The solar regulator (or charge controller) is a device that is only used in off-grid systems. It monitors and controls the charging of the battery array. Once the array is fully charged, it then disconnects the solar array so that your batteries are not damaged. Some regulators can redirect the power to a secondary source (such as a water pump or water heater) once your batteries are fully charged.
There are literally hundreds of manufacturers of power regulators (see here as an example list) ranging from $50 to over $1500, so what is going on here? Glad you asked.
For a start there are MPPT regulators and non-MPPT regulators.
MPPT stands for Maximum Power Point Tracking, and it relates to the solar panel itself. Each solar panel has a point at which the current (I) and voltage (V) output from the cell result in the maximum power output of the cell. The principle is that if the output from the cell can be regulated to the voltage and current levels needed to achieve a power output at this point, then the power generated by the solar cell will be used most efficiently.
A Maximum Power Point Tracking solar regulator will simulate the load required by the solar panel to achieve the maximum power from the cell. The regulator will work out at which point the cell will output the maximum power and derive from this the voltage and current outputs required for maximum power to be achieved. It will then calculate the load that it must simulate based on these voltage and current levels.
MPPT ensures that you get the most power possible from your solar panels at any point in time. It is particularly effective during low light level conditions.
Voltage and Charge Current
The second aspect that changes the price of the regulator is the charge voltage it is designed for and the maximum charge current it can supply. The charge voltage selected depends on the voltage of your batteries and the higher the charge current the faster the batteries will charge to full capacity.