During the day, these energy efficient, low-maintenance Atlantic Solars™ lights harness the power of the sunlight to charge their internal Lithium battery. At night they automatically turn on, emitting a bright light for your fence post line. One Pre-Installed Exceptional 3.2V Lithium-Ion Battery Per Unit, LED: 5 Ultra Bright White LEDs, Can Last Up To 100,000 Hours Solar panel: New & Improved For Superior Charging Material: Premium Heat-Resistant Plastic Operating Time: Up to 10 hours (Depending on sunlight availability) Safe & Water Resistant Overall dimensions : 5.4″ (L) X 5.4″ (W) X 3.5″ (H) Mounting Base Inside Diameter): 3.75″ X 3.75″ (fit all 4×4 Solid Wooden/Composite Post)(Posts Measured Side To Side 4″ or Less)