here this should help you out Metal halide light fixtures produce large amounts of blue spectrum energy and are best for vegetative (beginning) growth. The light produced looks similar to the 'cool' fluorescent light found in schools, offices, stores, and other high traffic areas.
A single 400 watt metal halide bulb delivers about as much light energy as twenty 'cool' fluorescent 40 watt bulbs. What that means is that fluorescent bulbs use about twice as much electrical energy to produce the same amount of light as metal halide bulbs.
High pressure sodium light fixtures produce large amounts of red spectrum energy and are best for flowering (later) growth. The light produced looks similar to the 'warm' incandescent lights found in most homes but has a deeper orange-red color. The street lamps in many cities are hps lights.
A single 400 watt high pressure sodium bulb delivers about as much light energy as about forty 'warm' incandescent 100 watt light bulbs. What that means is that incandescent bulbs can use as much as ten times as much electrical energy to produce the same amount of light as high pressure sodium bulbs.
What light should you get?
In a perfect world both metal halide (mh) and high pressure sodium (hps) would be used during both the vegetative and flowering stages of growth.
For example, a garden with 15 plants or less would use a 250 watt hps and a 250 watt mh lighting fixture. A garden with 25 plants or less would use a 250 watt hps and a 400 watt mh lighting fixture.
This is not always possible because the cost of two light fixtures will be more that of a single fixture. Two lights will also use more electricity, take up more space, and produce more heat than a single unit.