How to Evaluate and Compare Grow Lights for Cannabis

A grow light is perhaps the most important piece of equipment for growing cannabis indoors. In this article, we explain what you need to know to evaluate and compare different grow lights and make an informed grow light selection. We dive into horticultural lighting science to explain the important grow light metrics: Photosynthetically Active Radiation (PAR), Photosynthetic Photon Flux (PPF), and Photosynthetic Photon Flux Density (PPFD). Using these metrics, we compare High Pressure Sodium (HPS) and Ceramic Metal Halide (CMH) with LED grow lights. We explain why we now prefer and recommend LED grow lights and offer suggestions to help you select a quality LED grow light for your home cannabis grow.

This article is part two of the Coco for Cannabis Grow Set-up Guide.

The Coco for Cannabis Grow Set-up Guide
  1. Grow Tents and Harvest Sizes
  2. How to Evaluate and Compare Grow Lights for Cannabis
  3. How Much Light (PPF) Do You Need for Indoor Cannabis?
  4. Grow Light Calculator (Coming Very Soon)
  5. Airflow, Ventilation and Exhaust Systems for Grow Tents (Coming Soon)
  6. Managing Heat and Humidity in your Grow Tent (Coming Soon)

Grow Lights for Cannabis

There are two types of High Intensity Discharge (HID) lights that are commonly used for flowering cannabis plants: High Pressure Sodium (HPS) and Ceramic Metal Halide (CMH). CMH lights are also known as "Light Emitting Ceramic" (LEC). These HID lights offer a good photon efficiency, an excellent cost efficiency and spectrums that are well-suited for flowering cannabis plants. For decades, these lights have dominated the indoor cannabis growing industry and even influence how growers think about things like harvest efficiency. However, all of that is changing.

Light Emitting Diode (LED) grow lights are rapidly transforming indoor horticulture. When they first came on the market, LEDs were less efficient than traditional HID lighting. Despite being less efficient, LED lights found a space in the market because they offered advantages in terms of convenience, heat, and longevity.

The science and the technology of LED grow lights have improved significantly. In the last few years, manufacturers have started producing LED lights that are significantly more efficient than HID lights. These LED lights are up to twice as efficient as HID lights, produce a full spectrum of light, and create less heat than traditional HID lighting. Unfortunately, the most efficient models have been too expensive for most home growers. However, as the technology of LED lighting has developed, the price has come down. Within just the last year, several manufacturers have released models which can compete with HID lights on cost and beat them in terms of efficiency.

We now recommend LED grow lights to all home growers. However, the LED grow lights available on the market vary tremendously in terms of their efficiency, cost and the quality of the light that they produce. The entry level LED market is still full of low quality and low-efficiency fixtures. In order to help you select a high quality, high-efficiency LED grow light, we explain how to evaluate grow lights scientifically, things to look for in a quality LED, and offer specific brand and model recommendations.

Evaluating Grow Lights Scientifically

The grow light market and the internet are full of conflicting and confusing claims about grow lights. It can be difficult to tell the difference between scientific fact, "broscience", personal opinion, and marketing tactics. Many manufacturers try to take advantage of this situation and use data about their lights in a misleading way. We recommend that you ignore the manufacturer's claims about coverage, equivalency, or efficiency. Instead, we explain how to use the proper scientific metrics to evaluate different grow lights.

Grow Light Metrics

With HID lighting it became common to measure the amount of light produced by discussing the wattage of the light fixture. Wattage worked as a reasonable proxy measure for light because all HID fixtures convert electricity into light at about the same rate. LED grow light manufacturers tried to take advantage of this situation by marketing their fixtures with an "equivalent wattage". These figures are usually grossly inflated to make the lights appear more efficient than they are. Wattage is still an important dimension of a grow light fixture's efficiency. However, we also need to consider the light that it produces with those watts.

Understanding the scientific metrics that are used to measure and evaluate horticultural lighting will allow you to make a more informed grow light decision. We explain the metrics and then use them to compare HID lights with select LED fixtures. To begin, it is important to recognize that we are not interested in "light" per se, we are interested in photosynthetically active radiation (PAR).

Photosynthetically Active Radiation (PAR)

Photosynthetically Active Radiation (PAR) is the region of the electromagnetic spectrum that plants can use for photosynthesis. PAR wavelengths are between 400 nanometers and 700 nanometers. Any light that is outside this range of wavelengths will not be used for photosynthesis. Therefore, when evaluating the efficiency of grow lights, we consider PAR rather than other measures of light, such as lumens (Lux).

