This Circuit uses three Transistors as Switches.
The left hand voltage source is the Solar Panel and the right hand voltage source is the battery.
In Short, when Solar V drop below around 1.9v the PNP opens to run the Light LED.
If Solar V is between 1.91 and Batt Voltage plus 0.5V the LED is switched off and excess power is dissipated through one of the 10k resistors.
When Solar Voltage Exceeds Batt voltage by 0.5V an NPN opens to provide a charge to the battery.
There is no charge regulation provided, so this is only recommended for a panel providing less than 200mA or so.
In theory all components are rated to well in excess of 36V so this could be used on a 6 or 12V system easily enough, provided the solar panel doesn't exceed 3W
