Here on EBAY you can find many different grow/plant lights.  Some are worth the money, some, for various reasons are not, and should be avoided at all costs.  Of course it's fun and rewarding to use these lights indoors to grow your own plants that you couldn't otherwise grow outside, but sometimes it's not worth the risk, or the discomfort you may experience using these lights.  I don't sell these kinds of lights, but I have used them a lot, and have learned alot about them that I can share...

HID, High Intensity Discharge, lights are commonly sold on EBAY.  True, these are the light setups used by nurseries and many "home gardeners" with success.  The two types usually seen are HPS, High Pressure Sodium, and MH, or Metal Halide.  A good setup using these lamps will definitely give good results, if used properly.

HPS lamps are the lamps commonly used as streetlights.  You are probably familiar with the pink-orange light they produce already.  MH are also used as streetlights, they appear as a bright white, when new, to a sickly greeenish color when old.

WHAT TO LOOK FOR.  When you consider purchasing an HID lamp setup, the first thing you want to avoid is "do it yourself" kits.  These kits are dangerous, and potentially fatal. They usually do not provide the proper housing that the electrical hardware needed for the "guts" of the device.  Protection from water is necessary, and the components that would be "internal" components on any other setup are exposed.  You wouldn't buy a computer like this, would you?

When you are looking, see if a tempered glass lens, or shield is included with the unit.  No unit without a glass shield is UL approved, on the contrary, these units are dangerous.  The big bulbs that are used by these light setups are containing high pressure gas.  When running, these lamps are hot - above 500 degrees, and by alot!  One problem with these lights that is not completely uncommon is "catastrophic failure", this is where for no apparent reason, a bulb simply explodes....shooting very hot shards of glass everywhere - fire is entirely possible.  This can also happen if one droplet of water should strike the glass envelope, like if you water, or mist your plants.  THIS GLASS SHIELD IS NECESSARY - REQUIRED!

MH bulbs can be very dangerous if not carefully inspected.  A small crack on such a bulb can allow short wave ultraviolet radiation to escape, which can cause blindness, or even skin cancer.  MH bulbs, like fluorescent tubes, contain sizable amounts of mercury, and therefor are considered hazardous waste

These lights can heat a room! between the electrical parts and the bulb, a 400 Watt light can comfortably heat a small room in the winter.  Or unbearably warm in the summer. Using such lights, you'll have to water your plants much more often.  High humidity is often another byproduct.  If used under normal cicumstances, using one (or several) of these lights.  No, you don't need to rewire anything in your house to use them, and when used under normal circumstances no damage is ever done to a dwelling place - that's why many of them come with "UL" tags.  Don't believe the hype.  

Helicopters (with infrared detection) used by National Guard and police easily find these (HPS AND MH)  lights in homes where they are being used. You may be visited with a warrant, even if you're abiding by the law! 

Balanced Spectrum / Full Spectrum Bulbs.  These bulbs have a different spectrum because different metals (such as mercury) are added to the bulbs to alter the output spectrum.  Truly, plants need basically blue and red light to grow.  They do not use green light, and use little, if any yellow light, and orange light.  Many of these bulb sellers will show the bulb's spectrum - look and see if there is a lot of yellow or green in the spectrum...

Bulbs which are used in horizontal position will shine more light on the area below, as compared to vertical bulbs.  Also, horizontal type relectors are better also, sending more light into the growing area.  Relflectors which reflect light back into the bulb shorten the bulbs usable lifespan.

A 400 Watt HID lamp should be adequate for a square meter of space, maybe a little more.

Really, there is no "better" between HPS and MH, both do a good job.  MH will produce a shorter, stockier plant, though, and less space between leaves on the stem. No, HPS doesn't really resemble "harvest sun".  The sun doesn't change color during the year, it's always the same.

One difference is in bulb life time.  On average, a metal halide bulb will last about 12,000 hours before the bulb must definitely be changed.  On a high pressure sodium system, this life time is 24,000 hours. 

