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 could use a 250 watt hps and a 250 watt mh lighting fixture. A garden with 20 plants or less could use a 400 watt hps and a 250 watt mh lighting fixture (or 250 watt hps and a 400 watt mh).
This is not always possible because the cost of two light fixtures will be more that of a single fixture. Two lights will also take up more space and might produce more heat than a single unit.
If you can only afford a single fixture, get a high pressure sodium fixture with a high pressure sodium bulb and a metal halide bulb (both bulbs rated for the same wattage as your light fixture). Use the metal halide bulb for vegetative growth and the high pressure sodium for flowering. When you have enough money, get a proper metal halide fixture.
- A 250 watt fixture will supply enough light to cover a 2.5 foot by 2.5 foot grow area. (6 plants or less)
- A 400 watt fixture will supply enough light to cover a 4 foot by 4 foot grow area. (12 plants or less)
- A 600 watt fixture will supply enough light to cover a 5 foot by 5 foot grow area. (18 plants or less)
- A 1000 watt fixture will supply enough light to cover a 6.5 foot by 6.5 foot grow area. (30 plants or less)