Hello,
SAM calculates the PV array's output assuming that the array is a single unit with the same physical and electrical characteristics. When you use the backtracking or self-shading options, SAM makes adjustments to the array's output using the parameters that you specify for each of those options. For example, the self-shading and backtracking options each have their own separate parameter for spacing between rows.
To represent the array, all SAM needs to know is how many modules are in the array, and the orientation of the modules in the array. The "Module per String" input helps you ensure that you choose a number of modules that will work with your inverter -- that the array voltage is within the limits of the inverter ratings. In your case, you should first set up the array with the number of modules that meets your capacity requirements, and modules per string that works with your inverter. (At this stage, the 2x20 configuration does not matter to SAM, here you are defining the electrical layout of the array.)
Next, if you want to model backtracking, you should choose a row width value that is consistent with the module you chose on the Module page and your 2x20 configuration: For two modules, the row width should be twice either the length or width of the module. SAM does not store the module length and width in its module libarary, so you will need to get those values from the manufacturer data sheet. (You can confirm that they are consistent with the module area value from the library, which SAM displays on the Module page.) Then, with backtracking enabled, you can experiment with different "Space between edges of adjacent rows" values to see how row spacing affects the array's output. SAM reports the axis rotation angle in the hourly results, so you also can see the hours when the array backtracks.
As for the number of rows, the backtracking algorithm estimates the portion of the array that is shaded by neighboring modules and how that affects its output internally with geometric calculations based on the sun's position and the two backtracking inputs (row width and space between edges of rows). You do not need to specify how many rows are in the array.
One note for the Beta version (to be released at the end of this month, November 2012) and later: SAM models backtracking of each subarray in the same way as above, except that it treats each subarray as a separate array. The four subarrays on the PV Subarray page are not intended to represent the array row sections for backtracking.
Best regards,
Paul.
