Normal household timers are not built to take the high inductive loads produced by HID ballasts. If used with a HID ballast, the contacts inside the timer that open and close the timed circuit will overheat and weld together, breaking the timer and leaving your grow lights permanently on.
Heavy Duty Timer’s, as the name suggests, is designed to take high loads and has no problem dealing with the inductive loads produced by 600w ballasts. This means no separate contactor is required, saving you money.
They boast 96 switching operations within a 24 hour period (every 15 min), a robust, inline case design with socket and timer dust covers and a high quality timer unit.