HyperVOC

Why HyperVOC
The main question asked regarding MPPT solar controllers is “How many panels can
I hook up in series?” Since we designed the OutBack MX60 a decade ago, we have
answered this question thousands of times!
This is the scenario:
Consumer:
I have a 150V MPPT controller. How many panels can I put in series. I plan on
using a panel with a VOC of 44.3V.
Technician:
How cold does it get where you live?
Consumer:
I live in Alberta, Canada. It gets mighty cold up here, Aye, -32
0
C in winter.
Technician:
The panels will output higher voltages in cold weather. The NEC says you
must multiply the VOC of your solar panel by the correction factor based on
your coldest conditions. In your case the correction factor is 1.23. Let’s see
what happens when we apply the 1.23 correction factor to 2 and 3 panels in
series. Two in series looks like this: 44.3V x 2 x 1.23 = 108.98V. Three in series
looks like this: 43.3 x 3 x 1.23 = 163.46V.
On a normal 150V controller you can put 2 panels in series. The highest volt-
age the controller will see is 108.98V. That is below the 150V max input volt-
age. If you put 3 panels in series on a standard 150V controller, it will see up to
163.46V on a cold morning. I suggest you read the directions and warranty
information on your controller. Go to the section of the instructions for your
normal (other than MidNite) controller that deals with PV input voltage. It will
read something like this regarding voltages above 150V. “Voltage above
150VDC will cause the controller to explode into a huge ball of ames leaving
a smoldering crater. This condition is not covered under warranty.”
I might have paraphrased a bit but you get the message, they do not want you
going over 150VDC on the input. Here is a little known fact about PV output volt-
age and charge controllers.
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MidNite Solar Inc
www.midnitesolar.com
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