MPPT charge controllers give approx 30% more power than a PWM charger and you can connect high voltage panels to lower voltage battery banks. 

In a normal charger the battery acts as a resistor reducing the amount of power you can get out of the panels, these chargers use Multi Power Point Tracking (MPPT) which keeps 2 separate circuits for the panels and batteries allowing upto 30% more power to be collected from the panels.

Using higher voltage on the panel side will also benefit by less volt drop and can use thinner cables.  On large arrays we run everything at 100V - 140V.

Always plan to run your panel VOC at least 10% under the max VOC for the controller to allow for the panels temperature co-efficient.

We did a side by side test between MPPT & PWM controllers, read the results here

To work out what size controller you need calculate by
(Peak Solar Panel Watts) / (Battery Voltage) = MPPT Amps (ie 240W/12V = 20A)

Please beware of cheap controllers on ebay claiming to be MPPT which are not, there seems to be many from china these days.  An MPPT needs a large transformer coil and Capacitors inside, it is a bigger heavier unit and will usually take 100V+ input. All of our controllers below are genuine MPPTs.