Formulas for off grid solar systems

Sharp230Sharp230 Registered Users Posts: 22
Can someone help me with the formulas for sizing a 24v solar panel system, as I have, (8) 6V AGM 200ah batteries, (4) 230W panels, 60A
Morningstar charge controller, 1000W Samlex inverter.  I would like to upgrade my system to accommodate 3000wh.
Formula sizing, panels, charge controller, inverter, and batteries? Had them at one time, however computer crashed, had to format disk.
Would like to thank all in advance!!


  • PhotowhitPhotowhit Solar Expert Posts: 5,987 ✭✭✭✭✭

    Just give us all your money!

    Ooops rather give us all your information.

    Load 3000wh? Load during the year (winters in northern hemisphere have less sun and often larger loads... then again if you run an AC in the summer time and heat with wood...)

    Days of autonomy(how many days of no sun do you want to withstand with out running a generator)

    Time of day of your loads?

    Will you have an alternation energy source?

    Direction of solar array, will it be shaded at any time?

    Location, where on the earth are you?

    Do you need a larger inverter?


    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, 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.
  • zonebluezoneblue Solar Expert Posts: 1,220 ✭✭✭✭
    Having experience with your current system puts you in a pretty good place to know how well solar performs at your location, azimith . I find i can simply the math for a given location, by figuring out the Wh/d per Wp ratio from practice. In other words if you have 1000W solar, and it consistently produces 2kWh/day then that figure is equal to 2. Thus to scle up to 3kWh/d you need  3kWh/d /2= 1500W solar. Etc.
    For the battery you want to aim for a 0.1C charge rate, and a 0.25C discharge rate. That combined with the forum rule of thumb reratings:

    PV+CC 0.77
    Inverter 0.85
    Battery 0.8

    Gets you pretty much everything you need.
    1.8kWp CSUN, 10kWh AGM, Midnite Classic 150, Outback VFX3024E,

  • Sharp230Sharp230 Registered Users Posts: 22
    Thanks a lot Zoneblue, really appreciate your help!!!  God bless you!
  • Sharp230Sharp230 Registered Users Posts: 22
    Thanks Photowhit for all your help!  God Bless You!
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