Micromole (µmol)

PAR values are expressed in micromoles (µmol). A micromole is a standard unit for counting particles (e.g. atoms, molecules, ions and electrons). In this case, we are counting the number of photons in the PAR wavelengths. Micromoles are used to express two different measurements of PAR: Photosynthetic Photon Flux (PPF) and Photosynthetic Photon Flux Density (PPFD). It is important to understand the difference.

Photosynthetic Photon Flux (PPF)

Photosynthetic Photon Flux (PPF) is a measure of the total number of PAR photons emitted by a light fixture per second. PPF values are properly denoted as micromoles per second (µmol/s).

PPF is a measure of the total amount of usable light that a fixture will produce. As we explain in our guide, "How Much Light (PPF) Do You Need for Indoor Cannabis?", PPF values are the best way to determine the total amount of light that you need for your grow space. As we explain below, it is also the value that we must use to determine the efficiency of different grow lights. You will often find PPF data reported as number between 0 and 4 as "µmol/j". This is a measurement of photon efficiency. To convert a figure reported as "µmol/j" into PPF you need to multiply by the power draw of the fixture in watts (µmol/j x watts = µmol PPF). However, PPF values are often harder to obtain than the other measurement of PAR, which is PPFD.

Photosynthetic Photon Flux Density (PPFD)

Photosynthetic Photon Flux Density (PPFD) is a measure of the number of PAR photons that strike a particular target per second. PPFD values are expressed as micromoles per meter squared per second (µmol/m2/s).

PPFD is used to measure the intensity of light at a particular distance and position. PAR meters and the “PAR maps” from LED manufacturers both report PPFD values. It is the correct measurement to use when you are determining the intensity of the light at canopy level and the proper hanging height for LED fixtures. However, PPFD values cannot be used to evaluate the overall efficiency of different light fixtures.

PPF vs PPFD

PPF measures the total amount of light. PPFD measures the intensity of light hitting a target. Both PPF and PPFD measure the number of PAR photons per second. PPFD divides this number by area whereas PPF is just the total value. Although PPF is properly µmol/s and PPFD is µmol/m2/s, both PPF and PPFD values are often reported simply as "µmol". Thus, when you see values reported as "µmol", you need to know whether it is a measure of intensity (PPFD) or the total quantity of light produced (PPF). When adjusting your lights and determining light height, the important value is intensity as measured by PPFD. However, when you want to compare the efficiency of different fixtures (see our Grow Light Calculator) or determine the total amount of light that you need (See our Grow Space Calculator), the correct value to use is PPF.

How to Compare Different Grow Lights

To evaluate and compare different grow lights there are several factors to consider. Convenience, heat, light distribution, and light spectrum all deserve some attention. However, the most important factors to consider are grow light efficiency in terms of electricity and cost. In order to accurately estimate the efficiency and relative cost of a grow light, we first need to accurately estimate how much light a fixture really produces, its "Estimated True PPF".

Estimating True PPF

As we explain in our guide, "How Much Light (PPF) Do You Need for Indoor Cannabis?", there are three different ways to measure or calculate PPF. The value that is most relevant to us as growers is "True Measurement" PPF. However, True PPF data for LED grow lights is nearly impossible to obtain. Manufacturers often provide "Calculated Values" which are typically about 20% higher than True PPF. The third type of PPF data is "Field Measurement" data. Typically, field measurements under measure PPF by about 15-20%. Field measurements can vary widely between different testers, so they are the least reliable form of PPF data. Despite this, field measurements from reliable testing may be more accurate than calculated values.

The Data we provide in the table below include all three types of PPF data. For HID lights, we use True PPF measurements (including reflector losses). These data come from sophisticated laboratory testing by Nelson and Bugbee. For the LED lights, we use data from independent field measurement by Migro because we are confident in their testing procedures. We rely on manufacturer's calculated values for fixtures that have not been tested by Migro.

