Please help me understand voltage and limits.

[Soho]

Hello,

I am a little confused about how voltage works and the limitations of a system. A 40 amp controller and a 24V battery bank have a limit of 960 watts or so. With that in mind, If I have ten 100watt panels wired in pairs. Does that mean that I am only sending 500 watts at 24volts? So I could essentially put more panels if I wanted to as long as the total does not exceed the 960 watts? It is just mind numbing because the panels at 12 volts wire in serial add up to 1000 watts. I imagine that there is a limitation in amps also is it not? In which case I can neither exceed the voltage and or the amps whichever comes first?

The second question, If said pannels where wired in strings of three for a total of 4serial and 4parallel making the grand total of 12 100w panels in a 24volt system. What happens to the extra voltage above 24? I sit simply used by the house on sunny days or does it somehow helps expedite the charging rate of the batteries? My guess is that the excess power is wasted if not used, but I would like a clarification.

These questions are to help me understand the concepts, I am certain that a 48volt system would be preferred in this case.


Thank you

[sensij]

The answer is somewhat specific to the charge controller, but for most controllers of decent quality, you can look at it like this:

The "power limit" is telling you the maximum PV power that can be utilized at any moment, *not* the maximum allowed DC rating of the array. There are many applications in which it makes sense to "over panel" the charge controller, which allows for more morning, evening, and cloudy day production, at the expense of a small amount of non-utilized mid-day power due to "clipping".

The ratings you generally need to watch are the Voltage input limit and the Current limit. The voltage input is calculated from Voc, adjusted for the coldest ambient temps you might see, while the current limit is calculated from Isc.

If you wired all 10 panels in series, you would go way over the voltage limit and the system would shut down or fail.

If you wire 10 Renogy 100 W panels in series pairs, your PV circuit electric design would be based on
Vmp = 37.8 V (18.9 V x 2 in series per string)
Voc = 45 V (22.5 V x 2 in series per string)
Imp = 26.45 A (5.29 A x 5 strings in parallel)
Isc = 28.75 A (5.75 A x 5 strings in parallel)

If you were producing at STC conditions, you'd get 37.8 V x 26.45 A = 1000 W

Your charge controller would receive take that 1000 W and convert to the battery voltage. Let's say you are at 28 V, that 1000 W becomes 35.7 A if the CC is perfectly efficient (it is not), or something more like 33.2 A at 93% efficiency.

If your battery is charging at 24 V, the 1000 W would be converted to 41.7 A (100% efficiency). However, since the CC has a 40 A output limit, what happens is that the PV voltage is increased slightly, which decreases current and overall power generated. The charge controller would increase the PV voltage above Vmp until the converted output was equal to 40 A.

Once the battery moves out of bulk stage (usually when it reaches at least 28.8 V, some mfg's recommend higher) and into absorb, the current will taper down as the voltage is held, and unless other controls are used, any additional generation potential is lost. There are systems that employ "opportunity loads" like water heaters to capture some of this excess generation potential.

[Soho]

Thanks for the reply senji, now is the loss due to increase negligible enough to not worry about it? In this case then if I wanted to fix the issue then it seems the only options aside from going to a 48v system, would be to go to a 30 amp cc. However doing so would also lower the capacity correct? Or the second option would be to go to three panels wired in series to bump the vmp = 56.7v (18.9 V x3) correct? However, if I did that anything past the 40 would either be used by house loads or just wasted correct?

Thanks for the crash course.