The Morningstar Sunsaver MPPT charge controller is a versatile device.  There are many advantages to using this controller, as noted below:

1. The ability to use higher voltage panels, up to 70 volts.  The advantages of using higher voltage panels are many.  You will notice that the batteries receive a charge earlier in the day and later in the evening.  The reason behind this is that 70 volt panels running at 40% voltage provide 28 volts to the charger.  If you were to use 35 volt panels under the same solar conditions, you would only have 14 volts.  This is not enough to charge 24 volt batteries.  Another important reason is current.  The higher the current, the larger the wires you need to bring the power to the charge controller.  390 watts @ 70 volt panels will run at about 60 volts and 6.6 amps under full sun.  390 watts @ 35 volt panels would be 13.2 amps under the same sun.  You might use 50 feet of 12 awg wire for the 70 volt panels, or you might need 8 awg wire for the 35 volt panels.

2.  MPPT (Maximum Power Point Tracking)  MPPT charge controllers hold the panel voltage at the point where they produce the most power.  Non MPPT charge controllers typically connect the panels directly to the batteries.  This will drop the panel voltage to the level of the batteries.  While this method works well with panels that are closely matched to the batteries, the panels are unable to produce at their maximum potential.  MPPT will harvest the most power possible out of the panels.

3.  Load control.  The Morningstar MPPT charge controller is able to control most loads.  The notable exception according to the manual is "inverters".   Since our 24 volt inverter has a remote power switch, we are able to use a 24 volt relay tied to the load control to switch the inverter.  Why is this important?  Battery life!  The last thing you want to do is drain your batteries beyond a set low voltage.  This would greatly reduce the life of your expensive batteries.  Our goal is 5-7 years out of a battery bank, so we watch battery voltage closely.  A second reason for load control is that it permits the use of a smaller battery bank.  You can have a "dump load" to utilize the excess power you produce.  Our dump load is a dehumidifier.  When our batteries reach 28 volts, the load control (custom set points) turns on the inverter and runs the dehumidifier until the batteries reach 25 volts, at which time the "low voltage disconnect" removes power from the relay and turns off the dehumidifier.  The batteries we use are $80 each (including tax), so four 6 volt batteries are $320.  We would need to add four more batteries, or just let the extra power go unused if we were unable to control a dump load.

4.  Monitoring.  The Morningstar Sunsaver MPPT has a modbus interface, and we are able to monitor our solar system with a Netbook PC.  The netbook uses about 6.5 watts while monitoring the system.  Future plans include using a LabJack U3 to control the dump load.  This would give us the ability to use several different dump loads, such as a freezer.  There are times when the basement just isn't all that humid.

5.  Budget.  Initially our plan was to use the Morningstar Sunsaver MPPT as a 12 volt system with one 130W 24 volt panel.  since the controller is rated at 200 watts at 12 volts, it was unlikely that we would have expanded to a second panel.  Then some fate set in.  On eBay we found an Exeltech 1100 watt 24 volt inverter for a little over $200.  These things retail for $800 so I couldn't pass it up!  Next thing I know, we are buying more batteries and installing more panels.  At 24 volts, the Morningstar Sunsaver MPPT will output 400 watts.  Everything came together at this point, and we ordered two more 130W panels for a total of 390 watts.  Part two of this journey is that we are able to wire the panels in series for a maximum of 72 volts.  The controller typically holds the voltage of our solar panels in the mid 50's or low 60's under load.  Since price per watt is about the most important number in a solar system, it is more economical to run the Morningstar Sunsaver MPPT with a 24 volt battery bank.

6. Simplicity.  If you can read, you can set this thing up.  There are 4 dip switches and a jumper.  For flooded batteries, we removed the jumper check the dip switches and we were ready to go.  The Morningstar Sunsaver MPPT automatically adjusts for battery voltage.  There are screw terminals for the panels, for the batteries and the load.  simple hookup and great protection for your expensive batteries.