Everything You Need To Know About Indoor Grow Lights

It's time to talk about indoor grow lights. As more people start growing inside, there are more advanced options than ever before for the indoor cultivator. Choosing the perfect grow light for cannabis has gotten a lot more complicated, but the options are a lot more effective and efficient. From LEDs to HIDs to everything in between, where do you start?

The following guide breaks down the different types of grow lights suitable for growing cannabis indoors. Most importantly, it covers the basics of LEDs and why more growers than ever before are adopting them. 

And, if you have questions about PAR, PPF, and PPFD, we also get into the details (and why they matter for growing weed). We aim to answer all your frequently asked questions about indoor grow lights for weed, including how high to hang them and if they are bad for your eyes.

What's Covered in this Grow Light Guide?

• Types of Indoor Grow Lights
• Fluorescent Tubes
• High-Intensity Discharge (HID)
• High-Pressure Sodium (HPS)
• Metal Halide (MH)
• Ceramic Metal Halide (CMH)
• Light Emitting Diodes (LED)
• How Do LED Grow Lights Work?
• Why are LEDs Taking Over?
• Light Spectrum 101
• Grow Light Efficiency 101
• Grow Light Output 101
• Photosynthetic Photon Flux (PPF) & Photosynthetic Photon Flux Density (PPFD) 101
• How to Choose the Right Grow Light?
• Frequently Asked Questions About Grow Lights for Cannabis

Types of Indoor Grow Lights 

Wading into the field of indoor grow lights is a bit daunting for the uninitiated. Originally, the industry had just one style, High-Intensity Discharge (HID), but now there are several popular technologies, including LED. What's more, each has several types bundled under the category.

Fluorescent Tubes

Fluorescent tubes, of the variety that you might see in offices and retail stores, are glass tubes set inside a ballast which controls electrical currents. Electricity flows between pins set at both ends of the glass tube, which creates a reaction with the gases contained within. This reaction lights up the phosphor powder coating the inside.

Compact Fluorescent Lights (CFLs) are also included in this category. They are a version of traditional fluorescent tubes in a compact spiral shape. Although some growers use them for supplemental lighting in the grow room, these are most common for residential spaces.

Fluorescent lighting is affordable, comparatively long-lasting and efficient (roughly 50 to 100 lumens per watt). 

But, they make poor grow lights for cannabis — with a few exceptions. The spectrum and intensity are comparatively poor; as a supplement to other lighting solutions, they can be useful. If you choose to grow only with fluorescent grow lights, stick with T5 bulbs and keep the plants short and small.

High-Intensity Discharge (HID)

The longtime standard for indoor grows, HID lights are gas-discharge arc lamps. They give off an intense light by sending an electrical discharge between two nodes within an ionized gas or plasma-filled chamber. HID fixtures sit within a ballast, which generates the electricity required.

There are three popular HID styles, each differentiated by which gas the electrical charge travels through:

High-Pressure Sodium (HPS) 

Commonly used within industrial applications, HPS fixtures are suitable for growing cannabis indoors, specifically for flowers. HPS lights use highly-pressured sodium gas and a sturdy ceramic arc tube that can withstand the pressure. In combination with Metal Halide (MH) fixtures, these were an industry standard for growing cannabis. Outside of the grow room, they are widely used for parking lots, roads and other industrial settings.

As with all HID fixtures, the HPS varieties emit a substantial amount of energy as heat — even more than MH fixtures. According to most assessments, you can expect more than 2,000 British Thermal Units (BTUs) from a standard 600w HPS. High-Pressure Sodium lights last longest among all the HID types, often upwards of 24,000 hours. 

Metal Halide (MH)

Metal Halide fixtures contain a mixture of vaporized mercury and metal halides. Over the last few decades, they've evolved a much better efficiency than older models. These HID lights also have a better spectrum than their predecessors, with a range leaning into the red. For indoor cannabis, cultivators traditionally use MH for the vegetative stage. Outside of cannabis, these bulbs are typically installed for commercial and industrial applications, such as street lights, retail environments and stadium lighting.

