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piomark avatar image
piomark asked

MPPT calculator voltage recommendation

Hello,

I'm preparing to deploy new solar panels on my roof and I'm trying to chose correct solar charger/panels pair.

In my location the temperatures can go down to -5 deg (for last 4 years the temp did not go below 0 for longer than 1-2h), therefore to keep it safe I've put -10 as the lowest temp.

With 3 Ja Solar 410W panels I get 149.9V max in coldest contitions.

The calculator suggests 150 versions of mppt.
Somehow my gut feeling says that this is too good to be true, what safety margin I should add to what the calculator shows?

Any help would be much appreciated.

Piotr

mppt calculator
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kevgermany avatar image
kevgermany answered ·

I remember one winter in the UK when we hit -30C. It was over 40 years ago, though.

With the effects of climate change, extreme weather is becoming more common. It's impossible to put a completely safe number on your safety margin. It really depends on how willing you are to replace the MPPT if it is damaged. And on the impact if you can't replace it quickly.

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Jason - UK avatar image
Jason - UK answered ·

@piomark Cutting it a bit too close if you ask me. Not knowing what exact JA Solar panels you are looking at, the ones I found had a open circuit voltage of 50.12v so that would be over. If the MPPT fails, and it record a voltage greater than the max, you'll have a hard time getting it replaced under warranty. Do you have a link to the panel data sheet?

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piomark avatar image piomark commented ·

Hi @Jason - UK ,


The model i was looking is jam66s30-490/mr, it has an open circuit voltage of ~46V


Datasheet: https://www.jasolar.com/uploadfile/2022/0928/20220928044113690.pdf


According to the mppt calculator, 3s configuration has 149.1V open circuit voltage at - 10deg.

As the 150 is the absolute max having just 1V margin sounds risky, the other thing is is it really possible to get such a high value?



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ejrossouw avatar image ejrossouw piomark commented ·
The calculator now incorporates a search function using a global panel database. I'd personally be fine with the recommendations given that it is under ideal lab conditions and once you take voltage drop etc. into account, you will very unlikely put the MPPT at risk or ever reach that voltage.
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Jason - UK avatar image Jason - UK piomark commented ·

@piomark The panel performance is based on 1000w/m2 so you could factor it down based on the pitch and angle to south. On the other hand, I have a 16 x 360w south facing roof with panels at 35*C. I've had periods where I have had over 6kW from my 5.76 rated array, when you get a sudden burst of sunlight on a cold panel, you can get situations where the panels exceeds the rated output, for me in excess of 4%.

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