Solar System Calculations

JYanke Registered Users Posts: 39 ✭✭
Looking to see if Im correct on my system design with the below calculations. In reference to potential battery bank and panel upgrades

(2) 365 W panels wired in series:

730 Watt PV Array
Vmp Max: 68.6V
Voc Max: 81.6V
Imp Max: 10.65A
Ioc Max: 11.32A

Victron 100/50 MPPT controller

PV power max for 12v system: 700W
PV max Voc: 100V
PV max Ioc: 60A
Eff: 98%
Charge Current: 50A

12V battery bank: 450aH

I realize the Victron calls for 700 watt array which im slightly over with the 2 panels in series, but it also notes it will limit power if larger array is connected. 

So if I want to have close to a 10% charge rate for the 450aH bank I calculate the following:

450aH * 0.10% = 45A charging

45A * 14.4V * 1.2999 = 840W min Array

450aH * 14.4V * 1.2999 derating * 0.10% = 840 Watt minimum array

So with these calculations it puts me closer to a 8-9% charge rate with only 730 watts of PV power. I think Im ok with the Victrons allowable current and voltage with the exception of the larger than 700 watt max array. Does this sound correct?




  • littleharbor2
    littleharbor2 Solar Expert Posts: 2,054 ✭✭✭✭✭
    You have to run those panels in parallel due to their high voltage and the 100 volt  max limit of the controller.
    You can over panel MPPT controllers to some extent. I wouldn't worry with your setup.
     As long as you are not drawing the batteries down on a everyday basis you should be fine. You need to be able to replace whatever you draw from your batteries daily if you are using this system regularly.

    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.

  • JYanke
    JYanke Registered Users Posts: 39 ✭✭
    The panel voltage and amp specs I listed are for both panels wired in series, not individually. The Voc max for ine panel is only 40.8 volts. I doubled that for the series panel configuration 
  • BB.
    BB. Super Moderators, Administrators Posts: 33,468 admin
    And were is your rough location for this system? How much energy (WH/AH @ 12 volts/etc.) do you want from the system (by season?).

    Will the system stay "small" or need to grow (i.e., 12 vs 24/48 volt battery bank)...

    Guessing flooded cell lead acid battery bank?
    • 450 AH * 12 volts * 1/2 days storage * 0.50 max "planned" discharge = 1,350 WH per day DC energy from battery bank suggested
    • 1,350 WH DC battery storage * 0.85 AC inverter eff = 1,148 WH per day of 120 VAC power
    >1,000 WH per day is pretty nice for a full time off grid cabin (LED lights, RV water pump, laptop computer, cell phone charging, etc.)...

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • BB.
    BB. Super Moderators, Administrators Posts: 33,468 admin
    Can work "backwards" on minimum hours of sun per day for "break even" battery usage and 730 Watt array:
    • 1,148 WH per day of AC power * 1/0.52 off grid FLA system eff * 1/730 Watt array = 3.0 hours of sun per day "break even"
    Amount of sun for your location?

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • JYanke
    JYanke Registered Users Posts: 39 ✭✭
    Thanks Bill, 

    we’ve discussed some of those factors at great length in some of my other posts (which I greatly appreciate) but this is for my small offgrid system at my cabin in southern ohio. The system now only has 225aH flooded 12v bank and 240 watts of pv. From our previous discussions Im getting about a 5% charge rate currently and seems to maintain the batteries quite well for my weekend use (led lights, rv pump, radio, tv etc.) I got about 7 years out of my last set of flooded 6v golf cart batteries.  Want to upgrade eventually just to have more available power for longer stays, growing family and peace of mind really with system capabilities. 

    I guess Im leaning more towards; are my calculations correct while Im essentially doubling my battery bank and upgrading to the 700 watt pv array in order to get closer to the 10% rate of charge? Assuming I didn’t really change much from my current power demands and obviously the amount of sun at my location in Ohio. 

    Also, any thoughts on the Victron 100/50 MPPT controller? I currently have the Morning star sunsaver mppt but it cant handle the voltage of the new panels in series. As for potential for system growth in the future it’s hard to say. Knowing me Im sure Ill go down this path again. 
  • BB.
    BB. Super Moderators, Administrators Posts: 33,468 admin
    Yep... Your calculations are fine. 

    Using the 2 panels in series--The Voc-cold @ 0F is around 96 volts... Close, but under the 100 VDC max for the controller you are looking at.

    The Midnite string sizing tool gives quick detailed answers:

    My quick guess (I don't have your panel specs):

    PV Array
    Rated PV Array Power:730Watts
    Anticipated Array Power @ 104F:681Watts
    Rated PV Array Current:10.64Amps
    Battery Charging Current @ 14.4 V:50.7Amps
    VMP (Maximum Power Point Voltage) :68.6Volts
    VOC (Open Circuit Voltage):84Volts
    VMP @ 0 F°:81.8Volts
    VOC @ 0 F°:95.8Volts

    I am not in the solar biz--But Victron seems to be popular and is sold by our host NAWS (they stand by their products). And Victron seems to be very popular with RV/Van conversions.

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