A
ballast is what fires the bulbs. Be it fluorescent or
metal halide. They're the big "box" between the plug and the actual lighting fixture. Some are integrated into the lighting system, some are on longer cords so they can be located nearby, but not IN the fixture itself.
Watts per gallon is not a very good way of measuring light. It fails to take into account a number of important factors, like the depth of the tank, intensity of the particular lighting system, etc.
PC,
VHO and
T5 are all flourescent technologies. They consume less power and produce less heat, but they tend to produce less usable light per watt than
metal halides. i.e. 250 watts of
PC light is NOT the same as a 250 watt
metal halide light.
