Finding the right ballast for a fluorescent light requires knowing the number and type of fluorescent bulbs in the fixture and the voltage the fixture uses. Using this information, the installer can select a ballast compatible with the type, number of lamps and voltage.
Ballasts work by regulating the amount of voltage the light fixture receives at any give time. While fluorescent bulbs require a high amount of voltage to initially light up, if they continue receiving such high voltage, they quickly overheat and burn out. A compatible ballast supplies the bulbs with the large amount of voltage they initially need, but it then greatly reduces the voltage to the minimum amount the bulbs require.