Matching Panels to Charge Controller

[kittysmitty]

Just trying to understand. Solar panels are sold by watt rating, charge controller by amp rating. Help me understand the math. I have a 30 amp controller. Here are the spec on a panel that I have modified slightly to make the math work.

• Maximum Power at STC(Pmax): 92Wp
• Maximum Power Voltage(Vmp): 18.34V
• Maximum Power Current(Imp): 5.00A
• Open-circuit Voltage (Voc): 21.92V
• Short-circuit Current (Isc): 5.43A

So with my 30 amp controller can I have 6 of these panels  ( 30 amps/5amps= 6 panels, or

 5 panels ( 30 amps/5.43 amps= 5.52 panels rounded to 5 panels )

[Flux]

If it is a diversion controller then yes 6 panels will be ok There few places on earth where you get the rated power. For series control or diversion you should be fine.

If it is a mppt controller then there is not really enough information to pass judgement.

My experience is based on the UK where I have never seen rated power. If you are in a cold clear sunny area then perhaps you may be close to the limit. If it is a cheap controller the 30A may be questionable, if reputable it will take a bit of overload so there are a few factors here.

Flux

[Mary B]

Today the sun bouncing off the snow pack onto the 25 degree panels had the voltage up over 90, above rated for my string of 4 panels.Below ratings on my CC of course because I planned for it. It is really evident when it drops to -15f with lots of sun off the snow.

[DamonHD]

I have occasionally seen edge-of-cloud effects taking my panels (and inverters, because they can briefly do it by design) over their rated output here in murky London, but it's rare.

One of the reasons I suspect, along with my E- and W- panel orientation, that the local electricity distribution operator was happy to waive the limit in the amount of solar PV that I was allowed to connect to the grid without vast expense and paperwork (G83).

Rgds

Damon

[clockmanFRA]

When I am sorting out PV panels for a NON GRID Tied arrangement, ie battery charging or AC coupling, I have to accept what panels I get. I am a skinflint, so its best watts per buck.

I now use MPPT controllers, and get the voltage as high as possible in relation to what the controller can handle. This saves in line/cable losses, and also means I still get the MPPT controller shoving juice into my batteries or AC Coupling grid when the sun is poor.

If your 30 amp controller is a MPPT, then how much voltage can it take?
A MPPT Tristar can take about 120vdc input, yea it says 150v but believe me it won't on that odd occasion when you have a crisp clear sunny day.

Therefore 6 x VOC = 131.52vdc as the panels are series connected then the amps will be 5.00amp.

If your controller is good for 70vdc input only,  then a string of 3 x VOC = 65vdc  at 5 amps in series connection and a another string of 3 x VOC = 65vdc at 5 amps.
This then gives 65vdc at 10amps into your controller.

Split the panels into 3 strings of 2 panels each,  so that's 43.84v at about 15amps going into your controller.

NB, I now normally allow a over and under panel production of 10% this stops my controllers from smoking. Some controllers now, like the Midnite stuff has a built in allowance for overvoltage, on those crisp sunny days, which at this time of year are eagerly awaited.

Trust this helps.   
 

[DamonHD]

Indeed "eagerly awaited".

Here February tends to get the best astonishingly good clear cold days for PV.

Rgds

Damon

[mab]

Have to say I've frequently seen 125%Imp from my panels and once 140% (very briefly) due to cloud lensing and being on trackers so panels are square to the sun. On bright cloudless days I get ~Imp.
 
That said, I don't use MPPT so a small portion of that would be due to operating at below Vmp, and i'd have to allow another small amount for possible calibration error on the meters, but even so I always size my dumpload/controllers for 120% Imp minimum.