specific PV module / charge controller question

Hey Travis - there are two "gotchas" in your scenario. So,

1)Technically speaking the C40 charge controller can take in up to 120V DC without blowing up (ok, maybe fizzling?), but it would be wildly inefficient to use it at its maximum for a 48V nominal battery bank. You'll probably lose about 40-50% of the array's potential power because the solar panels won't be operating at their optimal voltage point (Vmp for each panel is probably 42V DC or 126V DC for them in series). That gets to the 2nd "gotcha"

2) The nominal voltage of these are 32V. The point where the produce the maximum amount of power is likely to be ~42V. And then the Open Circuit Voltage (Voc) is probably around 52V. Open circuit is the voltage when the panels are first connected or when you throw the breaker/disconnect off/on between the charge controller and the array. The problem is that three of them in series gives you a total voltage of 156V - well over the voltage limit of the C40. You never want to the maximum open circuit voltage of your array to be more than the charge controller can handle - and this scenario would definitely mean you would fry the C40.

By the way, here's one of our articles that goes into more details on all those different voltages and what they mean:

http://www.altestore.com/howto/Solar-Electric-Power/Design-Components/The-Big-e-Demystifying-Voltages/a92/

also see:

http://www.altestore.com/howto/Solar-Electric-Power/Design-Components/Solar-Panels-PV-and-Voltages/a98/

So what to do?

Well, this where MPPT charge controllers come in - they address both issues 1 & 2 above. First, they automatically allow the solar panels to operate at their peak power voltage (which is almost always different than the voltage of the battery bank) and the battery bank to work at whatever voltage it's at. So immediately you'll see more power production than you would with a traditional charge controller like the C40 (ignoring for a moment that it would be fried above 120V DC).

Then second, several newer MPPT charge controllers have maximum (open circuit) voltages at 150V or above. We need to get you to probably 160 to 170V or above, for "no-fry" clearance. . You have a couple of options now in the market:

Apollo High Voltage T80:
http://www.altestore.com/store/Charge-Controllers/Solar-Charge-Controllers/MPPT-Solar-Charge-Controllers/Apollo-Solar-Charge-Controllers/Apollo-Solar-T80HV-High-Voltage-MPPT-Solar-Charge-Controller/p7377/

Midnite Solar Classic 200:
http://www.altestore.com/store/Charge-Controllers/Solar-Charge-Controllers/MPPT-Solar-Charge-Controllers/Midnite-Solar-Charge-Controllers/Midnite-Solar-Classic-200-MPPT-Charge-Controller-200V-79A/p8754/
(my personal favorite because it's the only Charge Controller there that currently provides arc fault protection)

Schneider (Xantrex) XW MPPT 80A:
http://www.altestore.com/store/Charge-Controllers/Solar-Charge-Controllers/MPPT-Solar-Charge-Controllers/Xantrex-MPPT-Solar-Charge-Contollers/Xantrex-XW-MPPT-80A-Solar-Charge-Controller-up-to-600V-DC-input/p8992/

If you could use those Sanyo's in series strings of 2, you could use MPPT charge controllers rated up to 150V that tend to be less expensive. Check out the MPPT Charge controller section for all of your options:

http://www.altestore.com/store/Charge-Controllers/Solar-Charge-Controllers/MPPT-Solar-Charge-Controllers/c474/

Hope that helps!

Yeah, the C40 has been the work horse charge controller for many years - so makes sense you'd consider it.

The Suntech 190W modules would likely be a less expensive solution overall. Using 24V nominal solar panels means you could get to 48V easily (2 in series) using a less expensive controller (like the C40) without have to down convert voltages with an MPPT charge controller. The Sanyo's do squeak out a few more watts per square meter, so you would potentially lose some power if you're trying to milk every square foot of roof space.

Sizing for the XW-MPPT80-600:
There are two thing you need to do when sizing for a charge controller - 1) size the array to not go over the maximum input voltage and 2) size the array so the maximum potential current that can comes out of the controller (going into the battery bank) is not more than the controller's ratings.

For a 48V battery bank, according to the spec page (http://www.altestore.com/store/Charge-Controllers/Solar-Charge-Controllers/MPPT-Solar-Charge-Controllers/Xantrex-MPPT-Solar-Charge-Contollers/Xantrex-XW-MPPT-80A-Solar-Charge-Controller-up-to-600V-DC-input/p8992/) you can have a maximum open circuit voltage on a single string of up to 600V.� However, you have to take into consideration that in colder weather the open circuit voltage (Voc) on a solar panel actually increases. So it's best to stay clear of the 600V by a good margin (ideally do the Voc calculations based on the coldest temps in your area, using the modules's temperature coefficient and size appropriately).

For those Sanyo's the Voc is 50.9V (at standard operating conditions - which means 25C) the temperature coefficient derate is -0.142 V/ °C (see the brochure in the Documents section of the spec page). Now we have to figure out 1) what's the max open circuit voltage a single panel would produce in your coldest temperature and then 2) divide 600V by this max panel voltage to determine how many we can put in series.

Max Open Circuit Voltage of a single panel = (Voc at Std Test Conditions) - (Derate Coefficient x (Std Test Temp - The Coldest Temps for your Area))

So,
Max Voc =50.9V - (-0.142 x (25C - the coldest temp for your area))

Let's take an example - let's assume the coldest it has ever gotten in your area was -20C (-4F). Here's what we'd get:

Max Voc = 50.9 - (-0.142 x (25C - (-20C))
Max Voc = 50.9 - (-0.142 x (45))
Max Voc = 50.9 - (-6.39)
Max Voc = 57.29V

Then we'd take the max voltage input of the XW MPPT charge controller and divide it by this number:

Max Panels in Series = 600V / 57.29V = 10.5

We have to round down (no half panels allowed , so that'd be a maximum of 10 panels in series.

Now a quick and dirty way we can estimate how many panels we can load up on the controller in total is to:

1) Figure out the maximum wattage output of the charge controller (Power = Voltage x Current), which would be 80A x 48V = 3840W

2) Divide that amount by the wattage of the solar panel, which would be 3840W / 210W = 18 Panels.

That works our neatly then for you. You could have two strings of 9 panels in series and run those strings into a combiner box (then on to the controller). For the combiner box, if you have an asphalt shingled roof I'd recommend the Soladeck combiner box - makes it super easy to both combine and do a roof penetration if needed:

http://www.altestore.com/store/Enclosures-Electrical-Safety/Electrical-Enclosures/Combiner-Pass-Through-Boxes/Soladeck-ACDC-Combiner-Box/p9520/?from=featured

Or, alternatively the Midnite Solar PV3:

http://www.altestore.com/store/Enclosures-Electrical-Safety/Electrical-Enclosures/Combiner-Pass-Through-Boxes/Midnite-Solar-MNPV3-Combiner-Box/p4567/

In both cases, because you are working with such high voltages you have to use the fuse add-ons instead of the breakers for these combiner boxes. I believe the breaker options only go up to a max voltage of 150V DC.

And that gets to the last point - be careful! Above 48V DC can stop a human heart. You'll be playing with near 600V. Please take all appropriate cautions so as not to put yourself or others in danger.

Oh, and if you're still wondering on more of the details of what's all needed for a complete system, please do call our very knowledge technical sales folks. They know all the in's and out's and help you be sure you got all the components needed for a safe and successful install on your home. Good luck!