Confused about how much solar I can use

[BroomJM]

When overpaneling (or not) you must be sure to never exceed the max PV input voltage of the SCC nor the max input Isc of the SCC.

I think you're very close to getting through this thick skull of mine...

When you "overpanel", you're bringing in more watts than the SCC is technically rated for, but if you don't exceed the maximum amps OR volts, it will only take in the wattage for which it is rated? Is the excess wattage dissipated as heat? I would like to keep charging to roughly .2C which is why I bought a 20amp SCC for my 100ah battery.

I only have one panel up right now. I checked it several times during near-perfect conditions and it showed 13.3 to 13.5 volts and 10 to 11.5 amps coming in, so roughly 150 watts out of a 180w panel. I can live with 83% efficiency. If I put the other panel up and connect them in series, the voltage should double to around 27 and the amps stay at the same 10 to 11.5, right? That would result in 270 to 300 watts, which is scarcely more than the 260 watts for which it is rated. Those numbers seem very safe, to me. It's also worth mentioning that these panels are mounted with a lot of trees around, so they won't both be getting full sun for very long on any given day.

I'm going to mount the 2nd 180w panel, connect them in series and monitor the output very closely. If it scales in a linear fashion, I should get a potential of around 275 watts for short periods of time, which is just barely more than the unit is rated to handle. Does that sound reasonable?

 

[rmaddy]

When you "overpanel", you're bringing in more watts than the SCC is technically rated for, but if you don't exceed the maximum amps OR volts, it will only take in the wattage for which it is rated?

Overpaneling is simply adding more panel wattage than the SCC is rated for. The SCC can't bring in more watts than it is built to handle.

Don't exceed the max PV input voltage.
Don't exceed the max short circuit current input.

Is the excess wattage dissipated as heat?

No. The potential excess wattage simply isn't taken advantage of. The SCC draws power from the panels. The panels do not push power into the SCC.

If your battery is full and you have no loads then no matter how much solar you have and what the current solar conditions are, the SCC won't take any power and the panels just sit there idle. No "waste heat" is being produced just because the panels could be producing more power if it was needed. Overpaneling is no different.

The advantage of over paneling is getting more power when solar conditions aren't as good. Let's say you have a big load on your system such that you can take the max from the SCC. Let's say that's 40A on a 12V system charging at 14.4V. That means the most watts the SCC can use is 40A x 14.4V or 576W. If the sun is low at the moment you might only get 50% out of your panels, for example. If you didn't overpanel such that you had 550W of panels then you would only be getting 275W and 19A of charging current. But if you over paneled with 800W of panels then you'd be getting 400W and 28A of charging current. Eventually you would get great sun and the 800W of panels could produce more than 576W. But at that point the SCC tops out at 40A and it simply doesn't pull any more than 576W from the panels. Or your load drops and the battery fills up and now the SCC only needs 5A. So the SCC only draws 72W from the panels even if they could produce 800W or 550W or any other amount of power above 72W.

I would like to keep charging to roughly .2C which is why I bought a 20amp SCC for my 100ah battery.

If all you want is 20A of charging then you'll never use more than about 290W. 400W of panels let you produce 290W is lower light conditions giving you the full 20A longer in the day. If you only added 300W of panels then you would almost never get the full 20A of charge current.

If I put the other panel up and connect them in series, the voltage should double to around 27 and the amps stay at the same 10 to 11.5, right?

Correct.

That would result in 270 to 300 watts, which is scarcely more than the 260 watts for which it is rated.

But getting 83% out of your panels is near the summer solstice. Wait 6 months and you'll be luck to get 20% out of the panels.

I'm going to mount the 2nd 180w panel, connect them in series and monitor the output very closely. If it scales in a linear fashion, I should get a potential of around 275 watts for short periods of time, which is just barely more than the unit is rated to handle. Does that sound reasonable?

Quite reasonable.

 

[12VoltInstalls]

Did I make a poor choice in charge controllers,

I'm going to be buying a 40 amp Rover

I would just skip renogy products

Epever, MPPSolar, Victron, Ampinvt, powerMR whatever. But avoid Renogy imho

 

[Robkh]

Started with Renogy Rover40A controller. PV array four 100w panels in series-parallel with calculated Imp 9.8A Vmp 40.8V, well under load specs for the Rover 40A. After 6 weeks the unit melted down, the display failed, the PV wires on a 20 foot run were very hot.
I shut down the system, removed the Renogy controller, reached out to Renogy who declined to help as I bought the kit from the Renogy store on Amazon.
I replaced the controller with a Victron SmartSolar MPPT 100V 30A which runs on the the same PV array as cool as a cucumber.