OK, here goes:

**Disclaimer: I do not have a bus (yet), or an off-grid home. I **__am__ responsible for two off-grid communications sites with photovoltaic wired in the 1990's. I have had to learn a few things about solar electricity in order to keep them working and deal with contractors.
First, let's understand your proposed panels.

Voc stands for Voltage, open circuit. It is the maximum voltage each panel is rated to put out in "standard" sunlight with no load (nothing but a sensitive volt meter connected). If you look at a photo of that model, you will see it is a matrix of 36 cells (6 wide by 10 high). Each cell in the panel puts out about 0.6 volts, and they are added in series to make the 36.4 volts.

**[brain-dead edit: 6 x10 is 60, not 36. 60 x 0.6 volts = 36 volts. duh!]**
Isc stands for current (amps), short circuited. This is the maximum current that each panel is rated to put out in "standard" sunlight with the positive and negative wires shorted together through an ammeter shunt. The rating for these is 8.4 amps.

If you get a data sheet for these panels, look at the power curves and you see the rated power curve (blue line) produces the 8.4 maximum amps when loaded down to between zero and up to about 26 volts. With lighter loads, less current is drawn, and the voltage approaches the maximum of 36.4 output volts. I would consider these to be "24-volt" nominal panels, for direct connection to 24-volt systems without MPPT.

Since power (watts) is volts times amps, at short circuit the output is 0 volts x 8.4 amps = 0 watts. At open circuit the output is 36.4 volts x 0 amps = 0 watts. The maximum power is neither of these. According to the spec, maximum power is 28.4 volts x 7.8 amps = 221.52 watts, hence the 220-watt rating.

You can use the open circuit voltage and the short circuit current to figure the panels in series or parallel, and should use these maximums when sizing electronics on the system. You have already used Morningstar's calculator to do this.

I do not know enough to tell you how many panels for how many batteries you should have. Handybob had a figure, and I have seen others here. I myself am searching for a good number that I can trust.

Second, the question is parallel, series, or series-parallel.

You will have to make a decision on this yourself.

If you put 4 panels in series, you get a nominal 96 volts, or 145.6 volts maximum open circuit.

**[edit: That is at standard insolation and temperature. On a crisp, cold day, it could be even higher.]** Some controllers will blow their transistors if the input goes that high. If you touch the hot wiring without putting a blanket over the panels first, you can get a fatal jolt just like with AC wiring. But the current will always be 8.4 amps or less. The size of wiring between the panels and the controller is much less likely to have resistance problems that will burn up current as heat. As far as MPPT efficiency, I think I remember from Outback manuals that the higher the input to output voltage ratio is, the less efficient the controller is.

If you read Handybob, he has great concern about shadowing a portion of any of your panels (turning off a number of cells). If all 4 panels are all in series, and have internal bypass diodes to skip failed cells, then blocking one cell drops the maximum voltage from 145.6 to 145.0 (without the cell) or 144.4 (with diode insertion loss). The MPPT should theoretically change gears and keep on chugging along.

If you put 4 panels in parallel, you get a 24-volt system making up to 33.6 amps. You now need heavy wire, shorter distance, and cleaner connections between the array and the controller. The voltage is closer to battery voltage, so based on my memory of efficiency figures, less power will be lost in the MPPT. You may want to double-check that. With low voltage, there is less danger of electrocution, but greater danger of fire from high current heating a high-resistance connection.

Now we come to shadowing. As I recall, panels may have internal diodes to prevent the electrical "pressure' in the batteries from flowing out through the panels when the sun isn't shining. If so, cover one 0.6-volt cell on one of the four panels, and the up to 36.4 volts from the three panels in full sunlight will cause a "back pressure" that will shut down the flow from the shaded panel operating at less than 35.8 volts, so your 880-watt (maximum) system is now rated at 660 watts.

Finally, why use MPPT?

Looking back at the power curves, if I connect one panel direct across a 12-volt battery, we see that each panel puts out maximum current up to about 26 volts. A battery discharged to 11 volts will short the panel output to 11 volts, so the output is 11 volts x 8.4 amps, or 92.4 watts maximum from a 220-watt panel. When the battery is charged up to 14 volts, the output increases to 14 x 8.4 or 117.6 watts.

So, the deeper your batteries have discharged, the less power a panel will put out to refill it. If shorted by a battery discharged to 10 volts, your 220-watt panel can only manage up to 84 watts with a direct connection.

With an MPPT controller, it acts kind of like a DC-DC converter. Your 220-watt panel runs at 28.4 volts, and considering 90% efficiency, puts (220/11 x 0.9) = 18 amps instead of 8.4 amps into your battery discharged to 11 volts. Theoretically, the power will remain roughly constant but the amperage will go down as the battery voltage comes up, so maximum current is provided to a deeply discharged battery, and less to a battery nearing full charge. This is in addition to the fact that the current would slow down as the "pressure' in the battery begins to equal the "pressure" of the charging system. This is like Handybob's example of pumping up a truck tire.

Handybob says he does not discharge his batteries below 85% full if possible, so the advantage MPPT has on nearly-dead batteries does not help him much. He instead uses a direct-connection controller that throttles back the output when the batteries are full, and instead eliminates inefficiencies wasting the generated electricity in controllers or in long or undersized wiring and terminals.

Anyone considering direct connection control should select a different model photovoltaic array than the one you listed, unless they have a 24-volt system. A panel for direct connection to 12 volts would have a Voc of about 18 volts, and a Vmp of about 15 volts.

OK, so I did not answer your questions, so you do not owe me a beer. But I hope the information gives you a better ability to make your own informed decisions.

Good luck and have fun!