In our previous post A Guide to Grow Lights we explained why it is necessary to use specialist grow lamps to grow cannabis indoors, and ran through the various types of artificial lighting available. Here we look at the most popular choice in a little more detail and describe the component parts.

HID Grow Lights

HID stands for High Intensity Discharge  and this type of grow light is based on the principle of delivery a large amount of light over a small area. The key point of an HID light system is the large bulb (around 30cm / 1 foot) in length. This bulb is filled with gas and two electrodes arc an electricity current which ignites the gas and emits a powerful light. Bulbs are filled with two types of gas, Metal Halide and High Pressure Sodium, which is explained further down. To supply the electrodes with the right sort of current a ballast is used which converts your regular electricity into the type necessary for the bulb. A third component in the HID Grow Lamp system is the reflector or hood which acts both as a lampholder and a means of directing the light downwards towards the plant canopy.


This is the specialist piece of equipment necessary to run a high density light system. The reflector (hood) with self-contained bulb socket plugs into the ballast, and the ballast is plugged into your power socket. It converts this electricity into a form usable by the bulb to produce large amounts of light. Ballasts come in a range of “sizes”:

  • 250 watt – good for young plants or a one plant grow
  • 400w – small personal grow
  • 600w – in Europe, the most commonly used grow light size
  • 1000w –  in USA 1000w light systems are often used.

Bulbs come in these sizes and may need to match your ballast. Most reflectors are “one size fits all” but some have a small/medium/large version e.g. CoolTubes.

As well as the different sizes, ballasts come in 2 types:

  • Magnetic
  • Digital


Advantages of using a Magnetic Ballast in your grow room:magnetic ballast cannabis grow room

  • cheaper to buy
  • simpler, with fewer components
  • will work with any bulb as long as it is the corresponding size
  • not fussy about the quality of the power source – they can cope with power spikes and fluctuations
  • easily repairable
  • bulbs last longer
  • longer warranties


  • usually heavier and bulkier due to the electromagnetic core (although improvements are being made by some brands)
  • very cheap versions can have quality issues – and for this translates as a safety issue. A faulty ballast is a fire hazard.
  • not energy efficient and produce more heat as a side-product
  • use a small starter circuit which means that as the ballast starts up the bulbs flicker.
  • hum

digital ballasts for the grow room

Why a Digital Ballast is worth the extra money:

  • Higher output than the same size magnetic ballast (up to 30% more): which means for the same amount of electricity drawn, your plants will get up to a third more of that light they need to convert into growth/flower production.
  • Produce less heat and the power given to the bulbs is more stable so they burn with less flickering
  • Less noise
  • Adjustable wattage: this is very useful. When plants are young or in veg stage, they need less light than in full flower. Turning down the power on the ballast means you save on unnecessary electricity usage.


Disadvantages to using a digital ballast

  • Super sensitivity. If your electricity supply is variable and “spikes” the ballasts will switch off – and sometimes burn out bulbs in the process. Even if you don’t lose the bulb, coming in to find your room has been in complete darkness when it should have been lit is a bad thing.
  • Some ballasts have inadequately shielded reflector cables causing RFI (radio frequency interference), possibly effecting nearby TVs and radios
  • Cost
  • Less dependable in the long term


To sum up:

Generally it pays off to buy a branded digital ballast. Not only do they provide a better light, but more of it. The possibility of delivering less power at stages in the plant’s life cycle will help to offset the extra cost. However, if you’re just starting off on a budget, or if you have uncertain electricity supply, then the digi ballasts extra sensitivity will prove problematic. One thing is certain: beware very cheap unbranded Chinese makes as part of bargain basement lighting kits: the biggest cause of grow room fires is a faulty ballast.


Reflectors or Hoods

Reflectors do just as their name implies: they reflect the light down from the bulb towards the plants. They also contain the lampholder into which the bulb is screwed, and the cable connecting the lampholder to the ballast.