As with most HID fixtures, the MH varieties emit a significant portion of energy through heat. For example, a single 400 watt MH fixture can emit more than 1,300 BTUs. Some estimates put the wasted energy as high as 30 percent of input.

The high heat and vapor pressure within the bulbs slowly degrade the bulb itself. An MH bulb's expected lifespan is roughly 20,000, with many commercial growers having a 10,000-hour replacement rule. Color and intensity diminish throughout the light's lifespan.

Ceramic Metal Halide (CMH)

In what is essentially a combination of HPS materials, and MH ingredients, CMH fixtures are popular because they are a more robust option, translating into a longer life span. A CMH fixture uses a ceramic arc tube borrowed from HPS technology but relies on metal halide and other gases for a better light spectrum for cannabis.

The combination means CMH lights are not only longer-lasting (24,000 hours) but also have a better spectrum balance. There are now specific CMH bulbs designed with spectrums suitable for both bloom and veg. Although it should be noted the range is not as intense as other options for bloom. 

Light Emitting Diodes (LED)

The most recent invention in lighting is also the most energy-efficient and long-lasting. Light-emitting diodes, or LEDs, are tiny semiconductors that light up when electricity passes through them. While each individual LED is small, but combined in larger fixtures of hundreds (or thousands), this technology rivals many HID fixtures' intensity. 

LED fixtures use a power driver instead of ballast. They also do not require a reflector. LEDs also come in a wide spectrum of colors. The semiconductor material and the separation of energy bands within each diode predict the color emitted. LED grow lights typically contain several different color-emitting diodes to suit the spectrum required for cannabis (and any other plant).

Many growers are switching from traditional HID for growing cannabis. One of the benefits, especially for small-to-medium size grows, is the lower heat emissions and higher system efficiency. While it's isn't true that LEDs don't emit any heat, they can be placed as close as 6" from the canopy, with similar headspace from the top of the tent. 

LEDs have a better light focus as well, which translates into a better total efficacy. Traditional HID lights loose 50 percent or more efficiency because they emit light 360 degrees around the tubes. LEDs focus the beam to 180 degrees or less.

While LEDs have come down in price over the years, they still require more upfront capital expenditure than conventional lighting. However, as the most energy-efficient fixtures with a lifespan of upwards of 100,000 hours, the operational expenditures promptly make up the capital costs.

• Bar LED 
  

Many popular indoor LED grow lights are a bar-type design with all LEDs positioned along bars connected along each end: the more bars, the more powerful the light fixture. Bar LED fixtures like the Growers Choice ROI-E680s are extremely useful within small spaces, as they require minimal headspace and are themselves only a few inches high. The intensity increases with the more bars added into the light.

• Board LED 
  

Similar to bar LEDs in height, board LEDs merge individual bars into a broad board and sometimes incorporate COBs. Because of the increased surface area, these board-style LEDs have better output than the simple bar style because of the available surface area, which means more LEDs. Keep in mind, this may mean increased prices because there are also more components. Horticulture Lighting Group (HLG) is known for extremely high quality board style LED lights.

• Chip on Board (COB)
  

COB LEDs are several diodes bound together into a single module. Fixtures using this technology look more like traditional lights than an LED. COBs can also fit many more individual diodes together for better light intensity than bar and board style. Cultivators frequently use COB LEDs as supplemental lighting to either HID conventional grow lights or LEDs.

How Do LED Grow Lights Work?

Although briefly touched on above, more explanation is warranted. How do LED grow lights work? 

LED grow lights are technically made up of hundreds (or thousands) of individual semiconductors called light-emitting diodes. An electrical current flows through tiny light bulbs (the semiconductors), which creates light.

Electron holes within the semiconductor are required to emit light. Essentially particles called electrons move inside an atom to produce photons. Each diode is of solid material instead of the inert gases contained in HID style lights.

With a few recent technological advances, LEDs now come in a full spectrum of color. The color differences come down to what material each semiconductor contains, impacting its conduction of electrical current. 

The basic option is aluminum-gallium-arsenide (AlGaAs), but in a process called doping, LEDs may have impurities added, which change conductivity and color. Doping changes the size of the gap between the electrons (known as the electron holes). With the introduction of newly discovered materials and the impact on the electron holes, LEDs now come in both blue and red spectrums. Both of which are critical to growing cannabis indoors.

