When using high intensity discharge bulbs such as the metal halide or high-pressure sodium light bulbs, it’s important to remember that a ballast is integral to proper functioning of the bulbs. The ballast and the HID lights also produce quite a bit of heat, which can result in overheating your room and burning your plants, so many growers use an air-cooling sleeve or tube around the bulb of their light to reduce the amount of heat that vents directly into their grow room.
There are two types of ballasts to choose between: a magnetic ballast or a digital ballast. Standard ballasts of either type are matched to both the bulb type and the wattage they support, (i.e. a 400-watt MH needs a ballast for a 400-watt MH and a 400-watt HPS needs a separate ballast specifically for a 400-watt HPS) so if you buy your lights and ballast separately make sure you get the right gear.
Magnetic ballasts are generally cheaper to purchase, as well as being cheaper, easier and faster to repair in the event of a failure or malfunction. With a magnetic ballast, the power consumption may be marginally higher and the ballast may produce slightly more heat, but the upside is that magnetic ballasts are tough, built to last and very affordable.
By comparison, digital ballasts run slightly cooler, generally consume a bit less power and can often be used more flexibly for MH or HPS bulbs. Many digital ballasts, such as those carried by Galaxy, are also switchable (i.e. the ballast can support differing wattages, for ex. a switchable ballast may support a 400, 600 or 1000 watt bulbs). Also referred to by some manufacturers as dimmable ballasts, a dimmable ballast also has the potential to run light as different wattages.
Ergo, in a dimmable ballast you might be able to run a 600-watt light bulb at only 450 watts, or you could run a 1000-watt light bulb at only 600 watts. Running a light at partial power isn’t recommended, though. A 1000-watt light, running at only 600 watts, won’t preform as well and over the long-run you will shorten the life of your bulb; so if you want to run a light at 600 watts, it’s best to purchase a 600-watt bulb. Similar damage is done if you overpower a bulb, by running a 450-watt bulb at 600 watts for instance.
The best use of a switchable or dimmable ballast is when you think, or know, that you will be moving up to a move powerful bulb in the future. This might mean that you’re currently running your system on 400-watt lights, but you know ymaou’ll be moving up to 650-watt lights soon; so you get a dimmable ballast and continue to use your 400-watt lights with the ballast tuned to 400 watts, then tune up the ballast to 650 watts when you get your new lights.
Alternatively, some dimmable ballasts support both MH and HPS lights; so if you veg your plants under a 450-watt MH but flower them under a 600-watt HPS, you can simply switch the bulbs when it’s time to flower and dial up the ballast accordingly. Unfortunately, digital ballasts can cause RF interference, with many growers reporting more issues with cell phone and/or Wi-Fi reception in the area around their digital ballast; magnetic ballasts do not have this problem.