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Old 05-29-2021, 04:27 PM   #1
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Charge controller sizing

We are interested in getting (4) 445 w panels = 1780 total watts. We read it will require 178 amps at 12v. We havenít found a charge controller that big. Is the information we read true? If so, would it be better to lower our solar panel watts or use 24v? Are there other options? Thanks!

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Old 05-29-2021, 05:18 PM   #2
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We are interested in getting (4) 445 w panels = 1780 total watts. We read it will require 178 amps at 12v. We havenít found a charge controller that big. Is the information we read true? If so, would it be better to lower our solar panel watts or use 24v? Are there other options? Thanks!

If you get max output from 1780w at 12v you are at 148 amps. 445 watts at 12v is 37amps.

Half those numbers for 24v.

There are very few compelling reasons to go with 12v systems once you get into numbers like this, so absolutely go with 24v if you donít have any real reason not to. Itíll save you half on charge controllers, even if you have to rebuy a pump or lights or something itís still likely worth it.

That being said, you can pair two 100a charge controllers or four 40a charge controllers if you want to stay at 12v, or my vote would be to look into 24v and two 40a charge controllers. I would consider a 48v system too, but you have to look at components too, as 48v parts are not as plentiful as 24v or 12v.
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Old 05-29-2021, 09:12 PM   #3
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Agree 100%.

24V.
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Old 05-30-2021, 08:41 AM   #4
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Best efficiency is one MPPT SC per panel.

Victron SmartSolar line is excellent, very flexible.

100/30 model should be fine for a 12V bank

75/15 for 24V
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Old 05-30-2021, 09:02 AM   #5
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Best efficiency is one MPPT SC per panel.

Victron SmartSolar line is excellent, very flexible.

100/30 model should be fine for a 12V bank

75/15 for 24V

If I did the math right (question everything!) 30a and 15a may not be enough. Not sure that youíll practically hit full power but in theory a 30 amp charge controller should be limited to 360w, same for the 15a@24v, so you should likely step up to the next level for 445w panels.
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Old 05-30-2021, 09:15 AM   #6
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For a 100/30, it is 100V on the input, 30A on the output. Iím sure you knew that already but just in case.

If you are hell bent on a 12V system, youíll occasionally max out a 100/30 converter with those panels, but itíll happen rarely. Bulk absorption by LFP batteries happens around 13-13.5V, so with 30A youíll be able to touch 400W.

It is not uncommon to oversize panels and undersize microinverters in modern residential installations, because that - if dimensioned right - can give you peak efficiency.

The Victron controllers are indeed great, I love their stuff too.

But as it has been said before. 12V is too low at those power levels. You will lose efficiency in the cabling, internal resistance of batteries and BMSes. It is not worth chasing the last couple procent by spending $500 extra charge controllers.
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Old 05-30-2021, 10:12 AM   #7
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For a 100/30, it is 100V on the input, 30A on the output. Iím sure you knew that already but just in case.

If you are hell bent on a 12V system, youíll occasionally max out a 100/30 converter with those panels, but itíll happen rarely. Bulk absorption by LFP batteries happens around 13-13.5V, so with 30A youíll be able to touch 400W.

It is not uncommon to oversize panels and undersize microinverters in modern residential installations, because that - if dimensioned right - can give you peak efficiency.

The Victron controllers are indeed great, I love their stuff too.

But as it has been said before. 12V is too low at those power levels. You will lose efficiency in the cabling, internal resistance of batteries and BMSes. It is not worth chasing the last couple procent by spending $500 extra charge controllers.
I just read up on this a bit, John and Bert are indeed correct. I'm very literal with specs much of the time but going over wattage a bit on the charge controllers is fine, just not over voltage. I was afraid it might heat things up in there but it seems to be able to just ignore the extra watts somehow.
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Old 05-30-2021, 05:39 PM   #8
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If I did the math right
Nope.

Amps rating times your charging voltage, not nominal.
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Old 05-30-2021, 05:43 PM   #9
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And your **input** side can go way over the amps spec no worries

but you should stay well below the voltage spec.

Note you will **very** rarely get anywhere close to the rated Watts output, like freezing bright days with reflections off snowy mountains

At 12V figure 150Ah per day per panel actually into your depleted bank, in ideal insolation conditions

half that at 24V
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