Why are LEDs Taking Over?

Until recently, few commercial cannabis cultivators would consider switching from the powerful HID style lights to LEDs. The spectrum, intensity, and output just weren't enough to grow the best possible bud. Not to mention, LEDs were much too expensive for small-to-medium start ups.

Flash forward to today, and LEDs have not only evolved to the point that they rival the power of an HPS fixture, but they are affordable enough for even the home-grower to consider. Of course, heap fixtures made overseas from unknown manufactures may not live up to the new LED promise, but if you do your research and buy from reputable companies, there are real reasons to make the switch to LED grow lights. 

Long story-short: LEDs are worth it.

• Lifespan

Both MH and HPS light fixtures have a relatively short life span, especially considering how long you run lights for commercial cannabis production. Generally speaking, these fixtures have a reported life of 20,000 to 24,000 hours, respectively, but commercial growers often replace as soon as 10,000 to avoid the diminishing outputs.

LEDs, on the other side, often have a life expectancy pushing 100,000 to 200,000 hours. That's four to eight times longer than any HID on the market. Furthermore, the half-life is also much better than the alternatives. As per UpStart Farms, a typical LED fixture operates at 100 percent for eight years and decreases to 95 percent. 

• Operational Expenditures

LEDs are measurably cheaper to run on an ongoing basis, with some running with 70 percent less energy. Going back to the explanation from UpStart Farms, LEDs can save as much as 50 percent of ongoing expenses. These savings come from energy inputs, size and energy for an exhaust system, and the bulb replacements needed for HID. 

• Space considerations

Because LEDs run much cooler than conventional HID varieties, they can sit much closer to the canopy without burning the plants. You can also hang LEDs without much headspace. Furthermore, HVAC and heat exhaust requirements are drastically reduced, another space saver.

Light Spectrum 101 

According to the Encyclopedia Britannica, the electromagnetic spectrum, or light spectrum, "the entire distribution of electromagnetic radiation according to frequency or wavelength." The spectrum ranges from ultraviolet to visible to infrared, and nanometers (nm) measure the wavelength. 

Humans can only see the visible spectrum with our eyes, which is between 380 to 750 nanometers. Plants use a broader range that often means infrared and ultraviolet wavelengths.  

Cannabis requires a broader spectrum to thrive — depending on the stage, blues, reds, and ultraviolets all play a role in quality bud development. Cannabis requires more blue during vegetative, and as they transition to flower, reds are more valuable.

Different grow lights will emit different wavelengths of light. For example, MH runs blue, which is why growers use it for veg. HPS tends to run red, and therefore is used in bloom rooms. LED grow lights often come with a specific combination of blue, red, and white diodes (with the option to switch between) to cover the full life cycle.

Grow Light Efficiency (System Efficacy) 101 

Before the invention of LEDs, comparing grow light efficiency came down to comparing watts, watts per area, or watts per dollar. Talking about watts is talking about electricity the fixture uses, but this is hard to compare between technologies. Just think, HIDs and LEDs pull drastically different electricity. Even fixtures within the same category can pull different inputs (this is especially true for LEDs). 

A bigger problem? Every manufacturer will measure efficiency using different metrics, especially between LEDs and HIDs. 

Electricity doesn't grow plants, however. Plants need the light emitted by the fixture. Therefore, efficiency is about the output, based on electrical inputs. The more usable light a fixture emits (μmol) and lower inputs (watts) it needs, the more efficient it is. Want to calculate this for yourself? Here is the basic formula:

Usable PPF (µmol) / watts = system efficacy

Look for manufacturers' information on system efficacy, which is a number expressed between 0 and 3. The higher the number, the better the grow light efficiency.

Grow Light Output 101 

What sorts of outputs and in what measurements should you look for in grow lights? The spectrum for photosynthesis is called PAR, or Photosynthetically Active Radiation. Roughly speaking, PAR encompasses the range between 400 to 700 nm. Grow lights focus on the PAR spectrum measured by Photosynthetic Photon Flux (PPF) and Photosynthetic Photon Flux Density (PPFD).

