During the day, the solar panel collects the sun's energy and charges the easily replaceable, but long-lasting batteries. At night the sensor automatically turns on the super bright LED. With multiple units your dock will provide an elegantly lit pathway. Run time is 18+ hours on a full charge. Fits post sizes 1-5/8" OD, 1-7/8" OD, 2" OD, 2-3/8" OD and 3-1/8" ID.