Check my math

97TJ

Hello,  It's been some time since I've asked a question here.  One of my 200 watt panels gave up the ghost and seeing that they are over 10 years old I thought I would replace them all and use the good ones on another project.  I upgraded to a MPPT 150 volt 60 amp charge controller last year when I had to replace my old lead acid crowns with 3 100ah lifepo4 batteries.  This last winter was brutal due to the lack of sunshine in my area.  Instead of (4) 12 volt 200 watt panels in parallel for 800 watts, I want to series/parallel (6) 12 volt 200 watt panels for 1200 watts total. The panel I am looking at is rated at 22.5 volts VOC and 12.21A ISC each.   So if I series two panels together then parallel the three sets of two in my combiner box will I have an array of 45 volts at 36.63 amps?  I live in a cold snowy climate.  While I'm not sure what it is, I know that there is an adjustment concerning the voltage for those cold sunny winter days but if my figures are correct I should be with the safety parameters.  Thank you in advance.

Dave Angelini

Old model number and make?  From my experience, you really need more solar. It does depend on your requirements but to me 1500 watts is entry level for a climate you describe  Good Luck!

BB.

As always, refer to the manual for safety limits (voltage, current, etc.)...

Just a quick set of math for sizing the array... Nominally, for an off grid system suggest 10-13% rate of charge, and upwards of 20% for areas with "dark" winters (I.e., not much sun, so over paneling does not "waste" much energy with an XX Amp controller)(Note: 300 AH @ 12 volt battery bank assumed):

• 300 AH battery bank * 14.5 volts nominal charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 565 Watt array minimum suggested off grid array
• 300 AH battery bank * 14.5 volts nominal charging * 1/0.77 panel+controller derating * 0.13 rate of charge = 734 Watt array (more or less optimum for Lead Acid Battery banks)
• 300 AH battery bank * 14.5 volts nominal charging * 1/0.77 panel+controller derating * 0.20 rate of charge = 1,130 Watt array suggested maximum for 300AH @ 12 volt battery bank

Note that 1,200 Watts is "close enough" for 1,130 Watt array "solar math".

For a 60 amp MPPT controller... The maximum "most efficient" solar array for a 12 volt battery bank would be:

• 60 Amps * 1/0.77 panel+controller deratings * 14.5 Volts charging = 1,130 Watt array "typical" cost effective maximum array for 60 amp MPPT controller on a 12 volt battery bank

Again, solar math is "fuzzy math"... More or less, a 1,130 Watt array may "safely and reliably" clip the charging current to 60 Amps a few hours a year on a cool/clear day near solar noon... A larger array will simple "clip" more hours (waste energy) more often... But perfectly OK to "over panel" a system in a "dark winter" location. During summer, you will have "lots of harvested" energy.

Just for a quick summary of some other quick rules of thumbs... Normally would suggest that a battery bank is sized to deliver 1/4 of its storage capacity per day (i.e., 2 days of "no sun" stored energy, to 50% planned state of charge--And on rare occasions to 20% state of charge).

So, for a 300 AH @ 12 volt battery bank:

• 300 AH * 0.25 discharge = 75 AH per day (or over night) discharge
• 75 AH * 12 volts * 0.85 AC inverter eff = 765 Watt*Hours per day/night discharge

And to calculate the hours of sun per day by month (assuming no trees/mountains blocking sun part of the year). Guessing Munising Mi, 47 degree from horizontal fixed array facing south (using 1,000 Watt array as starting point for software):
https://pvwatts.nrel.gov/pvwatts.php


Worst case December with 1.85 hours of sun (not much) and a 1,200 Watt array (a little more clipping on cool/clear days):

• 1,200 Watt array * 0.77 panel+controller derating * 1.85 hours of average Dec Sun = 1,709 WH per average December day (or a little less in December and "over paneled")

For the array--With an MPPT controller, this "class" of solar controller will typically have a maximum solar array input voltage of 100-150 Voc-cold (check manual, ask more questions if needed).

Need to have the exact specifications for your 200 Watt panels... Voc/Vmp/Isc/Imp... You would typically put 2 or 3 panels in series (check Voc-cold for your panels) and parallel the strings (2s*3p or 3p*2s)... 2s*3p would work with almost any solar panels and charge controller.

If you have three or more parallel strings, you are supposed to used a "Combiner Box" with fuses or circuit breakers (help reduce the chance of fire if one panel/wire run becomes shorted)... If you can go with 2s*3p, you can save the price of a combiner box.

Lots of details to work out--In general, size the battery bank to your loads, and size the solar array to your battery/loads/hours of sun. And in the "far north", a Genset, possibly a wind turbine (lots of issues), etc. are usually needed for those dark/stormy days...

-Bill

97TJ

Thank you for your help.  My kilowatt meter shows that over the last 79 days my average usage has been 682 watts per 24hr. Day/night.  By time I add what it takes to run the inverter I'm just under 1050 per 24hr day/night .  I do have a generator to top off the batteries as needed.  So, I will go with 6 200 watt panels in 2S3P into the combiner box I already have.  Thanks again.