Jan White wrote:Hmmm...
I think our batteries are 90Ah. The only thing it says on them is 220 minutes runtime 25A @ x temp, etc. So 220/60*25=91.6 repeating.
Which, going by your rule of thumb, means 9A charging @ 13V x 4 batteries equals 468. With fudge factor for poor conditions, that would bring us up to 6 panels. A 60A controller won't cover that many on a 12V system. Can I ignore the poor condition fudge factor somewhat if I'm using an mppt controller?
Here is what happens in the real-world. Your 100W panel will NOT produce 100W unless it is under perfectly ideal conditions, which are almost never present. If you point your 100W panel directly at the sun and measure it's total output, you are not likely to see more than 85W, or 85% (0.85X). That's because the panels are all tested in artificial sunlight at 1000W/meter squared. So, that's where the fudgefactor comes in. I could have told you to divide by 0.85X, but it seems that most people get confused by math with multiple divisions and multiplications. So, I try to keep it simple.
So, for your batteries (sorry, I initially missed the 4) the math would be 90Ah X 4 batteries X 0.125C X 13V charging X 1.25X FF = 730W. Maybe three 245W grid-tie panels. I just paid 55$ each for those last season. That works out to be [(245W X 3 panels)/13V charging] X 0.85FF =48A.
A little high for a 40A controller, but for a few $ more you could get a 50A controller. Look at Epever's 50A Tracer. They come in both 150 and 200V models. I'm guessing that most likely the controller you are looking at has a 100V limit. That's not very high by today's standards. Let's say you wire a total of three 30Vmp (37.5Voc) 245W panels in series. You'll get 90Vmp when the panels are producing power, but 112.5Voc when the controller shuts off. Voltage that high might fry a 100V controller. But, the Epever 50 would handle that just fine. Another combination would be six 100W panels (18Vmp; 21Voc). Six in series would put out 108Vmp. When the controller switches off the voltage would go back up to 126Voc. That might fry the cheaper 100V controllers.
You get a lot more flexibility when you go with higher voltage. You save a lot of money buying thinner copper wire. You also have more choices of where you can position the panels because you can place them 100-150 feet away from the system. No, you can't ignore the fudgefactor for MPPT. In fact, that fudgefacter is better for MPPT. For PWM controllers, the FF is more like 0.6-0.7 instead of 0.85.