[see
Forum Thread this is taken from for more]
MPPTs and String inverters can be quite perplexing at first as you try to figure out what the panel configuration should be. If you're not sure what's meant by
serial and
parallel in regards to solar panels check this
link out first.
Easiest of course is to use a program like
SAM (a big learning curve, but let's you play with a lot of what ifs)/
For this example let's use a
DUAL 2X 80A 3-stage MPPT with a
max PV input 145V
Because this MPPT has a dual input, we'll want at least two identical parallel strings (or multiples of 2) so the power is balanced on each input.
So, let's run through an example!
Let say you take a shine to Panasonic's 330W HIT panels. So you google to find the
datasheet and look up a few things:
Voc = 71V
Isc = 6.08
Temperature Coefficient (Voc) = -0.17C
You want 5 kw and the panels are 335W, so divide it out (5000/335) and that's 14.92 panels. They don't sell fractional panels, so you could round up to 15.
But alas! Your want to have an equal number of panels on each string, so let's bump it up to 16!
So, 16 panels/2 strings = 8 panels in series per string. Each panel has 71V, so with 8 panels that's 8x72= 576V --
OOPS! 576V is way over the 145V maximum of the MPPT....That's not going to work!
The voltage on the Panasonic's is fairly high; what would happen if you used something with a lower voltage like the LG Neon2's? Here's that
datasheet:
Voc = 41V
Isc = 9.8
Temperature Coefficient (Voc) = -0.27C
So two strings of 8 panels would give you 328V and 9.8 amps each. Nope... Well, how about 4 parallel strings of 4 panels? Nope each string is 164V. Still too high! Could you make it work with two strings of 8 panels in parallel? That would be 34V and 78.4 amps... that's just under the limit. It would work, but how about an even lower voltage?
How about those
AltE 150W panels? To get 5000W we'd need 30 panels (4.5kW)! 30 panels is nearly twice 16, that's a lot more racking and wire so it's gonna cost more!
Voc = 22.32V
Isc = 8.86
Temperature Coefficient (Voc) = -0.32%/C
With 30 panels we could do 6 parallel strings of 5 panels in series, 5x22.32 = 112V! Whew, made it! 112 < 142! Sweet, so you'd put 3 strings on the first MPPT input and 3 on the other.
But what about the amps? Each MPPT can take 80 amps, and string is 8.86 amps, so when 3 are combined it would be under 27 amps...no problem there!
Or is there?
Temperature Correction
What's the coldest on record it's been there? Your county will have a magic number for this and you'll have to meet it in order to get your system approved (There's a similar "hot" number for when you get to calculating the wiring).
You see, the colder it gets the
more power the panel produces. If you're in Colorado it might be -40°C. So, that's a temperature delta from STC of 65°C (25°C - (-40°C)), Which we multiply by the Temperature Coefficient, so 65°C x .32=20.8%
(100% + 20.8%) x 22.32 Voc ~= 27 Voct, and with 5 panels in series 27 x 5 = 135v Whew! Made it! Less than 142V on the coldest possible day of the year!
There's also a temperature coefficient for
current, but it works the same way.
Hopefully this was both entertaining and informative for how you can figure out your panel configuration based on your MPPT settings, the panels you pick, and local conditions.
