# Different numbers from MPPT calculator. What is system voltage?

I'm installing a total of six 210W panels. The panels are 24V each and I plan to connect pairs in series so that I have a 48V system with 3 branches in parallel. Per panel characteristics are Imax = 6.24, Vmax = 33.54, Voc = 39.66, Isc = 6.55. The system will charge a bank of 12V batteries.

On the MPPT calculator I need to enter system voltage, is that the 48V solar system or the 12V battery system? When I enter 48V it recommends the 100/20 MPPT controller, when I enter 12V it recommends the 150/60 MPPT. There's a huge cost difference between the two and of course I want to design for safety. Thanks in advance!

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The system voltage is referring to the battery system voltage.
Even the 150/60 will be over paneled with 6x 210 watt panels. I’d look at something closer to 100amps if you don’t want the charge controller to bottleneck your peak output in full sun.
210x6=1,260watts

1260/12=105amp theoretical maximum output

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As you're at the planning stage, consider configuring the batteries for 24 or 48V. Cuts the current and saves problems with keeping batteries balanced.

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Thanks for the feedback! It will be difficult to configure the batteries to 24 or 48V due to downstream wiring changes so I'm going to drop the system down to 4 panels so I'll end up with 2 parallel + 2 series. The online calculator recommends a 150/60 controller even though I could theoretically have 70A???

210 x 4 = 840W

840 / 12 = 70A

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I guess it will depend somewhat on your location.

I have 810W (3× 270W) flat mounted on our RV roof here in NZ and opted for a single 75/50A solar controller. This limits the solar ouput to a maximum of 712W into our 300Ah LiFePO4 but with perhaps 10 hours over the summer months where it would exceed 50A we have lost nothing in reality.

I have observed a peak 64A on a perfect day with twin controllers but only for a few seconds.

It has all survived 7 years of 24/7 operation.

I am sure a 60A controller would be more than adequate.

Good point, I'm also flat mounting panels on my RV and probably won't notice the difference. I get plenty of sun in southern California. I was more concerned of burning up the unit but if it only limits output power I should be okay

So here's a follow up question - can I use 2 charge controllers in parallel? I can get 2 40A cheaper than 1 70A.

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You can use two charge controllers. I am using two on my system. The roof of my RV would only fit 4 of my 48v panels and I was able to sneak 6 24v panels. The wattage of course was different, so I went with two charge controllers to not lose the higher wattage from the 48v panels in a series/parallel setup. My setup is 4x48v panels paralleled and 6x24v panels in a 2 series 3 parallel to give me 48v on those panels.

To add to your setup from above, when you series, you increase voltage not amps/wattage, so if you were to series your 210w panels, 2 in a series would still be the same amps as one. Only in parallel would you increase the wattage/amps. Honestly the VOC is the main thing you want to look at.