High Pressure Sodium (HPS) Grow Lights
What are HPS Lights?
High Pressure Sodium (HPS) grow lights are High Intensity Discharge (HID) lights. They operate and emit light when an electrical current is passed through a tube filled with the inert gas xenon and a gas that is a mix of sodium and mercury vapor. The resulting light is strong in the orange/red spectrum at around 2200K. This spectrum is ideal for flowering marijuana plants.
HPS grow lights provide the yellows 570-590 (nm), oranges 590-630 (nm) and reds 630-750 (nm) color spectrum which is great for the marijuana budding and flowering stages.
The exact spectrum of an HPS lamp will vary between different bulbs and different manufacturers but generally will look something like this.
Pros – HPS Grow Lights?
- HPS grow lights are pretty powerful, producing over 140 lumens per Watt. This is a lot for light for your plants.
- They emit light in the ideal spectrum for flowering plants.
- HPS Lights are easily available. Although we recommend built-for-purpose grow lights, the orange tinted lighting used in car parks, arenas and sports halls are all HPS systems, although the really orange ‘old school’ street lamps are actually Low Pressure Sodium.
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Cons – HPS Grow Lights?
- HPS grow lights generate a lot of heat. This can be a serious problem in a confined grow space. Good ventilation is essential when using HPS grow lights (see also Cool Tubes below).
- “Grow room 2x3x72 his 400 hps raised his temperature by 30 degrees.”
- “A 400 Watt lamp can raise the temperature of a room by 15 to 30 F.”
- “ You will need excellent ventilation and possibly air conditioning”
- The heat generated means that it is necessary to place the lights at some distance from the plant tips. Because light obeys the inverse square law, the further away the source of light is from your plants, the weaker it is. So light that is 2 feet away from the bulb is 4 times weaker than light at 1 foot away, and light at 3 feet away is 9 times weaker.
- HPS lights require a ballast to regulate the flow of electricity to the bulb. Older ballasts work on a magnetic system and can be very bulky and heavy and are separate units to the light itself. Luckily modern HPS bulbs come with digital electronic ballasts which are much smaller, lighter and often built into the lamp housing. Ballasts will inhale another 50 – 150 Watts of power.
- HPS bulbs have a limited bulb life. Not only this, but their efficiency can tail off quite dramatically. Even though the light might still seem bright to your eyes, your HPS grow bulb will have lost a lot of its lumen output after just a few grows. Many growers make a point of changing their bulbs after about 3 grow cycles, regardless of the life expectancy given by the manufacturers. Typically think around 2,500 hours vs LED grow lights which run at about 50,000 hours.
- All that light power comes at a price. The heat inefficiency of HPS lights means that they can be expensive to run. A 1000W HPS light has a 100 to 150Watt ballast and uses the same amount of energy 10 75 inch (MASSIVE) LED TV’s. You will see this on your power bill. LED around 55% of this cost.
Calculate the cost of running your HPS Grow Light?
Step 1 – Find the unit price for your electricity. This will be marked on your electricity bill as the price per kilowatt hour (kW/h). This is the price you pay for using 1kW of electricity for 1 hour. 1 kilowatt (1kW) = 1000 Watts.
Step 2 – So if, for example, you pay 12 cents per kW/h, a 1000W light will cost you 12 cents per hour to run. For a 400W bulb you just pay 40% of that (4.8c) and a 600W would be 60% of that (7.2c) etc.
Step 3 – Multiply that cost per hour rate by the number of hours you run the HPS. Remember to add the Watts of your bulb + the ballast.
Step 4 – 1000W bulb + 100Watt ballast = 1,100 Watts per hour. Using the example above that is 13.2 cents per hour. Vegging @ 24 hours a day will cost $22.18 per week. Flowering 12/12 light cycle will cost half that.
Although it might seem that there are many more disadvantages than advantages to HPS grow lights, their power and effectiveness cannot be denied and they have been industry standard for many years.
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What Wattage HPS?
There are a range of standard sizes of HPS bulbs available to the marijuana grower. The most popular are:
The size of grow light you get will dictate how many plants you can grow underneath it. Of course, this can vary quite widely if you decide to employ different training techniques. For example, you will obviously be able to fit in more individual plants using a SOG set-up than you will with a SCROG system.
The table below gives you a rough guide, growing small to medium sized plants, of how many plants you can grow under each sized grow light. Remember, this will also vary massively with different strains.
|Wattage||Number of Plants||Distance from Canopy|
|250W||3-5||6” – 8”|
|400W||6 – 9||9” – 12”|
|600W||9 – 12||12” – 18”|
|1000W||12 – 16+||18” – 26”|
We have also added into the table a guide to how far you should hang your light above the plant canopy. However, when doing this use your senses not the table. If the light feels uncomfortable on the back of your hand, it is too hot for the plants.
What many growers do is to start with their lights way above the plants and gradually lower them day by day, carefully observing the topmost growing tips. As soon as you see the first signs of leaf burn, back the lights off a few inches and maintain them at this distance from the canopy.
Keeping Your Grow Area Cool
As we have discussed, HPS grow lights operate at high temperatures and removing that heat from the system is essential, especially if your grow is in a confined space like a closet or a tent.
