LED Grow Light FAQ - Your Questions Answered

LED Grow Light FAQs for Indoor & Hydroponic Growers

How much money will I save if I switch to LED grow lights? What size LED fixture do I need for my grow tent or grow room? We get these questions a lot from indoor and hydroponic growers, and we’re always happy to dig into the details. Below you’ll find answers to the most common LED grow light questions we receive from GrowersHouse customers.

How much money can I save by switching to LED grow lights?

If you have a small grow room or grow tent that measures 4' x 4', you typically need one grow light with about 500 watts of power for efficient indoor or hydroponic cultivation. You’ll usually spend a bit more up front on a quality LED grow light compared to a CMH fixture, but you save money over time on both electricity and bulb replacement.

Over the course of 10 years, you would end up spending about $4,800 running and maintaining your CMH lights, and only around $3,900 on a comparable LED grow light setup. We have discussed this LED vs CMH grow light cost comparison in-depth here.

Can too much LED grow light kill a plant?

Yes, too much light from any source—including LED grow lights—can damage your plants, but it’s extremely rare for light alone to kill a plant. You’ll usually see clear visual stress signals before it gets that far.

Signs of too much LED light include lightening or yellowing of the leaf tips on the uppermost leaves. As this stress progresses, the leaves can become crisp and burnt. Another classic sign of excessive light intensity is bleaching of the buds near the top of the canopy, where they turn almost white.

If you notice these symptoms, raise your LED grow light by about 12 inches and observe the plants over the next few days. That adjustment alone usually solves the problem. Just remember: plants grow up toward the light, so keep increasing the fixture height as your canopy gains vertical growth.

What size LED grow light do I need for my grow tent?

This is one of the most common questions we get from indoor and hydroponic growers, both online and in-store. The ideal LED grow light size depends on two main factors: the square footage of your growing area and the actual power draw and efficiency of the LED fixture.

As a general example, a 5' x 5' grow tent can be fully covered by a single high-output 750W LED grow light designed for flowering coverage. 

What are the best LED grow light brands?

The “best LED grow light” isn’t the same for every grower. What’s best for you depends on your goals, grow space, and budget. Some growers define “best” by total output (PPF), others by efficiency (µmol/J), and others by features like fixture thickness, weight, modular design, or controllability.

We know “it depends” isn’t the answer you’re looking for, so we’ve broken down top LED grow lights using measurable performance data.

You can read full details on our top LED grow light recommendations in different price ranges in our lab-tested comparison here: Best LED Grow Lights Comparison & Review Test.

Best LED Grow Lights by Efficiency

Below are some of the most efficient LED grow lights we’ve tested, based on µmol/J (micromoles per joule), a standard measure of grow light efficiency:

1. HLG Scorpion Diablo 650W @ 2.86 µmol/J
2. HLG 650R @ 2.75 µmol/J
3. Gavita Pro 1700e LED @ 2.70 µmol/J

Best LED Grow Lights by Total Output

For growers pushing for maximum production per fixture, total output (PPF) is a key metric. Here are some of the highest-output LED grow lights in our testing:

1. Growers Choice ROI-E720 @ 1,895 PPF (µmol/s)
2. HLG Scorpion Diablo 650W @ 1,874 PPF (µmol/s)
3. Gavita Pro 1700e LED @ 1,764 PPF (µmol/s)

Should I run my grow lights at night? Can I leave grow lights on 24 hours a day?

Running your grow lights for part of the night can absolutely help accelerate both vegetation and flowering, especially in warmer climates where night operation helps control temperatures. Light cycles are expressed as X/Y, where X is hours of light and Y is hours of darkness. Common cycles include 18/6 for vegetative growth and 12/12 for flowering.

A reliable light timer can automate your LED grow lights, turning them on and off at fixed times every day.

Do not leave grow lights on 24/7. Plants need a dark period to respire and reset. Continuous light dries plants out, stresses them severely, and will at least stunt growth—if not eventually kill them.

What color should my LED lights be at night?

Most newer, high-quality LED grow lights are full-spectrum fixtures designed to closely mimic natural sunlight. This means they include a balanced mix of blue, green, yellow, and red wavelengths suitable for all stages of plant growth. Always check the manufacturer’s specifications to confirm spectrum details for your specific LED grow light.