There are many types and makes of reflectors on the market but here are some of the most common:

1. Stucco Reflector

basic reflector grow room

The most basic reflector is simply a thin metal sheet made from Stucco and containing the lampholder. These are usually very cheap – around 15€ – but the savings you make on purchase cost may well lose you a significant amount of yield: these reflect the light over the minimum area possible, do not reduce heat, and have only 80-85% reflectivity.




2. Air Cooled Reflectors

cooltube reflector

These fall into two types: the Cooltube and the Air cooled Hood. If heat build up is a big problem within your grow room (for air cooled reflectorexample in a small confined space, or when sing multiple lights) then using these makes a big difference. Ducting is attached to the reflector and the extractor pushes air through, pushing out the heat produced by the bulb. Depending on the number of lights you’re using you may need to use a separate extraction unit for this. If you are running a sealed room environment using CO2 where you don’t extract the air, using a cooltube system still enables you to pull out the heat. The more basic cooltubes have very small reflectors so obviously do not reflect as much light downwards, whereas the air cooled hoods feature a normal sized reflector. Because the light is cooler than using an open-ended reflector, the lights can be hung much closer to the plant canopy.


  • lose some light energy through dissipation through the glass
  • can create hotspots and potential light burn


3. Open-ended Reflectors

Adjust-a-Wing professional reflector

Similar to the basic Stucco model but with refinements in design and build materials. Probably the best of these type is the Adjust-A-Wings range which claims to increase yield by up to 50% and coverage areas up to 75%. (read more about the Adjust-A-Wings here).


reflectors for the grow room4. Closed end Reflectors

Tend to create the best spread of light with uniform coverage. However, heat stays trapped in the reflector area and therefore they need to be positioned higher up. Great in combination with a lightrail.



To sum up: there’s a huge price difference between the bottom end budget reflector and the highly designed specialist models.



Bulbs for an HID Grow Light system are large – nearly a foot in length. Like the ballasts, they come in a range of sizes:

  • 150w (uncommon)
  • 250w
  • 400w
  • 600w
  • 1000w

and you need to match your bulb size to the ballast you are using.

As was said at the beginning, these bulbs work because the electrical current ignites the gas within them.  Metal Halide bulbs (MH) use Halogen and produce a light at the blue end of the spectrum, High Pressure Sodium (HPS) uses the same principle with a sodium gas, producing a reddish orange light. In theory, you will need a Metal Halide bulb (which appears bright white, and covers the blue end of the spectrum) for marijuana plant growth, and a HPS (High Pressure Sodium) bulb for flowering. HPS bulbs have twice the lifespan of a MH bulb (around 18,000 hours as opposed to 10,000 hours), but their red-orange focus means that they can produce spindly, stretched vegetative growth. They are however ideal for flowering.

The best scenario when using HID grow lights is to run with a Metal Halide bulb for vegetative growth, then switch to HPS for flowering. Alternatively, you can operate both lights at the same time – but this will shorten the lifespan of the system and increase electricity consumption and heat whilst at the same time not giving a comparative increase in bud production.

But, unless you intend to grow your plants in vegetative stage for a long period, it is perfectly feasible to produce a beautiful crop using a Dual Spectrum bulb. This is an HPS grow light system with a small percentage of blue light included to prevent over stretching and closer node space. Commercial cannabis growers have for a long time added one MH system to every 9 HPS systems to get the same effect. Alternatively you can grow your seedlings or cuttings on under a CFL then put straight away into flower at around age 2 weeks under a standard HPS. This method of growing is ideal for hydroponic growing, and is commonly called a “sea of green”.

Like everything else, quality costs. A budget buy bulb may cost a quarter of the top end model, but produce less light even though its rated for the same amount of watts. Bulbs should be changed every 3 seasons or so as although they still function to the human eye the amount of light diminishes.


HPS grow lights

How many plants can I grow under my HID lighting system? Or, what lights do I need to cover my grow room?

