I currently have a C40 (similar to the C35) and a C12 running. If I were building a small system that would never grow beyond 12 amps for lighting or a small load under 12 amps, then the C12 is dirt simple to hook up and run, and has been reliable for the few years it has been installed - and it can function as the charge controller and the load controller at the same time.
If, on the other hand, I was building a small system for now, but wanted to buy more controller than needed for future growth, then I would probably by the larger controller. Keep in mind that the C35 can function only as a charge controller, or a load controller, but not both at the same time. With only the solar connected, you will not need a dump load using it as a charge controller, as it will not over-charge the batteries. Additionally, I have not had good luck using the C40 as a dump load controller with a smaller sized battery bank as for some reason the way that it is set up seems to let it discharge smaller banks too much. I don't know how the C35 compares but my guess is that it functions similarly. Thus, I would use the C35 in its charge control mode.
If I wanted to build an even simpler controller that would handle my 80 Watts for now then there is this fellow from Ohio who has a simple dump load circuit ready to assemble with all of the parts included. It can be configured as a dump load controller to dump excess power if the panel is connected to the batteries directly through a diode, as a low voltage disconnect, or even to disconnect the batteries once the battery voltage has risen past a certain point. Seems a reliable enough fellow for me to recommend: http://ghurd.info/dc.html (As a disclaimer, I should note that Ghurd is a friend of mine, and of course he is a regular poster here - and he's slowly getting rich off of these - I suspect he's taken in at least 48 bucks from selling those by now.) If you utilize the C35 as a charge controller, then you could utilize one of these to automatically shut off your loads once the batteries discharge to a certain point, thus protecting the batteries from discharging too deeply, which can be a problem for unattended loads, or inattentive usage, both of which are frequent modus operandi of battery murderers.
Otherwise, you could likely build your own dump load controller or Low Voltage Disconnect (LVD) something like the op amp comparator circuit 14 here:
http://www.uoguelph.ca/~antoon/gadgets/741/741.html
With an output something like figure 9d except that I would replace the 2N2222 with one or more Power N-Fets capable of sinking the small amount of current that I would need to dump or control to start with for the smaller panel, and I would connect a resistor back from the output to the voltage divider to provide some hysteresis so that it did not continuously flip on and off around the cut off/turn on voltage. You would have to set the dump voltage or disconnect voltage at a level suitable for the type of battery that your system was utilizing.
Rich
