I am trying to create an automatic solar powered water pump. The system will be battery powered so that the battery will be charged by a solar panel during the day and when the sun goes down, the battery will power a water pump. I thought about taking the circuit from a solar light which already does what I want but I will need 2-6A of current and 12V which will probably be two high for an existing circuit. Can I use Darlington transistors to allow the existing circuit to control the higher current circuit or is there a standard controller circuit available?

 

It does get a bit more complicated than that.
You need to also incorporate charge and discharge control so you do not damage the battery by over charging or discharging below a safe level.
There are many solar panel charge controllers on Ebay that will do just what you need.

 

Yes there are solar battery charge controllers but first need to know power required by pump & how many hours a day.