Need help with a grid tied system

AmazonWizAmazonWiz Registered Users Posts: 12 ✭✭
edited April 2016 in Solar Beginners Corner #1
I want to connect 8 x 250W panels to a home, in Riyadh, Saudi Arabia. These will be used to run all the lights, fans, misc equipment at home. So I was wondering, how do batteries fit in this grid tied system? I want to monitor the savings I make after installing the 2kw panels. 

Our task:
To design a 2kw system for home use (without any load considerations). Just need to use 2kw of panels. The load needs to be run at night as well on batteries. How will batteries fit into this?


  • BB.BB. Super Moderators, Administrators Posts: 30,712 admin
    More or less, an Off Grid 2kWatt system will cost about 4x the cost of an "equal capability" Grid Tied system...

    A GT system is roughly 77% of the panel rating (panel and controller derating) vs an Off Grid system which runs around 52% of the panel nameplate rating (additional losses due to battery and charge controller losses).

    If you want to see what a 2,000 Watt solar panel system would look like that runs "off grid" power every day... I would suggest 10% rate of charge (13% maximum "cost effective" rate of charge). If this was a backup/weekend system, you can get down toward 5% rate of charge (i.e., 2x larger battery ban).

    Start with a 24 volt battery bank (minimum voltage I would suggest) using standard deep cycle lead acid batteries and 10% rate of charge:
    • 2,000 Watt array * 0.77 panel+controller derating * 1/29.0 volts charging * 1/0.10 rate of charge = 531 AH @ 24 volt battery bank (full time off grid)
    Such a battery system, if you use it 25% per day for two days, and 50% maximum discharge would supply (very nicely):
    • 531 AH * 24 volts * 0.85 AC inverter eff * 1/2 days storage * 0.50 maximum discharge (longer battery life) = 2,708 Watt*Hours per day (night)
    • 2,708 Watt*Hours per night / 5 hours per night power usage = 542 Watt average 230 VAC load (for 5 hours per night)
    Based on hours of sun per day:

    Average Solar Insolation figures

    Measured in kWh/m2/day onto a solar panel set at a 65° angle from vertical:
    (For best year-round performance)
    Jan Feb Mar Apr May Jun
    Jul Aug Sep Oct Nov Dec
    Lots of sun, on average... Worst case average month (December) and toss the bottom three month (5.46 hours for February):
    • 2,000 Watts of panels * 0.52 off grid system eff * 4.43 Hours of sun = 4,607 Watt(Hours per day (December)
    • 2,000 Watts of panels * 0.52 off grid system eff * 5.46 Hours of sun = 5,678 Watt(Hours per day (February)
    You can use ~2,708 Watt*Hours over night (2 nights of storage) and 5,421 Watt*hours per night (1 night of storage)--You have enough sun (on average) that you use quite a bit of energy during the day too (upwards of 4.6 to >6 kWH per day total (day+night usage).

    You could cycle the battery down to 50% state of charge daily--But the batteries will not last quite as long--And you may "deficit charge" the batteries at times (Lead Acid batteries do need upwards of 9 hours per day of good sunlight to fully recharge from 50% discharge--Can be difficult to get that many hours per day of current from a fixed solar array--Tracking will give you "more hours per day" of charging current and allow you to more deeply discharge your battery bank "every day".


    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • AmazonWizAmazonWiz Registered Users Posts: 12 ✭✭
    Thank you Bill.

    Could you tell me what is the 1/29.0 volt charging in the formula you used ?
  • BB.BB. Super Moderators, Administrators Posts: 30,712 admin
    I am not sure what you are asking... is it why I choose a 24 volt battery bank (vs 12/48 volt)? Or is it why I am dividing by 29 volts vs 24 volts? Or is it why I wrote * 1/29 volts as opposed to doing (numbers)/(numbers)?

    In order:
    1. I choose 24 volts (minimum) as taking a battery bank over ~800 AH means heavier cabling, possibly more parallel battery strings, and multiple solar charge controllers (typically the largest is around 80 Amps, or 10% rate of charge on an 800 AH battery bank--Larger battery bank means another solar charger in parallel--another expense vs running at 400 AH and 48 volt battery bank and 48 volt rate of charge--as an example).
    2. The reason I used 29 volts vs 24 volts as a divisor... There are two reasons. First, for larger battery systems which typically use MPPT type charge controllers--Remember that Power=Voltage*Current. So, the solar array+solar charge controller spends a lot of time near 29 volts when charging (over ~80% state of charge, battery bank is near 29 volts when charging most of the time). So, from a solar panel point of view, it takes more power to supply 100 amps * 29 volts (2,900 Watts) vs 100 amps * 24 volts (2,400 Watts). For PWM controllers--They do not "down convert" energy--But instead connect a 35 volt Vmp-array to a 29 volt charging battery bank--A "loss" of energy (PWM do not follow p=v*i equation). Turns out that MPPT conversion and panel losses are "close" to PWM and panel conversion losses--The mechanisms are different, but from a mathmatical point of view, the equations (and deratings/losses) end up being very similar (not worth the work to be "exact"--Solar power calculations are rough estimates vs real live performance and natural variables in weather/etc.).
    3. Lastly using * 1/29 volts vs a "proper" math equation with brackets (12*200*0.60)/(24.49 volts) -- I am trying to write the equations like an English "sentence" (Arabic is probably your native language, may not "map" as well for you) -- So it is easier to understand how stuff above the fraction {abc}/ make things "bigger" and things under the fraction /{abc} make the result smaller. Also, I use the same factors in many places. Like 0,77 derating--Less confusing that writing 1/0.77 as * 1.30 (same number, but have to "remember" both forms that have the same meaning--losses based on panel temperature and converter losses).
    Does that help/answer your question?

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • AmazonWizAmazonWiz Registered Users Posts: 12 ✭✭
    Thanks a lot Bill.
    Point 2 answers my question. Was wondering why it was 29 instead of 24.
    Also what would you have used If the battery bank system was 12 volts or 48 volts ?

    P.S English is my native language =)
  • BB.BB. Super Moderators, Administrators Posts: 30,712 admin
    14.5 volts or 58-59 volts.

    English is my native/only language too--So what is my excuse for such poor English :s

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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