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Old 05-24-2016, 05:14 PM   #20
twistedtree's Avatar
City: Gloucester, MA
Country: USA
Join Date: Jan 2013
Posts: 4,983
The rated max voltage from the panels is at 70F I think, and max sun light. The colder they get, the higher the voltage. I haven't done the calculations in a while, but as I recall you adjust for the lowest temp you think you will even encounter (not average, but lowest), then provide a safety margin like 10-15%. NEC in the US spells it all out, and I expect the CN rules are the same or very close.

At our place in VT I designed for either -20F or maybe even -40F. -20F definitely happens, even during sun light. -40F during sun light would be very, very rare.

In your case you have two mitigating factors. First, in the PNW the winter temps are actually quite mild. Some compensation will be required, but nothing like inland and at elevations. Also, I presume your panels are flat on the roof? In the winter the sun will be very low on the horizon even at mid day. Probably no more than 20 deg in your area. So there is a huge angle between the panels and sun significantly reducing output. In contrast, my panels in Vermont get tilted to be almost completely vertical in the winter.

So I'd run the calculations, but be sure to use realistic temps, and you can probably de-rate for light incident angle.

OK, rather than be lazy I went and checked. I used 125% of the panel's VOC as the design max. So you can use that to figure out how many panels you can wire in series before you exceed the input voltage of the charge controller. My charge controller's limit is 150V and I ended up with 4 panels in series for a 125% VOC of 110V. Electrically, 5 panels would have been possible, but I didn't have physical space to mount groups of 5, just groups of 4.
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