A great app note from STMicroelectronics on Solar lighting application.
The solar-LED streetlight controller described in this application note is designed to achieve an 85 W solar energy battery charger and a 25 W LED lamp driver. During the daytime the controller preserves the electricity energy gathered by the solar module (PV module), then stores it in the battery. In the evening the controller uses the battery energy to power the LED streetlight. When the battery runs out of power after several rainy days, the controller enables the external offline power supply (not included in this system) instead of the battery to power the LED streetlight until the system battery is fully charged again.