# question

## Sizing charge controller

Please may someone clarify something for me on sizing charge controllers:

They are identified as, for eg, 150/100 12/24/36/48.

150 is panel VOC max, 100 is Amps and the rest are the system voltage.

I have been taught 2 conflicting sizing methods:

The number of panels max useable is:

a) based on Amps/(n strings x Isc) i.e. 3 x 265Wp panels per string at 9.03A = 11 strings or 33 panels or 8745Wp (but the MPPT sizing calculator does not offer this answer)

b) based on system voltage x Amps i.e. 48Volt system x 100 = 4800Wp at 3 per string (based on VoC max) = 18 panels maximum. (but the MPPT sizing calculator says we can have 24, or 6360Wp)

Or a 24Volt system = 9 panels maximum, for the same charge controller.

Seeing as the MPPT calculator gives neither answer, then how am I going wrong, or is the calculator wrong?

Many thanks

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Hi Tal. I can't speak for the 'calculator', as I've never used it. I'd have to check it anyway, so why bother? :-)

You seem to be onto the Voc limit/string. Fine, but also watch for exceeding it in low temperatures. 3x 265W panels should easily comply into a 150/.

Your 9.03A is at the end of a panel string. It's at a different V, and has nothing to do with the 100A output limit of the mppt. Look at the mppt specs and you'll see a panel Isc limit noted:

70A for the 150/100 there, so 70/9.03 = 7.75 strings. Maybe the calculator has a fudge-factor to allow 8 rather than 7 strings (wire resistance, weather that's never perfect, etc.).

If you really wanted to max out the number of panels, a 250/100 could likely handle 4 or 5 of those panels/string. The Isc/string won't change.

The above is solely a 'panel side' calc, and the output side can feed 12V > 48V batts regardless. Note the nominal W output there is at a ~14.5V (per 12V batt) level, and typical of many users.

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