For accurate measurement and comparison, we adjust PPF values to an "Estimated True PPF". We adjust the calculated values by subtracting 20%. We adjust Migro's field measurement data by adding 15%. This is based on known examples where we can see that field measured data from Migro is typically about 15-20% less than a True Measurement of PPF. Using these adjustments, we arrive at similar figures for Estimated True PPF whether we start with Field Measured or Calculated Values. These adjustments will never be perfect, but it is better than trying to compare different types of PPF measurements without adjustment. For more discussion on accurately estimating PPF, see our guide, "How Much Light (PPF) Do You Need for Indoor Cannabis?".

If you need to calculate the "True PPF" that is optimal for your grow space, use our Grow Space Calculator. If you want to estimate the "True PPF" of a grow light fixture, use our Grow Light Calculator (Coming Very Soon).

Grow Light Efficiency

The lights that we run are the most expensive part of growing cannabis indoors. They are expensive to purchase, and they are expensive to operate. Choosing efficient grow lights reduces both operating costs and the environmental impact associated with electricity use. However, most home growers on a budget also need to consider the upfront purchase price. Therefore, we use two separate calculations to measure grow light efficiency: photon efficiency and cost efficiency.

"Photon efficiency" is a calculation for how efficiently the fixture converts electricity into usable light. To determine photon efficiency, divide the Estimated True PPF (µmol) by Actual Power Draw in Watts.

"Cost efficiency" is a calculation of how expensive the fixture is relative to the amount of PAR light it produces. To calculate cost efficiency, divide the purchase price by Estimated True PPF (µmol). In the table below, we use estimated market prices to calculate cost efficiency. The actual price that you find might be different.

Coming Very Soon! You will be able to use our Grow Light Calculator to easily calculate the photon and cost efficiency for any light fixture.

Comparing HPS, CMH and LED Grow Lights

In the table below we provide data to compare CMH, HPS and LED grow lights. We include our recommended LEDs along with a "Cheap" LED and a "Top of the Line" LED for comparison.

Approximate costs are based on market prices in the US gathered in January 2020. We use PPF data from the sources discussed above. You can expand the table to see the complete PPF data and our estimates for coverage area and benchmark harvest targets for the different fixtures. For more information about how we calculate data about grow lights see our Grow Light Calculator (coming very soon).

See a complete description of all our recommended LED grow lights in our Equipment and Product Guide.

Grow Light Analysis: HID vs LED Efficiency

The data in the chart above reveal some interesting relationships between HID and LED grow lights in terms of photon and cost efficiency. HPS lights still have the lowest cost per µmol (PPF). A double ended 1000-watt fixture boasts an incredibly low price of only $0.15/ µmol (PPF). These units still dominate in the commercial grow space because of their cheap cost and competitive photon efficiency. However, a 1000-watt double ended HPS is a large light that requires a large space and generates a lot of heat. It is overkill for most home growers. Smaller HPS lights are less efficient and more expensive.

The small CMH fixtures are more efficient than the smaller HPS fixtures. However, they are more expensive. Indeed, the CMH fixtures are more expensive than some of our recommended LEDs. If we compare the 315-watt CMH to the Mars TSL2000 or TSW2000, we see that the CMH light costs more, uses more electricity, and produces less light.

The Mars TS series are the lights that we recommend as the best value on the market. They boast photon efficiencies that beat even the most efficient HPS lights and they cost less than $0.50/ µmol (PPF). This is an incredible price for such a highly efficient fixture. I (Dr Coco) now use two of the Mars TSL 2000 lights in my 4' x 4' tent.

The Horticultural Lighting Group (HLG) lights are somewhat more expensive per µmol (PPF), but they offer elite efficiency. The HLG 260W V2 tops 2 µmol/watt and comes in at only $0.62 / µmol with the DIY kit at LED Grow Light Depot. The HLG65 QB120, is a small light with an elite photon efficiency of 2.18. It is over 45% more efficient than a 600w HPS light!

See all our Grow Light Recommendations in our Equipment and Product Guide.

The Advantages of LED Grow Lights

The cost efficiency of the large HPS lights still make them attractive to commercial growers. However, home growers started moving to LEDs even while they were more expensive and less efficient. Home growers have been embracing LED grow lights because they are easier to install, produce less heat and help make managing the grow easier.

Until recently, we still recommended HID lights to new growers. We knew that the HID lights were more efficient than the affordable LED grow lights. This is no longer the case. Now that LED grow lights are more efficient than HID lighting and can compete with them on cost, it is hard to make an argument for HID lighting. We now recommend high-efficiency LED grow lights for all home growers. See our Grow Light Recommendations.