Fluorescent lights are also available.  These require less electricity to run, but are also less efficient.  While these lights are adequate for small plants, they are at best a supplemental light for larger ones, used near a window in conjunction with some natural light.  But you don't need a special system for these, you could use a regular setup for these.  And forget about those purple or pink plant/horticulture ones - a combination of warm white and cool white is best when using these.  But remember - these lights contain mercury, too, and are also considered hazardous waste.

A lot of people have been using the new compact high output bulbs of this type, and with great results, for plants that grow to about one meter tall.  These aren't the little fluorescents that are up to 20 or so watts, but operate at 100, 125, or more, typically.  They do come in a variety of color temperatures, from the lower numbers (more red) to higher numbers (more blue).  These bulbs are becoming quite popular now.

There are also fluorescent "Envirolights" available now, too.  These have wattages as high as 200 Watts, and are available in several color temps, measure usually in degrees Kelvin.  These Kelvin numbers go from 2700 for a bulb that looks like the light is very yellow, or warm, to 6500K which is a very bright white light.  The lower numbers, like 2700, are very high in red light, and are said to be used for blooming, while the higher numbers, like 6500 are relatively high in the blue part of the spectrum, and are used to promote vegetative growth.  A couple of drawbacks to these lights is that the bulbs are pricey, and they require special fixtures (most of the time) which contain a small ballast, or transformer, used only by these Envirolights. But they do seem to produce good results, which is good, considering the wattage consumed by these bulbs.

Sure, there are all kinds of Very High Output, other fancy-shmancy types too, like compact (which are really for use as reading lamps), but if you're going to spend so much, the HPS/MH are better suited through economy, as well as results.

Incandescent Bulbs.  These are the bulbs that screw in to a normal light socket.  Not worth the extra money. Just funny colored lights suitable for your birthday party, or whatever... Most of these bulbs are only internally coated to reduce red light, which will make the plant grow spindly, if you've ever tried to grow a plant nder these kinds of lights, you know what I mean.  These are a "don't even bother" kind of item.

LEDs:  One promising new system is LED, or Light Emitting Diode systems.  These are pricey, but last much longer than any of the above kinds of lights.  One commonly sold one claims 130 Watts of light. It may not sound like a lot, but these lights are made of very precise spectrum LEDs, and the light produced is only of the kind actually used by plants, so while "only" 130 Watts, think of how may Watts of unneeded light doesn't have to be produced - all the light from turquoise colored to orange-red.  Maybe this 130 Watts produced is equal to almost 300 to 400 Watts of HID light.  And they're not so bright to human eyes, so there's less likely light "pollution". And they run only warm enough to touch, so you don't have to cut vents in your house. But be wary when considering purchasing LED units.  One must consider not only the wavelengths necessary, but also the amount of radiation (light) emitted by these.  You'll read about HID equivalents, but how close must these lights be to deliver that intensity?  Within 6 inches.  Horticultural scientists and the materials engineers they work with routinely have genuine belly laughs when they are confronted with some LED units for horticulture.  These people are using many 3 Watt plus lamps in the laboratory, not little, largely ineffective 1 Watt (or lower) LEDs for any purpose involving whole organism plants.  Tissue culture, yes, plants, no.  Many (all on eBay as of March1, 2008) of these units are good only for starting seeds  I have received three messages from people who have bought these professional-LOOKING units for HUNDREDS of dollars, only to be very disappointed in what they ultimately got.  But, the utmost in efficient, if only supplemental, lighting available now.
Expensive, but good only for the tech-nerd....some would have you think.

When you do decide what kind of light you will buy, remember, the wattage on the bulb is just that - the wattage for the bulb, and not an indication of the quality of the light.  Also the ballasts used by all of these lamps require some energy to operate.  An HID lamp of 400 watts will use closer to 500 watts than 400 watts.

But when you do decide what to grow, think of whether the cost of doing it inside is worth the money you'll spend doing it inside.  Also think about what kinds of poison metals may be in the lamps, and the careful disposal of them.  The less oil/gas/coal burned, and poison in the trash keeps the Earth greener, and people freer.

If my guide helped you, or if it let you know something you might not have, give me a vote! Thanks.