Photosynthetic Photon Flux (PPF) & Photosynthetic Photon Flux Density (PPFD) 101

The industry uses PPF or umol/s to measure PAR outputs. This measurement tells you how many photons (within the PAR spectrum) the fixture emits emitted every second. Typically the region is the designated canopy size that your light is supposed to cover at a specific hanging height. 

But, what is more important than PPF? PPFD (μmol/m2/s). PPFD tells you photos within the PAR spectrum delivered every second to a specific spot. From one corner to the next, the light changes intensity. Plants placed directly in the center of the light fixture will get more light than those at the edges. Many LED grow lights provide a PPFD map, which indicates a spread of PPF numbers. 

How to Choose the Right Grow Light? 

With all those acronyms, formulas, and numbers floating around, it is incredibly difficult to compare grow lights. Especially between LEDs and HIDs. Often, the numbers are incomprehensible to the average grower. 

What do you need to take into consideration when you go shopping for grow lights? As distilled from our Complete LED Buying Guide, here are the most important takeaways.

• Budget

Ultimately, any fixture you buy will have to be within your budget. Keep in mind, you'll want to assess the capital expenditure as well as the operational expenditure. For example, the cost savings over an LED grow light's lifespan help offset the initial investment.

• Grow Needs

Different growth stages require different intensities (PPFD) and different shifts in the spectrum for optimal development. Are you going to use the fixture for all stages, or just for one (veg, flower, seedling)? Identify what you need out of the fixture before going shopping.

• Space Restrictions

Small spaces may require smaller and cooler fixtures, which is where LEDs come in. Large warehouses in colder climates may actually benefit from hot-HID lights, but with the minimal hang height, headspace, and heat emissions, LEDs work best for small grow rooms or grow tents.

• Climate

Although LEDs are more energy-efficient, some growers prefer HIDs because they live in colder climates and actually need the heat. Instead of buying a separate heating unit, they use HID lights to maintain a consistent temperature. Hotter climates may want to switch to LEDs to reduce HVAC requirements.

• Additional Features to Consider

Of course, many other factors might go into your purchasing decision. These include onboard dimmers, automatic controls, and a warranty. Some fixtures come with dimmers built right in. Others will require an additional purchase, sometimes hundreds of dollars more.

What about automatic controls? This is a valuable addition to any grow room, which alleviates some of the day to day responsibilities. Many brands offer automatic light controllers, like the Growers Choice Master Lighting Controller. 

This controller is of particular note because it provides complete control of the environment, including timed on/off, sunrise/sunset, and precise temperature readings thanks to thermal probes and thermal-threshold auto dim-shutdown for additional protection.

Frequently Asked Questions About Grow Lights for Cannabis 

How Much Energy Do Grow Lights Use?

There is no magic number that describes all grow lights. Instead, you have to work out the calculation yourself.

• Dig into the specs provided for each fixture by the manufacturer, look for the input in watts. Compute this to kilowatts by dividing by 1000. For example, 680 watt light equals 0.680 kW. 
• Next, multiply by the number of fixtures in the grow room. For example, 4 fixtures x 0.680 kW = 2.72 kW.
• Check out your electricity bill. What's the rate in kWh? For example, $0.122109/kWh.
• Finally, how many hours do you run your lights and for how long? For example, during veg, you may run them 18 hours for two months: 18 hrs x 60 days = 1080 hrs.

If you put this all together, the formula is as follows:

(Total Kilowatts x number of lights) x kWh x (hours per day x days) = cost 

As an example with the numbers we used above:

(0.680 x 4 lights) x $0.122109/kWh. x (18 x 60) = $358.70

What are the Best LED Grow Lights?

The best LED grow lights pump out a perfect PPFD for weed, with the lowest energy requirements — and all for the lowest cost. When making a purchasing decision, it's useful to balance these three characteristics. 

Happy Hydro recently put several popular grow lights into a LED Grow Light Battle: Updated 2023. 

