Ready.. AIM!

bentherebenthere Solar Expert Posts: 113 ✭✭✭
Well, I think I'm about ready to bite the bullet. Currently we are averaging 16KWH/Day BUT that's in a rental. We are soon moving to our own house off grid (more like a cabin). I've run around with the Kill-A-Watt measuring what we use that will be coming with us. After getting rid of the forced hot air furnace, trading out the electric dryer for propane, going for an energy efficient washer, and looking to propane for refrigeration; I calculate our needs to average 3117 watt hours per day. Hopefully I'm not crazy to think that our requirements are under a quarter of what we use now!

After some research here and elsewhere, here's what I have in mind.

Batteries: 8 Crown L-16 in series for 395 amp hours at 48v ( Am I crazy to only run one string? I noted that 105s are cheaper per amp-hour but I don't want twice as many batteries or connections.

PV Modules: In my location, with 6 hours of daily sun available, I expect to need 700 watts of PV modules. I'm looking at the Evergreen ES-A-205-B ( This panel is missing from the Outback calculator I've been using, but if my calculations are correct I should be able to string up to 5 of these (down to -20 F) without overrunning the 150 volt limit on the FM60. I was planning on 4. Anyone care to check my math?

Charge controller: Outback FM60. I would go for the FM80 for the ability to grow, but I guess the FM80 has some unresolved firmware issues?

Inverter: Outback VFX3648. More than I need, but there doesn't seem to be a efficiency penalty, so we'll be future-proof.

Layout: I don't have to put the panels far from the battery bank and inverter, but thought I might as well minimize the loss by going 48 volts. Worth while for this small system?

Anyone care to point out my errors or faulty thinking?



  • icarusicarus Solar Expert Posts: 5,436 ✭✭✭✭
    Re: Ready.. AIM!

    If your guestimates are correct in terms of your loads...

    I think that you are about right on with the battery bank. With your loading you would have a nice reserve drawing the batteries down to around 70% in a few days of no sun.

    I think however that you are under on the PV side by about 100%. Given your 700 watts of PV, using my ball park 50% efficiency for battery based system here is my calc.

    700 watts X 6 hours X 50% = 2100 watt hours of PV. If you are looking at 3.1kw/day loads,, your gonna be pretty short pretty quickly.

    Your panels might put out something in the neighborhood of 12-14 amps into 48vdc.
    The recommended minimum PV array is ~5-13% Using your 395 ah batteries, 5% would be ~20 amps,,, 13% would be ~50. I would look to try to add some panels.

    Good luck,, off grid is a challange

    PS You will find that your loads will grow with time,, plan for it. Also if you are going to make this permanent,, consider buying a good energy star fridge rather than propane. With the cost of propane going up, and the efficiency of conventional fridges going up,, I wish I had done it. A good compressor fridge is bigger and cheaper to buy as well. (The math doesn't work if you are not going to be full time off grid however)

  • DapdanDapdan Solar Expert Posts: 313 ✭✭
    Re: Ready.. AIM!

    hey been,

    your cal on the battery storage looks sound, you have considered the 3 day and 50% max dod. I think though you maybe cutting it close on the panels and you may need at least 2 more IMHO. considering the 50% rule on average output of your panels into your bank on a good day of 6 hr you would have driven appro 50Ah into your 48v bank when you would need at least 65AH according to your 3.1kw daily needs. Also as it relates to charging rate (5-13%) at STC your max current would be nearly 17Amps at 48v which is below the min. you would not be able to equalise that bank.

  • DapdanDapdan Solar Expert Posts: 313 ✭✭
    Re: Ready.. AIM!


    I forgot how are you able to drop from 480kw monthly to 90kw monthly on electricity needs. It just seems like a really steep drop even though you are going propane and the like on some appliances. Are you sure of these numbers. An entire house running only 3.1kw of juice for 24 hours.

  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Ready.. AIM!


    you may also find that both flexmaxes have been reported to have problems at this time.
  • BB.BB. Super Moderators, Administrators Posts: 32,999 admin
    Re: Ready.. AIM!

    Regarding your usage estimations...

    FYI--it is Watt*Hours and kWatt*Hours of energy usage (100 watts * 8 hours = 800 watt*hours of energy)... Everyone makes this error--and I have caught myself even now making the error on occasion when typing distracted.

    3.3kWhrs a day (or ~100 kWhrs per month) is my "magic" number that I tell people to aim at when they go off grid. From years of reading (such as Home Power Magazine) -- it is achievable. But you have to work at it.

    I have, with a natural gas supplied home in the summer (no drier usage, no central heat required) gotten as low as ~175 kWhrs per month (4 people, computer, CFLs, TV, fridge, washer, line dry. etc.).

    Solar panels (and charge controllers/inverters) are cheaper and better now (which was one of the drivers to reduce power usage)--but it still keeps the overall system reasonably small and "cheap" (always a relative term).

