hopefully you will read this :

http://www.rvforum.net/SMF_forum/index.php/topic,113514.0.htmla few points to note here..

1. 1200 watts usually requires a higher voltage battery, however in this case with LFP we CAN think differently and forget FLA rules, but at a cost

as we shall see below..

2. 1200 watts of panel into a 12 Volt battery requires a ( 1200 / 12 ) = 100 Amp controller !!!

this is not a good idea and certainly expensive..

3. using a prime number of panels in series is not normal practice for off grid, however if the VOC is in limits then you can do this.

for 6 panels on an RV you only have two practical choices: 3S2P or 2S3P there is the option of 6S, but unless

you boon dock in the desert with no shade, then it's not recommended.

lets look at the panel output...

for 3S2P you will have 3 * 21 vmpp = 63 Volts at a 9.5 A * 2 = 19 Amps

the main issue here is the VOC voltage.. in cold weather the panels in series will max at approx 25 * 3 * 1.25 = 93.75 Volts.. that's uncomfortably

close to 100 Volts of your proposed controller. one really cold desert night with bright morning sun will damage a 100 V controller.

for 2S3P you will have 2 * 21 vmpp = 42 Volts at 9.5 Amps * 3 = 28.5 Amps

and a max VOC of 25 * 2 * 1.25 = 62 Volts. a much better fit for a 100 V controller.

ok, now lets talk MPPT controllers..

1200 watts is well above what the industry recommends for 12 V, also at this amperage ( 1200 W / 12 V ) = 100 Amps you will need

heavy cabling from the controller to the batteries. quite often with lower cost controllers the input terminals are sub standard and cannot

accept the correct size cables so beware...

your controller inputs are :

a) 63 Volts at 19 Amps with 3S2P

b) 42 Volts at 28.5 Amps with 2P3S

and your output is 12 V at 100 Amps max.

you can expect to pay upward of $700 for a 100 A controller. also there is the cabling issue.. for such high

currents you will need heavy cables to minimize losses and that adds to cost and weight.

let's do a simple example using 35 feet from panel to controller and 6 feet from controller to battery bussbar

we are going to design for 3% or better voltage drop in both paths. ill omit the calculations for now..

Using standard AWG cable resistance for copper:

for 3S2P we have 63 V @ 19 A, we could use a minimum of 10 AWG

for 2S3P we have 42 V @ 28.5 A, we could use a minimum of 8 AWG

for 12 V @ 100 A we could use a minimum of 3 AWG

ok so $700 for a controller is a bit high.. so what other options are there ?

well, in the 3S2P setup we could use two 50 A controllers..

each string is considered as a single array, the controllers would both feed the 12 V busbar.

only thing here is to setup both controllers with the same charging profile.

for 600 W we require 600 / 12 = 50 Amps..

a good 50 A 150 V MPPT can be had for $250 upwards..

battery capacity:

for LFP, use my calculations on the page I linked for lead acid and multiply by 0.75

( for 100 Ah FLA you need 75 Ah LFP )

ok.. what would I do ?

well i would do a power budget and determine my REAL needs first, then design the system around that..

if you intend to stay 12 V then I would suggest to lower the panels to 800 watts, increase capacity to 600 Ah and set the array

in a 2S2P format and use a Victron 150/70 controller. or expect to spend big bucks on a high current controller..

without the 12 V limitation then I would go up to 24 or 48 V..