Comparing price, space requirements, efficiency (umole/j), output (umol/s), coverage and more, the finalist ticked all the boxes and then some for growing weed indoors: Growers Choice ROI E-680s.

The best LED grow light boils down to crunching the numbers and brand reputability. Amazon is rife with cheap knock-offs, which have flooded the market and dragged the reputation of LED lights into the mud. But, if you stick with established brands, the quality, customer service, and efficiency is never in doubt.

How Many LED Grow Lights Do I Need?

How many grow lights you need boils down to how many plants you need to cover within an intended space. Generally, grow tents come pre-fabricated in grow light sizes (or multiples thereof). This makes the calculation easy. For example, a 2' x 2' AC Infinity Grow Tent will fit one 2' x 2' LED grow light. 

If the solution isn't as clear as grow tent size, estimate how large your veg and flower canopies will be. This is based on the number of plants, pruning and training techniques used, and grow space. Next, determine the size options for your chosen grow light (ex: 2'x 2', 5' x 5', 6' x 6'), and aim to buy a single bigger fixture instead of several smaller ones. 

As you expand your grow space, the calculations get more complicated. For larger grows, these basic calculations may need modifications. Work with the grow light manufacturer to develop individual schematics for commercial spaces.

How High Above the Canopy do I Hang My Grow Lights? 

Once again, there is no simple estimate because there are so many factors. What style of light (HID or LED) is the most critical factor. HIDs are exponentially more intense, which may burn plants if hung too low. You'll need to hang LEDs much lower to avoid stretching and produce a beautiful bud. 

On top of that, each brand will have completely different specs, so it's worth checking each manufacturer's recommendations. We cover this question in more depth here.

Generally speaking, the ideal hang height for HID lights is as follows:

Grow Light Wattage	Lowest Height	Highest Height
250W	10”	14”
400W	12”	20”
600W	14”	24”
1000W	16”	36”

The ideal hang height for LEDs is as follows:

Growth Stage	Lowest Height	Highest Height
Seedling	24”	36”
Vegetative	10”	24”
Flower	10”	24”

What's the Best Light Schedule for Growing Cannabis Indoors?

All cannabis (besides autoflowering varieties) require shifts in the light schedule to trigger the transition from veg to flower. In nature, longer days indicate summer months, when plants are pumping up vegetative growth to feed future flowers. As the days get shorter, cannabis moves into flower knowing fall and winter are fast approaching.

While every grower has a slightly different opinion, most keep lights on between 18 to 24 hours a day in the veg stage. To switch to flower, move to a 12 on, 12 off schedule.

What is the Best Spectrum to Grow Cannabis Indoors?

Plants require a specific spectrum to create energy through the process of photosynthesis. This spectrum is called PAR (Photosynthetically Active Radiation). This range falls between 400 to 700 nanometers (nm). 

Each stage of your indoor grow needs a slightly different application of light. The vegetative stage needs substantially more blue than red. The blue and white spectrum will encourage bushier plants, with lots of internodial development. 

Flower, on the other hand, requires a deeper red spectrum, which is partially how cannabis knows it's time to switch between the stages. Boost the red as high as you can during flower to increase yields. 

Is UV Light From Grow Lights Bad for Your Eyes?

The ultraviolet, or UV, light spectrum breaks down into three different kinds: UV-A, UV-B, and UV-C. All UV light is bad for humans eyes, especially UV-C. However, grow lights shouldn't emit UV-C, as it is also damaging to plants. But LED lights and auxiliary lights often include a UV-A and B component because it's useful for developing specific properties (THC) in cannabis.  

If your LEDs emit any light within the UV-A and UV-B spectrum, ongoing unprotected exposure may cause some damage. It's why grow room glasses with UV protection are so important. These ensure, no matter how much time you spend in your grow room, the UV light will not damage your eyes.

Not the End of the Story on Grow Lights

Of course, this complete guide on grow lights is just the beginning. There are hundreds of forums and thousands of commenters debating the technology and details of setups for growing weed indoors. There are as many opinions as there are strains of weed, but the numbers don't lie. Take the time to crunch the numbers for your specific space. Wrap your head around the calculations, make a few comparisons, and find the best grow light for your grow room.