Choosing a Quality LED Grow Light

There are quality LED lights available at an affordable price. However, the LED market is rife with incomplete information and false claims. There are many discount LED lights that grossly exaggerate their efficiency and HID light equivalence. Without good information it can be easy to end up with a low-quality light. If you want the best LED grow light for your grow tent and budget, consider our advice and recommendations below.

Cheap LEDs are not Cheap

Many growers are attracted to the low price tags on cheap LEDs. The Viparspectra V300 that we show in the table above is one such light. It is sold for only $70, but it is not less expensive per µmol (PPF) than the TS series. Both the Viparspectra and the TS series are just under $0.50/ µmol (PPF). However, the TS Series is over 60% more efficient. Comparing the Viparspectra to the HLG 65 QB120 is even more damning. Both lights produce about the same amount of light (147 vs 142 µmol PPF), but the HLG 65 uses exactly half the electricity! It is commonly believed that LEDs are more efficient than HID lights, but the Viparspectra gets lower photon efficiency than any of the HPS or CMH lights in the table. No matter which way you crunch the numbers, the cheap LEDs are not actually cheap.

Efficient LEDs Do Not Have to be Expensive

The Fluence Spyderx that we include in the table above is an impressive fixture. We estimate its photon efficiency to be close to 2.5 µmol/w. That is almost twice as efficient as the average HID light. The price on these lights has come down significantly, but the cost per µmol (PPF) is still very high. The Spyderx is more than twice as expensive per µmol (PPF) as the TS series and over 50% more expensive per µmol (PPF) than the HLG lights.

Although the TS series and the HLG lights are not as efficient as the Spyderx, they are high efficiency models that easily outperform HID lights and "cheaper" LEDs. Our recommended LEDs are quality fixtures for all home growers and they fit into any budget. If your budget is very tight, we recommend starting with one Mars TS1000. You can add additional units as your budget permits and end up with a high efficiency quality lighting system. See all our Grow Light Recommendations in our Equipment and Product Guide.

Smaller LED Fixtures Can Be More Efficient

With HID lights, the most efficient fixtures are the largest and most powerful. This means that it is better to have one large fixture rather than multiple smaller units. However, the most efficient HID lights are simply too large for many home growers.

With LED lights, some of the most efficient units are small fixtures. Indeed, the HLG 65 that we recommend covers only about 2 sq. feet and the Mars TS 1000 covers just over 4 sq. feet. We suggest getting several of these lights and running them in an array.

Using smaller fixtures in an array offers several advantages over using one large light. Small fixtures allow you to distribute the light better throughout the grow space. They also allow you to hang the lights closer to the plants, which saves precious vertical space inside a grow tent. With a large light, you need a higher hanging height because the light source is more intense, and the light needs to distribute. Another advantage of using several smaller lights is that it allows you to adjust the height of individual fixtures to optimize the hanging height above different plants. Finally, having small lights allows you to start each grow using just one light, which saves electricity. As the plants grow, you can simply turn on the additional lights.

Ignore the Marketing Hype

The amount of deception in the LED grow light market is truly remarkable and disturbing. There are no standards for the claims that many manufacturers make about how their lights compare to traditional HID lighting. As a result, many manufacturers grossly inflate the perceived efficiency, power, and value of their lights. For example, the Viparspectra V300 light that we show in the table above is advertised as being equivalent to a 250w HPS. However, using PPF, we estimate that it is only equivalent to about 113 watts of HPS lighting. We recommend that you ignore the equivalencies that manufacturers publish. Instead, use PPF data and our Grow Light Calculator to evaluate the total power, efficiency and relative costs of different lights.

Avoid Blurple Lights

When LED grow lights first hit the market, they were less efficient than standard HID lighting. In an effort to improve efficiency, LED manufacturers focused on red and blue spectrum light. This led to the "blurple" grow lights, which continue to dominate the low end of the LED market. "Blurple" light is more efficient to produce, but it offers an inferior spectrum for growing cannabis. We recommend that you choose only "full-spectrum" LED grow lights.