    I like 1 series string--very easy to diagnose problems (bad connections, bad cells) and no issues with parallel strings being unbalanced (one string carrying more load/charging vs the other because of wiring resistance differences, etc.).

    However, Windsun has said before that they see batteries from series and series/parallel strings and has not seen any lifetime differences (assuming the wiring / maintenance was good).

    48 volts--use it if you have no need for 12 volt or other devices. Lot smaller wiring and less distribution power loss. Also for Charge Controllers which are rated on Amperage--a single 48 volt controller will handle as many panels as 4x of the same controllers at 12 volts (cheaper and simpler).

    Lastly--look at the fridge. If you want propane--fine... But a modern Energy Star fridge is low cost, long life, ~1+ kWhr per day, and frequently the extra solar panels+batteries required to support the load is less expensive than a propane/off grid fridge. And you get frost free and an ice maker--probably a hit with your spouse and any kids.

    There is a chest freezer to fridge conversion thread which can get down to ~0.25 kWhrs per day--but that may be a bit out of your comfort zone.

    There is one sort of major issue with 48 volt battery banks and "high voltage" solar panels (Vmp~48-60 volts)...

    Not an issue for you (your panels are V~18 volts)--but for "High Voltage" solar panels, 1x 48-60 volt panel is not enough voltage to charge a 48 volt battery bank (requires Vmp~62 volts). And 2x panels in series may be too much voltage for the MPPT controller (Voc>150 VDC).

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • BB.BB. Super Moderators, Administrators Posts: 32,999 admin
    Re: Ready.. AIM!

    I don't remember where your cabin will be (if you said)... Assuming somewhere in Texas (as an example). Using the PV Watts calculator Website and 700 watts of solar panels...

    First, the PV Watts program only goes down to 1kW of solar panels, use that for now. And use 0.52 as a derating factor (basically, same as Tony's/Icarus' 50% derating). The results for Midland Tx (choose nearest to your location with similar weather):
    [FONT=Courier New]Results
    Solar Radiation (kWh/m2/day)
    AC Energy (kWh)
    Energy Value ($0.097 per kWhr)
    1      5.14          81        7.86   
    2      5.77          80        7.76   
    3      6.70          101       9.80   
    4      6.67          94        9.12   
    5      6.38          91        8.83   
    6      6.32          85        8.25   
    7      6.58          90        8.73   
    8      6.43          88        8.54   
    9      6.07          83        8.05   
    10     6.21          92        8.92   
    11     5.43          79        7.66   
    12     4.82          74        7.18 
    Year      6.04        1037    $100.59 [/FONT]

    As you can see, at least for Midland TX, 1 kW of solar panels is a bit light for a system to expect 100 kWhrs per month of power (assuming 80% efficient flooded cell batteries running a 85% efficient TSW inverter).

    It is usually easier to add solar panels (if you have the head room) than it is to add battery capacity. Plan accordingly.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • bentherebenthere Solar Expert Posts: 113 ✭✭✭
    Re: Ready.. AIM!

    Wow, that was quick. Thanks for all the prompt advice!

    My location is eastern Arizona.

    I shall recalculate my loads, look at where I botched the pv modules, and report back.

    I should have also mentioned that I have a Honda EB3500 for charging in case of extended poor weather, but hope not to run it much.
  • Solar GuppySolar Guppy Solar Expert Posts: 1,989 ✭✭✭
    Re: Ready.. AIM!

    Your will get more harvest with the XW-Mppt over either Flexmax version ... they have a bunch of issues yet resolved.
  • icarusicarus Solar Expert Posts: 5,436 ✭✭✭✭
    Re: Ready.. AIM!

    The whole idea is to avoid running the generator as much as possible. My hunch about that Honda is that is it not very fuel efficient, especially at lower output levels. Try to figure out how to run it 75% loaded if you need to use it to charge batteries. For example,, we run our shops off of generators and we can use the genny to charge at the same time keeping the genny loaded while we are waiting to run the saw again. The whole game is to use as little "outside" energy as possible,,, and then when you do,, use it efficiently.

  • bentherebenthere Solar Expert Posts: 113 ✭✭✭
    Re: Ready.. AIM!

    I believe that my usage calculations are correct. There are only two of us and we don't watch tv. Internet usage... that's another story!

    My mistake was in using an online calculator that didn't properly derate the panels.

    I'm now looking at the same panels, but with two strings of 4 and thinking of bumping the batteries to 420AH Trojans if realistic.

    I'm not sure that I'm exactly clear on the requirement to be able to charge at 5-13% of the 20AH capacity. In this case, with a MPPT charge controller, the actual amperage in my configuration to the charge controller will be maybe 22-ish amps, but at 72-ish volts. The charge controller will then step down to 62 volts (for equalization according to the Trojan site) at 26-ish amps (minus loss). How much loss do I need to account for here? Is it then the 26 amps (in this case, minus loss) that need to meet 5-13% of my 20 hour battery capacity?

    Also, do my strings look safe with a record low of -20F?

Sign In or Register to comment.