Originally Posted by tedsherrin
Yes, I believe it probably IS simple maths, but for something so simple I'm having a devil of a job getting simple confirmation of what this simple equation is. Is it the max amp size of the controller muliplied by the max volts? Or is it the max amp output of the panels
muliplied by the number of panels
Are you asking about ratings or actual output?
All of the MPPT
controllers I have seen will list max. input voltage (Voc) and max. input wattage and will be rated at continuous amps of output. They will probably be able to exceed that rating by 10% or slightly more for short periods, but a 30 A controller shouldn't be expected to hold up forever at 33 A output.
Every controller has a different Voc, even if they're rated at the same current
. I have one controller rated at 25 A, but it can't accept any higher than about 21 or 22v, which limits it to what people call "12v" panels, which generally put out about 18-20 Voc and around 16-18 Vmp.
So this particular controller could take 400w @ 18v (22.22 A) and will lose maybe .5 or 1 A in conversion, but put out about 25 A @ 14.8v. There are several things to consider. 400 w of solar panels
will rarely, maybe never, put out full power. Maybe for the first 2 weeks when it's higher than rated, but after it settles down you might only see 380, 360, maybe 350 w out of the panels. So you might end up with 23 or 24 A of current
out of the controller. If there is ANY shading at all, even a tiny amount, you'll see a much larger drop in solar
output than most people would guess. Don't forget that unless you go with monster sized cables
, there will be some loss between the panels and controller and controller to battery
Another example is a 20 A controller I have. It will take 300 w of power for a 12v system, or 600w for a 24v system. It can take up to 42 V input, which allows the use of higher V, lower current panels, which exhibit less power loss in the cabling. Although it's rated at 20 A, I've seen it put out 22 - 22.5 A for short periods because there is 410 w of panels connected to it.
Keep in mind that if you want to run a 24 V, 36 V or 48 V battery
bank, then you'll need to shop for a solar controller that can output those voltages and solar panels
that are even higher, like 60 V. Another option is to connect in a series/parallel configuration like 4 x 40 V panels, 2 in series, 2 in parallel for a total of 80 V.
Now here is something to ponder. I generally tell people to size their solar array based on the amount of power they need.
For example, let's say someone says they need 50 A of power, I'd suggest a 75 A controller (so the controller isn't maxed out all of the time), with enough panels to supply about 55 A peak, and less off peak. That might be 800 w to 1,000 w of panels, depending on Vmp. With this setup, early in the morning you'd see very little power, mid morning you'd see 25 or 30 A, midday you'd see peak power, midafternoon maybe 25 or 30 A, evening almost nothing. These are just round numbers for illustration purposes.
However, in my case, I've attached 410 w of solar panels to a controller that is rated to handle only 300 w. What happens, compared to an identical controller attached to 300 w of panels is that the output rises faster, earlier in the day, peaks at 22 A midday vs 18 or 19 A, and continues to produce more power (closer to 20 A for longer) toward mid to late afternoon.
I would still gain more power if I attached the 410 w of panels to a 30 A controller, but I've come pretty close to what a 25 A controller would have produced. Since the 20 A controller was only $100 and the 25 A controller is about $210 delivered, I thought it was worth the risk of burning up 1 $100 controller just to see what it could do and if it would last. So far, so good.
For the record
, the only way I could see these output numbers was to put an abnormally high load on the DC system. Normally these are sitting in storage
, just keeping the golf cart batteries topped off, so they're normally just loafing along in float charge mode. But it's nice to see what they can do at full load.