It's definitely efficient enough to operate when you are actually using equipment. We have a shut off switch to shut it down when not at our cabin. That way it is not slowly draining the battery while running nothing at all. It is not perfectly efficient, but the way I look at it is that we gained by charging at 24v we lose some of that in the conversion process. Also you pretty much have to go higher than a 12v battery bank at a certain system size. Specs on the unit say it is 85% efficient, and has a 0.2 (2/10ths) of an amp for a no-load current draw. The no-load draw is why we installed a properly rated shut off switch between the battery bank and the 24v to 12v converter.
As to the other question, the small charge controller would only handle your 300 watts at 24v. And yes you could run three 100 watt panels in series, I believe the max voltage to the charge controller will handle up to four 100 watt panels in series. (check the max input voltage in the manual to be sure).
If I were in your situation, I would probably get the 40amp charge controller, and an extra panel if you want, and wire them in series. Then just keep everything 12v until you want to replace the current 12v battery. At that point you could go to 24v. The larger charge controller would give you the flexibility to do whatever you find works best. Starting with that would give you time to learn your needs, and avoid purchasing another battery until you actually need it.
Renogy 100w get free delivery, 1.30 perwatt. Bigger panels no free delivery. Also easier to deal with. Sturdier. Victron Blue solar mppt, excellent European company. 5 year warranty. Website will build confidence.
+1, Love my victron, just got the cable recently for the full adjustability now, very nice. I have had some renogy products in the past, i feel they are okay, kind of the better end of the cheap stuff. Didnt pay too much more for my victron though and no comparison.
I have one of the EP 3215 model, (30 amp 150 Voc), with the optional meter. Voltage setpoints are adjustable with the external meter. It has done its job without any issues.
When you are thinking about adding additional panels remember you can double the input wattage when switching to 24 volt. Be sure to series wire your panels for higher than system voltage if switching to 24 volts and always connect your battery bank first , which allows the controller to sense whether 12 or 24 volt system .