Every metal halide and high pressure sodium lamp has an electrical system that requires conversion to higher voltage than is delivered through the electrical grid. The ballast converts house current to the appropriate voltage.
Ballasts are connected to a reflector/bulb by a length of 14 gauge electrical cord. This allows the ballast to be stored outside the garden area. The convenience of this is that the heavy ballasts are not hanging from the ceiling, only the much lighter reflector and bulb.
Ballasts come in either magnetic or digital format.
Magnetic ballasts use 20% more electricity than the bulb is rated to convert the electricity to an appropriate form. A digital ballast uses about 10% more electricity that the bulb is rated to convert the electricity to an appropriate form.
Magnetic ballasts contain an iron magnetic core so they are heavy – (1000w HPS weighs between 35-44 lbs.) but a digital ballast weighs about 8-12 lbs. for the same size.
Magnetic ballasts hum and emit a lot of heat, whereas a digital ballast is quiet and generally run cooler.
Magnetic ballasts are better suited when using a relay – (ping-pong 2 separate rooms 12hrs/12hrs with 1 ballast) than are digital ballasts.
Digital ballasts regulate currency more precisely than do magnetic ballasts. The bulb receives a more even electrical flow so it is stressed less. It lasts longer, emits a brighter light, and it doesn’t flicker.
Digital ballast are more expensive that magnetic ballasts, but they do save electricity, and generally produce more light, resulting in more flowers.