DIY Manual

19 | P a g e 10-543-1 REV A
DIY Manual
As you can see, selecting and buying PV panels comes last. So, let’s make this work with the
new 340W panels you bought!
1. Most PV arrays are about 80% efficient. If you need 8000W of panels, then you actually
need 8000W x 1.25 (the inverse of 80%) = 10,000W of PV.
2. 10,000W / 340W = 29 PV panels. 29 is an odd number, so that either goes up to 30,
maybe down to 27. Maybe 26. We don’t know yet, so let’s move on for now.
3. The Voc is 42V. The batt bank voltage is 48V. Since you plan to use the MidNite Classic,
you know it needs a PV input voltage that is at least 33% higher than the highest batt
charging voltage. So, look at the batt manufacturer’s spec sheet for your new batts.
They are sealed batts, so the highest will be Absorb, and that is at 59.2V, for example.
4. So, 59.2V x 1.33 = 78V. Thus, the PV string needs to be wired to offer more than 78V. If
you wire 2 of these new panels in series, that is 2 x 42Voc = 84V. That is enough for
now, but maybe not when it gets warm! At about 97F, that 84Voc will lower to about
78V, thus being marginally close to the point that the Classic will not work. Let’s get that
PV voltage higher then!
5. 3 x 42Voc (3 panels in series) = 126Voc. This is high enough that warm temps will not
impact it. But cold temps will raise the voltage. And when the string voltage exceeds
the input limit of the charge controller (150V in the case of the Classic 150), then the
controller will either rest and protect itself (like the Classic) or be permanently
damaged. At -5 to -13F, the PV string voltage of 126Voc will reach 151V. Bottom line …
if this installation location will not see temps colder than -5F, then these panels, wired 3
in series, will work fine with the Classic 150.
NOTE: PV voltage is influenced by temperature: cold temps raise the string voltage;
warm temps lower the voltage. The Classic charge controller has a special feature called
HyperVoc. The Classic will self-protect if the string voltage exceeds the Classic’s input
voltage limit + battery bank nominal voltage. Example: A Classic 150 on a 24V system
will self-protect if the PV input is between 150V-174V (150V + 24V).
6. Now we know the first element of the PV design – 3 panels will be in series to make the
desired 126V. Therefore, the PV array has to be in multiples of 3: 3, 6, 9, 12, 15, 18, etc.
7. Remember, we need 29 panels. So, 29 / 3 = 9.6 strings. How many panels will the
Classic 150 handle?
8. The Classic 150 in 48V can make 86A max. 86A x 48V = 4128W. Again, most arrays are
80% efficient, so 4128W x 1.25 = 5160W of PV wattage can be handled by one Classic
150 in a 48V system.
9. 5160W / 340W = 15.1 panels. With 3 wired in series, we now know the first array needs
15 panels, thus 5 parallel PV strings (3S/5P = 15 panels).