Since the electrical devices used in indoor cannabis cultivation have to operate in a specific time schedule, it is necessary to use relays and timers. These allow you to control the turning on and off of lamps, ventillators, CO2 dispensers, heaters, humidifiers, fans etc. Ranging from the simple plug and play kind to more advanced control stations, these will suit the need of every grower.
One important factor to look at while choosing a timer or relay for your cannabis plantation is the maximal load. Though e.g. 600W HPS lamp is in theory taking 0,6kW of energy, analog ballasts are actually taking much more for a fraction of a second to switch them on - enough, however, to damage the cheap timer. A wide margin of safety has to be thought of, as keeping your plants in the dark for several days might severely decrease the final marijuana yield.
Advanced relays offer multiple sockets, that could be used to control several lamps, fans etc. - perfect for commercial growers that have to be absolutely sure everything runs smooth.