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"'A 1000 watt setup good for about 20 square feet would cost more.'"

"Who are you growing for -- Orange county?"

I'm not growing for anyone. I do have a lot of gardening experience though and I have grown plants indoors under lights. I've only grown orchids though to full maturity, and I've started a lot of fruits and vegetables and herbs (basil, that sort of thing, not pot) indoors under lights to get a good start on the growing season before it's warm enough to plant outside.

Those thousand watt systems are not at all uncommon for people growing legal plants indoors, and they aren't uncommon for people growing small amounts of pot either. Anything under 400 watts is crap for plants with high light requirements because the light is not intense enough and light strong enough to promote good growth doesn't go far from the bulb. Light intensity drops off considerably the farther you get from the bulb. A fluorescent tube for instance is only going to provide good light for plants with high light requirements a few inches from the tube. A 1000 watt hps on the other hand is going to throw strong enough light several feet. You put it up pretty far from the plants so the intense heat doesn't burn them and there is still enough light intensity to promote growth for plants that grow a few feet tall. The light can provide enough light for the bottom of the plant as well as the top. That's why people often use them for things like tomato plants. With low wattage lights they have to keep the plants really short or there won't be enough light for the lower parts of the plant. A hundred watt hps will cover maybe a couple of square feet worth of very short plants packed together tightly. With a bigger light, the plants can be spread out some to let them breath and to minimize the risk of mold or plant diseases and pests that destroy plants from spreading before they are noticed.

It is true that a guy with a thousand watt system who packs his plants tightly and goes to a lot of effort to maximize production could produce a couple of pounds of buds five times a year or so if he knows what he is doing, and if he has separate chambers with more lights to house his mother plants and his clones that are maturing to the level they are ready to flower in the flowering chamber. But that involves a lot of work. Someone just growing for personal use though might like a 1000 watt system so he can just plant some seeds and have a few plants in his growing room and not have to mess with them so much. He wouldn't have to pack them together and worry so much about mold. He wouldn't have to check the plants everyday and keep raising the light higher as they grow. He wouldn't have to worry about having a chamber or room for mother plants or one for clones he is growing out to maturity. He could just pull the roughly 50% of his plants that turn male and let the remaining females grow out till they provide him enough pot for six months or a year, so he doesn't have to continually mess with plants, not being able to leave on vacations or business trips or anything like that.

Besides, thousand watt systems aren't much more expensive than 400 watt systems, and 100 or 250 watt systems can easily cost just as much as a 400 watt system. The ballasts and bulbs and reflector hoods and so on cost about the same for all of these setups because they cost about as much to make and because the lower wattage bulbs and ballasts are not used as much in other applications as the higher wattage stuff. The 400 watt bulbs are the ones most commonly used in non-growing applications followed by the 1000 watt bulbs, and the fact that so many of these things are produced to sell for lighting parking lots, factories, streets, and so on by so many different companies makes the ballasts and even the bulbs less expensive, provided people don't buy the fancy bulbs designed just for plants, which aren't really much better than the industrial bulbs for that purpose.

The power requirements are higher for higher wattage systems, but not as much as you might think because the higher wattage systems are much more efficient than the lower wattage systems. We use shop lights for orchids because they have low light requirement. Good T8 systems with electronic ballasts will produce about 100 lumens per watt. Ours though are cheapo T12 40 watt tubes that are not so efficient. A 1000 watt hps will produce about 145 lumens per watt. A 400 will produce around 125 lumens per watt, and a 100 watt hps will only produce around 95 lumens per watt and of course watt light it does produce will not be intense enough to provide the kind of light high light requirement plants need more than ten or twelve inches from the bulb. I looked into a 1000 watt system for our orchid room a while back and calculated the energy cots running that bulb twelve hours a day at about $25 per month. It would probably cost more now. A 400 watt system would cost about half in monthly electric bills watt a 1000 watt system would cost.

Running 18 shop lights in the orchid room burns about 1440 watts, not including the watts burned through ballast loss from the magnetic ballasts which aren't particularly efficient. For all that we are producing about 90,000 lumens, compared to 145,000 from a 1000 watt high pressure sodium lamp that would might be enough to light the whole room fine even though the plants are spread across the room on several wire shelving units. Shoot, two 400 watt systems would produce more light and be cheaper to run, but they'd cost a good bit up front. And of course these shop lights wouldn't be good for tall plants with high light requirements because the light they produce loses intensity required for those types of plants within a few inches of the tubes. When you start tomato plants or something with them you have to keep the bulbs right over the tops of the plants and raise the fixtures every day as the plants grow, and if you start your seeds too early and have to keep the plants growing under the lights too long before it warms up enough to plant outside, they'll get skinny, leggy, and unhealthy, and fall over because there isn't enough light.