This question is a bit like asking the length of a piece of string .. and yet its asked all the time. The answer is: “it depends on the size of the plant” .. probably the best answer is to consider the coverage area of each size light and grow accordingly. Information on the internet can get very confusing as writers talk of lumens etc. but as a rough guide, consider that the very best bud production happens at 60 watts per square foot. I’ve commonly seen area coverage guides suggesting that (for example) a 250 watt light will cover an area of 3′ by 3′. This would give a watt/foot ratio of 42: I think you would be better off reducing the area to 2.5′ by 2.5′ at most, with 50 watts/ square foot. I say this from personal experience : the nastiest, stickiest bud I ever grew was with a ratio of 62 watts/ square foot. The plant had big juicy buds which went further down the plant than when I grew the same cuttings with the same conditions at 40 watts/square foot. You’ll probably end up with less grams/watt using this ratio, and your electricity bill will be slightly higher, but believe me, if you are growing marijuana indoors for quality rather than quantity, this is the route you should take.

Unless you have unlimited height available, you probably want your plants to end up at around 3 feet tall. At this size, in pots big enough to give good root-ball space, you can grow 2 plants under a 250 watt HID grow light and get good bud from it. Alternatively you could grow 5 smaller auto-flowering plants that end up around 2 ½ feet tall.

Advantages of HID for indoor marijuana lighting

  • Relatively cheap to buy: you can get a usable 600 watt HID grow light for $100 on ebay.
  • Tried and tested. Lets face it, the fact is that the majority of cannabis growers won’t consider using anything else. We know that they work and work well.
  • Because they’ve been around for the past 15 years (or so) as the major source of lighting for marijuana, nutrients, additives etc. have all been refined to suit growth under these lights. Plants grown under different systems are showing mineral deficiencies with standard foods that don’t occur under HIDs.
  • This will change .. but at the moment unless it states otherwise, any information that you read about indoor gardening will assume you are using HID



Disadvantages of HID lighting for marijuana production:

  • The initial outlay may be low, but the shelf life of a HID is short: at 12 hours daily, expected bulb life is said to be 3 years. Most growers would however replace bulbs after 1 ½ years. Ballasts etc. also deteriorate in efficiency so rather than just replace bulbs, long term growers replace the whole system at this time.
  • Electrical consumption. We all know that a major factor in cannabis grows being discovered is as a result of suspicious increases in power consumption. Remember that as well as running your lights, you will also be powering extractors, fans etc.
  • Heat. This is another reason why growers get busted: tell-tale heat signatures coming through their attic roofs like a beacon to the bobby.
  • Heat also has major implications on the way you grow your weed. Healthy temperatures for an indoor grow are between 21ºc and 29ºc. Even in winter you will find that you will have to work in a way of reducing the heat in your grow room down to this level. Of course, for ventilation reasons, you should have an extractor anyway: with HIDs, this needs to be a reasonably large one (see ventilation for CFM ratings).Dealing with, and monitoring your heat levels becomes a part of your growing technique. And if you live anywhere where outside temperatures rise during the summer, you can forget about indoor growing.


LED grow lights

2 thoughts on “What are HID Grow Lamp Systems?

  1. Henry says:

    Just stumbled on your site. I like it! Especially this guide. A lot of growers have been switching to LED lights (I prefer them myself these days), but there’s still a sentimental attachment to HID lighting, so it’s nice to see a thorough explanation. Plus, HID lights do work really well. They’re just not as simple and efficient as the best LED lights.

    • weed widow says:

      Thanks! And yes, we’ve been using LEDS since 2011 and totally agree with your comments. You might want to read our first LED vs HPS trial grow report that we did back then. In the past year we’ve also been able to pit 6 x 600 HPS vs 6 x 400w draw LEDs – and guess what happened? But that’s a story for another day ….

Leave a Reply

Your email address will not be published.