Focus on PPF not the Brand of LED Chips

Many growers get caught up thinking about the brands and wattage of the chips used for LED fixtures. However, the chips are not a good indicator of how well a light fixture is going to perform. There are many variables that influence the ultimate efficiency of any grow light fixture. Using the latest technology could allow for more efficient lights, but that is not always the case. Rather than focusing too closely on the components, it is important to consider the entire light system. The best way to evaluate the efficiency of the total fixture is by measuring PPF. To learn more about PPF and how much you need, see our guide, "How Much Light (PPF) Do You Need for Indoor Cannabis?". If you have a grow light fixture in mind, use our Grow Light Calculator (Coming Very Soon) to evaluate the total PPF, efficiency, coverage, and harvest estimates it will provide.

COB Vs SMD

It is commonly believed that "Chip on Board" (COB) style LEDs are more efficient than "Surface Mounted Device" (SMD) style LEDs. However, this is not always true. COB fixtures use many individual light emitting diodes in close proximity to each other. This design can concentrate heat energy and lower the overall efficiency of the fixture. An SMD fixture can produce the same total PPF with fewer chips that are spread out over a larger surface area. The larger chips are less efficient, but the overall design may be more efficient because it allows heat to dissipate and avoids thermal runaway events. COB designs often offer lower price points and may be a great option for DIY fixtures but be sure to evaluate the efficiency of each fixture using PPF.

Quantum Boards: Printed Circuit Boards

The Printed Circuit Board or "Quantum Board" design has transformed the industry and is quickly becoming the standard for quality LED fixtures at the lower price points. These lights simply mount the diodes to a printed circuit board and use passive cooling to dissipate the heat. This streamlined design requires far few components and has allowed manufacturers to dramatically reduce the cost of high efficiency LED grow lights. Both the Mars TS series and the HLG quantum boards that we recommend take advantage of this printed circuit board design. See all our Grow Light Recommendations in our Equipment and Product Guide.

The Next Steps

We have two more important resources to help you make the best grow light decision. To determine how much light you need for your tent, see our article, "How Much Light (PPF) Do You Need for Indoor Cannabis?". In it, we dive into plant science to determine the optimal amount of light for the best growth. It also includes our Grow Space Calculator which you can use to calculate the optimal PPF for your tent.

When you know how much light you need it is time to turn to our Grow Light Calculator (coming very soon). The Grow Light Calculator makes it easy to evaluate the efficiency, coverage area, cost and potential harvest size of any grow light fixture.

The Coco for Cannabis Grow Set-up Guide
  1. Grow Tents and Harvest Sizes
  2. How to Evaluate and Compare Grow Lights for Cannabis
  3. How Much Light (PPF) Do You Need for Indoor Cannabis?
  4. Grow Light Calculator (Coming Very Soon)
  5. Airflow, Ventilation and Exhaust Systems for Grow Tents (Coming Soon)
  6. Managing Heat and Humidity in your Grow Tent (Coming Soon)

Need Grow Equipment?

Check out our Equipment and Product Guide for our Recommended Grow Lights, Grow Set-up Equipment, Germination Supplies and everything else you will need during the grow!

Get Support from our Community!

We have a wonderful community of grower's and you are welcome to join! If you are looking for advice and support throughout your grow, I encourage you to start a Grow Journal in our Grower's Forum. It is a great way to share your experience and ask questions along the way.

We also have a live chat room, which is a great place to ask questions, get quick answers, and chat with fellow growers. We welcome all growers who want to learn, share and grow together!

Citations:
  1. Nelson JA, Bugbee B (2014) Economic Analysis of Greenhouse Lighting: Light Emitting Diodes vs. High Intensity Discharge Fixtures. PLoS ONE 9(6): e99010.
  2. Migro LED Field Measurement: Mars TS1000 Test and Best Small Fixture 2018

Author: Dr Photon and Dr Coco

About Our Product Recommendations

At Coco for Cannabis, our mission is to help growers maximize the success of their cannabis crops by providing scientifically accurate information and sharing proven growing practices. The products that we recommend are the actual products that we chose to purchase and use ourselves. We do not accept sponsorship or advertising and will not recommend any product that we would not buy for ourselves. Please see our Product and Equipment Guide.

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Disclaimer:

The information on cocoforcannabis.com should not be considered as financial, legal, or medical advice.
You are responsible for knowing and following the local laws that pertain to cannabis cultivation, possession, and use. Decisions to grow cannabis should be made in consultation with a lawyer or qualified legal advisor. Decisions to use cannabis should be made in consultation with your doctor or medical professional.