PV panels, Schneider XW-MPPT60-150 Charge Controller, Schneider CSW4024 Inverter/Charger, Schneider SCP, 8S (25.6V), 230Ah Eve LiFePO4 battery in a custom insulated and heated case.
  • westbranch
    westbranch Solar Expert Posts: 5,183 ✭✭✭✭
    edited November 2018 #84
    Bill said\;   23 amps per string nominal charging current during bulk    on page 1.
    this is important to the amount of time the gen will run, low discharge from a battery = shorter run time.....deeper discharge = longer run time and possibly larger gen set.

    All XX Amp chargers are not equal in the max amount of power they will draw.... I have a 12V 40 Amp Xantrex TRU-Charge that looked like it might run off a Honda 1000i but will not as the checking programming needs an amount greater than  that little gen can output, so now I have a 2000W gen to power it..  check those data sheets before buying.

     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
  • tucsonjwt
    tucsonjwt Registered Users Posts: 36 ✭✭
    Could you tell me specifically what the "checking program is?"
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    I think Westbranch is thinking that the TC (or TC2?) charger is "checking" the battery bank to make sure it is healthy by sending high charging current (pulses?) and seeing how the battery bank responds (how much voltage does the battery respond with)--And after the charger does its "testing/checking" of the battery, it then decides to to bulk, absorb, or float.

    And these high current pulses are too much for his small Honda eu1000i to support:
    • 40 amps * 14.8 volts charging * 1/0.90 estimated charger efficiency = 658 Watts (or VA) estimated maximum charging current
    The eu1000i should support a load of ~900-1,000 Watts. But it is not for him.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • westbranch
    westbranch Solar Expert Posts: 5,183 ✭✭✭✭
    Thanks Bill, you said it far netter than  I would have.....  My 24 Volt charger does similar checks but more disruptively  throughout the charge cycle.
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
  • Surfpath
    Surfpath Solar Expert Posts: 463 ✭✭✭

    If it helps:

    I've owned three of these camp shower tankless units over the past 9 years. In fact the photo that Little Harb2 posted is the exact same as my #2. Each of the units looked different on the outside, but on the inside it's pretty much exactly the same.

    My newest version has an extra "summer"/"winter" switch (winter basically just turns on another, pretty pathetic, row of burner jets). I would agree with tucson, they are best in warm weather. My first 2 units lasted me about 2.75 years each. At $130ea, not a terrible deal for something that fits well into my offgrid life, but I am hoping that my 3rd unit will last longer.

    Outback Flexpower 1 (FM80, VFX3048E-230v, Mate, FlexNetDC) 2,730watts of "Grid-type" PV, 370 AmpHrs Trojan RE-B's, Honda 2000 watt genny, 100% off grid.