Am I doing the math right?

[sl dl]

Hello. I am new here.
I have been using 50 watts of solar for an off grid weekend cabin since 2009 but always 12v.
I usually run less than 4 amps total and do that mostly direct from the batteries not the load terminal on my controller. Some CFL or leds, occasionally a 12v tv or small 12v pump.
These new panels will let me increase the current watts by a factor of 7 so she can charge her phone, computer, hot spot and whatever else she has in mind and I can keep the batteries full. I'm afraid she will find out there are 12v electric blankets.
the new panels are two 2 year old 175W panels, suntech, for 50.00 each. They are 24V
Label says Pmax 175W, Vmp 35.2, Imp 4.95, Open Circuit 42V. Isc 5.2A
I can use them together or separate 1 bank or 2 banks. 2 banks are better since batteries are similar but 2 years apart. Or I can use 2 panels on 1 bank and swap out periodically. Banks are 2 @ 125AH AGM batteries each x 2 banks.
I am trying to figure out how to size the controller, by the panels or batteries.
In other words:
175W at 35.2v = 4.97A x 2 panels = 9.94A. But when i convert that to 12V it will become 19.88 Amps except that the controller will charge at 12V but more likely 13.7 prox depending on the battery type and status. so 12/13.7 = .876 x 19.88 total Amps at 12v = 17.41 total amps max at ideal conditions. From this it looks like I could use two 10 A controllers or one 20 Amp controller. I think I would rather have 2 controllers for 2 banks and only combine the banks if I had a larger load for a short time. MPPT.
Am I right to ignore the open circuit voltage and amps in this calculation?
Or:
I could series connect two batteries and get 24v and use a dc-dc inverter to make 12v to feed the loads. If I did that would I lose much using a PWM 24v converter and the inverter. could i run both banks into the same inverter,

thank you in advance for the help.

[solar pete]

Hi sl dl and welcome to Solar Panel Talk, lots of good info around these parts.

Loads, .......I wanta here more about the loads, we need more info on the location, more info about the loads, need to know how many kwh (kilowatt hours the system needs to deliver) or watt hours, I like the bit about the 12v electric blanket

Read up a bit in the off-Grid section of the forum, get a handle on what the system needs to deliver, always remember 12v sucks, if this set up is a permanent / semi permanent set up think 24 or 48 volts, there are others who know about this stuff and they might chime in but answer the questions to ensure you get a good outcome, cheers....I need coffee

[sl dl]

Solar Pete thanks for the enthusiastic welcome This system is permanent in a very rustic 100% off grid 1 room weekend cabin; hunting, sleeping, abusing alcohol, telling lies,peeing into the wind. That sort of stuff. I am already running 50 watts Mono at 12v and it is adequate but just adequate. I have to keep an eye on the two 125ah batteries and will not run them down below 12v. and usually they do not go below 12.2 or 12.3 in a weekend. I was there in November for 10 days and had to swap out the batteries after 6 days of very little sun. So, 50 watts is adequate for the loads.

I only bought the 2 year old 175 watt panels because I got them for 50.00 each and I am now spending more time there and want to max the life of the batteries. The loads are 4 12v 13watt cfl which pull about 1 amp each. Never more than 2 on at a time, and usually only 1 on and mostly at night. Other than that, i have a coleman propane on demand water heater and the igniter is on a small rechargeable 6v battery which I charge from the batteries when necessary. I have two batteries and usually just bring one home and charge it there. Other than the lights we charge cellphones. I have a small 1 gpm water pump for showers but i seldom use it off the system, preferring to use a separate battery or just heat water on the stove and "shower" out of a bucket. Ever dump hot water over your head on a 30 degree day? I have an am/fm/w/sw radio which i use with 4 D batteries. The only time I have a load which concerns me is if I want to watch the 7"tv, yes 7",and I have to hook up the 250 watt inverter because the antenna only works off 120. Very inefficient. I still never run the batts too low just have to watch more.

The loads are not enough to worry about in relation to the capacity i will have. I am increasing my current marginally adequate system by 7 fold and will have a lot of excess capacity. I will for the most part not be running the loads off the charge controller. I just need to know how to match the controller to the panels to generate electricity to be able to run what I have described above

My question is how do I size the controller to the panels and batteries given the parameters in my original post. and given that these are 24v panels into a 12v system. Everything is already given except the size of the controller.
Basically I am not sure whether to calculate the amperage size of the controller with the amperage from the batteries or the amperage from the panels.
Thanks again.
BTW she does have a 12v electric blanket in her car for winter emergencies. We went to the cabin mid Jan and it was 20 degrees inside when we got there but she was a trooper.