2000 lumens per square foot is about as low as you want to go when growing indoors. If you get under this mark, plant growth will certainly not go as fast as possible, and internode/stem length will increase. Also, light distance to plants will be much more critical. Daily adjustments to the lamps will be necessary, meaning you get no vacations. 2500 lumens per square foot should be a good target, and 3000 is optimal if your going to inject or enrich CO2 levels.
Higher Intensity Discharge lamps are the best solution for most indoor growers. The 3 main kinds of HID lamps are High Pressure Sodium (HPS), Metal Halide (MH), and Mercury Vapor. Metal Halide is an improved spectrum, higher intensity Mercury Vapor design. HPS is a yellowish light, maybe a bit pink or orange, the same as some street lights. HPS lamps can be used to grow a crop from start to finish. Tests show that the HPS crop will mature 1 week later than a similar crop under MH, but it will be a bigger yield, so it is better to wait that extra week.
The easiest HID to buy, and the least expensive initially are the flourescents and mercury vapor lamps. MV will put out about 8000 lumens per 175 watts, and 150 watts of HPS puts out about 15000 lumens, so HPS is almost twice as efficient. But the color spectrum from MV lamp output is not as good. HPS is high in reds, which works well for flowering, while the MH is rich in blues, needed for the best vegetative growth. Unfortunately, MV lamps provide the worst spectrum for plant growth, but are very inexpensive to purchase.
400-watt HPS lamps will put out around 45000 lumens. For every 500 watts of continuous use, you use about $20 a month in electricity, so it is evident that a lamp taking half the power to output the same lumens (or twice the lumens at the same power level) will pay for itself in a year or so, and from then on, continuous savings will be reaped. This is a simple initial cost vs. operating cost calculation, and does not take into account the faster growth and increased yield the HPS lamp will give you, due to more light being available. If this is factored into the calculation the HPS lamp will pay for itself with the first crop, when compared to MV or flourescent lamps, since it is easily twice as efficient and grows flowers faster and bigger.
400 watts=30k lumensMercury Vapor
400 watts=20k lumens
400 watts=36k lumens
High P. Sodium
400 watts=45k lumens
Notice the MV lamps are less efficient than the flourescent (FL), and can not be positioned as close to the plants, so the plants will not be able to use as much of the MV light. The light distribution is not as good either. MV lamps simply are not suitable for indoor gardening. Use flourescent, MH, or HPS lamps only. Halogen arc lamps generate too much heat and not enough light for the wattage they use, and are also not recommended, even though the light spectrum is suitable for decent growth.
There is a new type of HPS lamps called Son Argo, and is available in a 250, 400, and 1000-watt range. The 400 is actually 430 watts, they have added 30 watts of blue to this bulb. It is a very bright lamp (53k lumens) and is made for the greenhouse. These bulbs can be purchased to replace normal HPS bulbs, so they are an option if you already own a HPS lamp. The beauty of this bulb is that you do not give up most of the advantages of MH lamps, such as minimal internode spacing and early maturation, like most HPS users do, and you have all advantages of a HPS lamp, one bulb does it all.
Internodal lengths of plants grown with the San Argo are the shortest ever seem with any type of lamp. Plants grown under this lamp are incredibly bushy, compact and grow very fast. Son Argo bulbs however, do not last as long as normal HPS bulbs. There is something like a 25% difference in bulb life.
Metal Halide is another option, and is available in both 36k and 40k lumen bulbs for the 400-watt size. The Super Bulb (40k) is about $10-15 more, and provides an extra 4000 lumens. Halide light is more blue and better than straight HPS for vegetative growth, but is much less efficient than HPS. It is possible to purchase conversion bulbs for a MH lamp that converts it to HPS, but the cost of the conversion bulb is more expensive that the color corrected Son Argo bulb.
The Son Argo bulb will prove to be much better than the MH for any purpose. The MH bulb does not last as long, but is cheaper. Compare $36 for a 400 watt MH bulb vs. $40 for the HPS bulb. Add $15 for the Son Argo HPS. The HPS bulb life is twice as long. 10k hours vs. 21k hours. The Son Argo is about 16k hours. Still, longer and more light add up to more for your energy dollar in the long run.
Horizontal mounting of any HID is a good idea, as this will boost by 30% the amount of light that actually reaches the plants. Most HID's sold for indoor garden use these days use this horizontal mounting arrangement.
HPS is much less expensive to operate than any other type of lamp, but comes in the 70-watt size at the home improvement stores. This size is not very efficient, but blows away FL in efficiency, so they might be an alternative to FL when it comes to very small operations, like 9 sq. ft. or less. Over 9 sq. ft., you need more light than one of these lamps can provide, but you could use two of them. 70-watt HPS lamps cost about $40 each, complete. Two lamps would be 140 watts putting out about 12k lumens, so it's better than FL, but a 150 watt HPS puts out about 18k lumens, the bulb life is longer, bulbs are cheaper and the lamp is more efficient to operate. The biggest problem is that the mid size lamps like the 150 and 250 watt HPS are almost as expensive to buy as the larger 400's. For this reason, if you have room for the larger lamp, buy the 400. If your going pro, a 1080 watt model is available too, but you might find there is better light distribution from two 400's rather than one large lamp. Of course, the two smaller lamps are going to be more expensive to purchase than one large lamp, so most people choose the larger lamp for bigger operations.
Heat build up in the room is a factor with HID lamps, and just how much light the plants can use is determined by temperature, CO2 levels, nutrient availability, pH, and other factors. Too big of a lamp for a space will make constant venting necessary, and then there is no way to enrich CO2, since it's getting blown out of the room right away.
The bulb cost on the 70 watt HPS is $24, the 150 is only $30, and the 400 is only $40. So you will spend more to replace two 70-watt bulbs than you will to replace one 400 watt HPS. Add that up with the lower resale value on the 70's and the fact that they are being modified and are not suited to this application, and it becomes evident that $189 for a 250 watt HPS lamp, or $219 for a 400 watt, might just be worth the price. Keep in mind that for $30 more, you can have the larger lamp (400-watt) which puts out 20k lumens more light than the smaller one.
Son Argo 400
Super MH 400
HPS Argo 250