Many modern LED grow lights also allow spectrum adjustments, so you can fine-tune light color for propagation, vegetative growth, or flowering. Before setting spectrum, ask yourself: in what phase are your plants right now?

Propagation: More blue and some red support strong rooting and compact early growth.

Vegetative: A full-spectrum LED grow light works best. As plants get bigger and fuller, extra green and yellow wavelengths help penetrate the canopy and feed lower growth. Quality full-spectrum LEDs already include these.

Flowering: Increase red and, if possible, add some extra blue to tighten internodes and support quality flower development.

Do LED grow lights take longer to flower than CMH lights?

Both full-spectrum LED grow lights and CMH (ceramic metal halide) grow lights perform well for flowering. Flowering time is influenced by several factors: light hours, intensity, temperature, humidity, strain genetics, and spectrum.

Older “burple” LEDs that only use blue and red diodes typically underperform compared to CMH or modern full-spectrum LED grow lights. Both CMH and full-spectrum LED grow lights approximate sunlight, providing balanced blue, red, and key vegetative wavelengths like green and yellow.

Many newer LED fixtures let you adjust spectrum for propagation, veg, and flower. Lower-cost LEDs usually do not. For flowering under an adjustable LED grow light, shift toward more red with some added blue to enhance bud formation.

CMH grow lights are also strong flowering performers, but their spectrum is not adjustable. To boost red during flowering with a CMH fixture, one option is to temporarily replace the CMH bulb with a compatible HPS (high-pressure sodium) bulb. The HPS lamp increases orange and red output, which can accelerate budding and flowering.

How should I adjust my grow lights throughout the growing cycle?

Dialing in light spectrum and intensity at each stage—propagation, vegetative growth, and flowering—can significantly improve plant maturity, structure, and yield. Not all fixtures allow spectrum control, but many newer high-quality LED grow lights do.

Before you buy, look at the spectrum chart and PAR data for each LED grow light. Ideally, you want a full-spectrum fixture that closely mimics daylight.

In general, for optimum results, these wavelengths are preferred at each stage (with blue and red useful throughout the entire cycle):

Propagation: Extra blue and some red; a limited amount of ultraviolet (UV) can increase terpene production, but too much can damage plants.

Vegetation: Plenty of blue and green. As plants become fuller, yellow wavelengths help penetration into the canopy to feed lower branches and inner growth.

Flowering: Definitely more red, plus a little additional blue.

Adjustable LED grow lights make these spectrum changes straightforward. MH and CMH grow lights work well for propagation and vegetative growth, but you would typically switch to an HPS bulb for flowering to get more red and orange. Fixtures must be compatible with multiple bulb types if you plan to do this.

Fluorescent lights perform well in propagation due to their blue-white spectrum and can be used through vegetative growth. For flowering under fluorescent fixtures, you may want to replace a standard white tube with a red-enhanced tube, which is a relatively inexpensive upgrade.

How much will my LED grow lights add to my power bill?

Let’s look at real-world power consumption for an LED grow light suitable for up to a 3' x 4' veg space: the California Lightworks SolarXtreme 500. At the average U.S. electricity rate of 13.19 cents per kilowatt-hour and a 12-hour-per-day schedule, this LED grow light would add about $19 per month, or roughly $228 per year, to your power bill.

If you run it longer than 12 hours per day or your local kWh rate is higher, your cost will increase accordingly. As a rule of thumb, the higher the wattage on a light, the more it costs to run—so fixture efficiency (µmol/J) matters a lot for long-term operating costs.

What are the best grow lights for small closets and grow tents?

For ultra-small grow spaces, like 2' x 2' closets or compact grow tents, you want compact, efficient LED grow lights that deliver strong PPFD without overwhelming the space with heat.

Two of our favorite LED grow lights for these small areas are the California Lightworks SolarXtreme 250 LED and the Horticulture Lighting Group HLG 100 V2 LED. Both are well suited for tight spaces around 2' x 2', providing efficient, full-spectrum light for vegetative and flowering growth.