Sangamo Q550 Solar Dials come in 4 variations depending on where you are in the British Isles the switch is adjusted to match the slight differences in sunrise and sunset times. Simply locate where you are on the map and add the number on to the end of the part number or select the equivalent zone. Popular applications for solar time switches include street, stairwell and security lighting. Once set, the switches track SUNSET and SUNRISE throughout the year, without the need to sense light levels. This avoids the installation and maintenance problems associated with photo sensing devices and means they can be left to operate unattended for years. The Sangamo Q550 solar dials are available in 4 variations. The item you require is dependent on the area in the British Isles in which the unit will be installed. This is because the switch is adjusted to match any differences in sunset and sunrise times.