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Please....Check my math

Nobadays

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Dec 8, 2019
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Have some off-grid cabin friends who are currently terribly low on solar panel wattage. They now have 4 new Rolls Surette 6v 445ah batteries wired for 24v system. Where they live in south central CO we get about 6 hours a day usable sunlight.... but they do have a bit of shading periodically during the day, but not too bad. They estimate average usage at 2672w/day with a max of 4102w/day. Their SCC is a Morning Star MPPT-45. I believe right now they only have about 550 watts of solar.
I was able to get them 6 x 265 watt panels from SanTan Solar. The problem is as these are used panels the label has been removed and the name on the junction box on the back gives me a dead end when trying to look them up.... seems to be a universal junction box not specific to the panel manufacturer. That said, I do have the panel specs.

Morning Star has a string calculator but they require the panel manufacturer to use it. I did search though their list of panels until I found one with specs nearly identical to these. When I ran the calculations using these 265w panels 2s3p their calculator indicated in the "blue." Which they indicate is on the high side but should not cause problems. I would love to do the math by hand but try as I might I just can't seem to get the numbers right.... or I just plain don't understand the numbers I get! Which is more likely! I would really appreciate someone checking this for me so I don't tell my friends it's fine to hook them up, then they burn up their SCC!!!

Here are the particulars and panel specs.

High temp: 93 (very rare! Usually mid 80's max)
Low Temp: -33 (also rare)

Panel specs:
72 cell, polycrystalline
Weather resistant. Certified for salt mist and ammonia resistance
Anti-Reflection surface treatment

Rated Power: 265W
Open circuit voltage (VOC): 44.2 V
Max power voltage (VMP): 35.33 V
Short circuit current (ISC): 8.20 A
Max power current: 7.50 A
Power Tolerance 0/+3%
Maximum system voltage: 600V (UL)
Fuse Rating: 15 A
IP65 Junction Box w/MC4 Compatible Connectors

Thank you so much!
 
open circuit/short circuit (won't happen)
(2 * 44.2v) * (3 * 8.2a) = 2174.64 watts

Theoretical max (also won't happen)
(2 * 35.33v) * (3 * 7.5a) = 1589.85 watts

Reasonably you can expect to get 1400 -1500 watts full sun from a 2s3p setup with those panels but the theoretical max should be considered when sizing the charge controller.

That MPPT-45 will not handle the watts unless you go with a 48v battery bank.

1589 watts / 12v = 132 amps (too much)

1589 watts / 24v = 66 amps (too much)

1589 watts / 48v = 33 amps (ok)

The specs say 1200 watts max at 24 volts which works out to 50 amps so you would be ok with a 2s2p with those panels and that controller.

(2 * 35.33v) * (2 * 7.5a) = 1059.9 watts

1059.9 watts / 24v = 44 amps (ok)

 
Ouch!! Glad I asked! I was going off Morning Star's string calculator.... and using a "similar " panel. Using 2s2p will still nearly double their current wattage and should keep those new batteries healthy I hope.

Thank you so much! I will keep these calculations handy as they are way simpler than all the math I was trying to do... accounting for low temps, equipment losses, etc, rather involved formula I had found.
 

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