The optimum temperature for growing marijuana indoors is somewhere between 70oF and 75oF during the light period and 60oF to 70oF during the dark period. Whilst there is a lot of leeway here, try and aim for these temperatures as an ideal.
Remember that once your light goes off, temperatures may plummet quickly, especially if you have a set-up in an unheated space such as attic or a garage. You may need to consider having some heating on a timer for the dark period if you live in a cold area.
One good idea is to time the 12 hours light period for the middle of the actual night. Using the lights to generate heat at night time when it is cold, and having the dark period in the daytime when temperatures are higher, can help you to regulate the temperature of your growing environment.
It also helps to train your internal circulation fan(s) directly across the area beneath the lights and above the canopy. You should be using internal circulation fans on all your grows. A constant air current helps strengthen branches and stems and moves fresh air around, bringing fresh air loaded with CO2 to your plants. By training it beneath the lights you can also help to keep the canopy temperature down, allowing you to get the light closer to the plants.
Many modern HPS grow lights are fitted into cool tubes. A cool tube is a tube that envelops the light bulb and runs cool air over it from a dedicated cooling fan. A cool tube is just about the best way to deal with the heat generated by HPS lights. We totally recommend the use of cool tubes, especially if you are using a HPS bulb of 600W or more.
Some cool tubes also come with a built in reflector, like the one pictured below.
Metal Halide (MH) Grow Lights
As a High Intensity Discharge lamp, a Metal Halide grow light operates in a very similar way to an HPS. Light is produced by the same electrical arc through a gaseous mix, but in the case of MH the gas also contains metal halides which are compounds of metals with bromine or iodine. This changes the output spectrum. As you can see in the spectrum graph below, Metal Halides are stronger in the blue end of the spectrum, this promotes photosynthesis and is therefore great for vegetative growth.
MH grow lights are not as powerful as HPS lights, providing around 65 to 115 lumens per Watt.
The average lifespan of a MH grow light is about 10,000 cumulative hours, but as with HPS the bulb will decrease in efficiency over that time. You will need to change your MH grow light bulbs regularly to keep them working efficiently.
Actual nanometres of a MH Lamps? Well, that is always spiking in the blues and here is a very intriguing article completed by some true boffins at Penn State University show the graphs of light for different aquarium lights.
HID Grow Light Spectrum
We talked at the beginning of this article about the benefits of the HPS spectrum. Although the spectrum is ideal for flowering plants, it is not the best for vegetative growth. In the ideal HID set up you would use Metal Halide (MH) bulbs for vegetation and switch to HPS for flowering. Modern digital ballasts will run either bulb, so if this applies to you it is just a question of unscrewing the MH bulb and replacing it with a HPS bulb when you flip to 12/12.
If you have an old fashioned magnetic ballast then you will need to check whether it will run either bulb. On some of the older models you will only be able to run one type of bulb, either MH or HPS. You can get convertor bulbs for a reasonable price and these are designed to overcome this problem.
If for some reason you can only have one type of bulb, either because of the ballast, or because your budget won’t stretch to two types of bulb, choose HPS. Of the two different types of HID bulb HPS is the best all rounder and will deliver the best yields.
Many modern grow lights are now available with ‘dual spectrum’ and ‘full spectrum’ bulbs. These have been designed to optimize the output spectrum to fit exactly with the needs of your plants. There is a lot of hype and marketing out there and we would recommend sticking with a quality bulb from a reputable brand name. The company website should be able to provide scientific data to back up their claims.
Some of the leading grow light brands include:
iPower – iPower offer a basic selection of quality grow lights. 400W, 600W and 1000W in HPS and MH. They also supply digital electronic ballasts and a comprehensive range of cool tubes and reflectors.
Lumatek – Lumatek are one of the leading names in digital ballasts. Their ballasts are fitted as standard in many grow tent and grow box kits. Their grow lights are renowned for their quality and are engineered to deliver high PAR values. Lumatek also supply a great range of parabolic reflectors that is well worth a look at.
Sunmaster – Sunmaster offer a range of HPS and MH grow lights that cover 6 different spectra: 2100K, 3200K, 4000K, 5500K, 6000K, and 10000K. These can provide optimized lighting for any stage of plant growth and includes one full spectrum bulb (6000K). Bulbs are available from 400W to 1100W.
Solaar – Solaar provide a full range of high quality horticultural grow lights.
Hortilux – Hortilux are a well established Dutch company that provide commercial horticultural solutions. They have two ranges of electronic ballasts and a huge range of HPS, MH and conversion bulbs.
Philips – Philips has been the leading name in lighting for generations. They have a dizzying range of grow lights to fit all scenarios.
Osram – Like Philips, Osram have been making a huge variety of lighting applications for many years. The Osram name is usually a vote of confidence in the product.
Sylvania – This international company is another of the big players on the lighting scene. Sylvania manufacture a huge range of bulbs and their bulbs can often be found in grow light kits provided by other manufacturers.
The HPS grow Light Conclusion
- HPS grow lights are THE grow light of choice for many, if not most growers;
- They offer high powered lighting in a spectrum that is ideal for your plants;
- HPS grow lights generate a lot of heat and this needs to be taken into account when choosing HPS for your grow;
Only high quality LED grow lights can supply better performance and efficiency for growing marijuana.