Perhaps the most widely used grow lamp to raise cannabis is the metal halide lamp. It has been around for decades, so manufacturers are able to produce a high quality product with tried and true materials and science. This translates into a product that is cheap, reliable and should be available in some form all around the globe.
The metal halide lamp comes in many standard sizes, but the sizes most used in hydroponics are the 400 watt and 1000 watt power ratings. A 1500 watt lamp can be found with a little digging, but I’ve found the 1500 watt lamp to actually be too powerful – the tops of the plants were scalded but the bottoms were phototropic. This is due to the far height you have to keep the 1500 watt lamp at. It must be low enough for the plants to receive enough lumens of light but not so low as to scald the topmost growth. It’s very tricky, so this lamp isn’t used much.
You would use the 400 watt light for small crops, no more than four or five plants. Trying to deliver enough light to the plants will become a problem. If you were using the 400 watt lamp to grow more than four plants you will notice a peculiar trait: the foliage growth will be fuller on the inside facing side of your plants. You might think the easy solution is to turn them a few times a day, but this causes another problem, which is slow or stunted growth. This is because the plant has to use up precious resources to move the leaf bearing limbs into the path of the light.
The 1000 watt lamp is normally used with 5 or more plants and is the lamp I have used the most over the years. It is a perfect balance of light and heat. The spectrum is good for the all-important foliage growth stage of life as well.
Just about the only drawback to using a metal halide lamp is the fact that the ballast that has to be used is extremely heavy. It’s essentially a power transformer that increases the voltage delivered to the lamp. This voltage is in the tens of thousands of volts, which is tons of energy. Use large, high quality screws or bolts to attach the ballast to the ceiling. Drywall won’t hold it. You have to hit a support beam and attach it directly to that. Otherwise you’ll hear a great crash in the middle of the night, which will immediately tell you that this crop has been squished to death. Good luck.