LED grow lights: reading the real specs (PPFD, spectrum, watts)

On the same hardware store shelf, a 15 W bulb claims to be "200 W equivalent," an LED bar advertises 2,550 lumens and 6400K, and a third promises "full spectrum." Which one will grow your basil? Impossible to tell from those numbers. Plants do not see light the way we do, and most boxes speak to our eyes, not to their leaves.

Here is what really matters, in five numbers, then how to read them on real product pages. If you are looking for which device to buy instead, start with our comparison of smart indoor gardens.

Why watts and lumens say almost nothing

Watts tell you how much electricity the light uses, not how much light it produces. Two 24 W lights can perform very differently depending on the quality of their LEDs.

Lumens and lux measure light as perceived by the human eye, which is very sensitive to green and yellow. University of Missouri Extension's guide to grow lights is blunt: these units represent light that human eyes perceive, not the light plants use to grow. It advises avoiding lights that only list lumens, lux, candelas or a colour temperature in kelvins, and not buying a light if the manufacturer provides neither a spectrum graph nor PPF or PPFD values.

As for "equivalent watts," they compare an LED with an old incandescent bulb, a notion that means nothing to a plant. Case in point: a 15 W grow bulb sold at Home Depot in the United States claims "200-Watt Equivalent" in its title, and "250-Watt Equivalence" in its description. For the difference between LEDs and incandescent bulbs around the house, see our comparison of LED vs incandescent bulbs.

The 5 numbers that matter

For a gardener, the most useful one is the DLI. Purdue Extension compares it to a rain gauge: instead of measuring intensity at one moment, it adds up all the useful light received over the day. So it depends on two things you control: intensity, set by the distance between the light and the leaves, and duration, set by your timer.

How much light for herbs and lettuce

For lettuce, the University of Missouri guide gives an intensity of 250 to 350 µmol/m²/s, and its worked example targets a DLI of 16 moles per square metre per day.

For basil, trials by Kellie Walters and Roberto Lopez at Michigan State University are enlightening. They grew basil, sage and mint hydroponically at DLIs of about 5 to 19 moles a day: basil's branch number increased with light, from 5.5 to 13.2 moles a day, and so did its fresh mass. The researchers note that, according to other work, basil keeps benefiting from light well beyond 11 moles a day.

At home, then, a reasonable benchmark for herbs is at least 10 moles a day. Here is what a few intensities give, depending on how long the light is on:

A modest light left on for a long time can therefore deliver as much as a powerful light on for a shorter time. But not indefinitely: Iowa State University recommends 12 to 16 hours a day, not 24.

Decoding a real product page

We went through a few product pages for lights sold in Canada and the United States on October 21, 2026. The lessons are instructive.

The bulb that publishes its numbers

The 15 W SANSI grow bulb (PAR25 format, E26 base) sold at Home Depot in the United States at least has the merit of publishing a PPF of 26.1 µmol/s and a PPFD of 152.83 µmol/m²/s at 30 cm (1 ft). Let us run the numbers:

• Efficiency: 26.1 µmol/s ÷ 15 W = 1.74 µmol/J. That is below the 2.5 µmol/J benchmark for current LEDs cited by the University of Missouri.
• DLI: 152.83 µmol/m²/s × 16 h × 0.0036 = 8.8 mol a day, and only directly under the centre of the bulb, where the light is strongest. Toward the edges of the lit area, it will be less.

Bottom line: fine for one or two pots placed right underneath, not for a full tray. And forget the "200 W equivalent" in the title.

The bulb that contradicts itself

On the same platform, a 9 W GT-Lite bulb shows "12.85 PPF" in its title and a PAR/PPF of 6 in its specifications table, with no unit in either case. When two numbers on the same page do not match, ask the manufacturer for its spec sheet before buying.

The bar that promises without numbers

The 24-inch SunBlaster LED bar, a brand widely sold in Canadian garden centres, is described by its distributor Hydrofarm with its 24 W, 6400K, 3,400 lumens and "Improved PAR, PPF & PPFD," but without a single value. At a Quebec garden centre, the page for a 24-inch model lists 24 W, 6400K, 2,550 lumens and a 50,000-hour lifespan, but no PPF either. These bars may well grow seedlings and herbs; it is just impossible to compare them on paper with a light that publishes its numbers.

Spectrum: white, "blurple" and kelvins

Colour temperature in kelvins (3000K, 4000K, 6400K) describes the hue of the light to the eye: warm or cool. It says nothing about the amount of light useful to plants, hence the University of Missouri's advice to be wary of lights that list only that.

For herbs in a kitchen or living room, a broad-spectrum white LED does the job and has a practical advantage: under white light, you see the true colour of the leaves, and therefore any yellowing, spots or insects. Red and blue lights, often called "blurple," give off a purple glow that makes that inspection harder. Either way, ask for the spectrum graph: it is one of the two pieces of information the Missouri guide recommends getting before you buy.

Distance, duration and timer

PPFD depends on distance: the closer the light is to the plants, the higher the intensity, the University of Missouri points out, and good manufacturers provide a PPFD map for several heights. In practice, Iowa State recommends placing the light 15 to 30 cm (6 to 12 in) from the leaves and plugging it into a timer for 12 to 16 hours a day.

Two simple habits:

• Raise the light as the plants grow, to keep the same distance from the top of the leaves.
• Group your pots under the light rather than spreading them out: light that falls beside the plants is wasted.

And since electricity costs money, we calculated what each light costs for a full winter in what a winter of herbs under LEDs costs. Spoiler: much less than you think.

The DLC list: a shortcut, mostly for professionals

In North America, the DesignLights Consortium (DLC) keeps a list of qualified horticultural fixtures, used mostly by greenhouses and vertical farms. Products are tested to the ANSI/ASABE S640 standard for their PPF and efficiency, and must meet the UL 8800 safety standard. The minimum efficiency threshold was 1.9 µmol/J in 2022, according to Professor Erik Runkle of Michigan State University; version 3.0 of the DLC requirements, slated for 2023, raised it by 21%, which works out to about 2.3 µmol/J.

Countertop lights and bulbs rarely make that list. But if a model is on it, its numbers have been verified.

Your checklist before buying

1. Does the manufacturer publish the PPF in µmol/s? If not, be wary.
2. Is there a PPFD map at different heights, for the area you want to light?
3. Calculate the efficiency: PPF ÷ watts. Over 2.5 µmol/J is a good current LED.
4. Calculate the DLI you will get: PPFD × hours × 0.0036. Aim for at least 10 for herbs and around 16 for lettuce.
5. Insist on a spectrum graph, and prefer white light in a living space.
6. Ignore "equivalent watts," and do not rely on lumens or kelvins to judge growth.

To build your own system around a good light, see our DIY hydroponic system for under $100, and for the care of plants brought in from the garden, which herbs survive indoors.

Sources

• Understanding Grow Lights (G6987), Juan Cabrera-Garcia and Matt Ernst, University of Missouri Extension, 2025.
• Measuring Daily Light Integral in a Greenhouse (HO-238-W), Ariana P. Torres and Roberto G. Lopez, Purdue Extension.
• Modeling purple basil, sage, spearmint, and sweet basil responses to daily light integral and mean daily temperature, Kellie J. Walters, Sean Tarr and Roberto G. Lopez, PLoS ONE, 2023.
• Culinary Herbs: Balancing Light and Average Daily Temperature, Kellie Walters and Roberto Lopez, Produce Grower, 2021.
• How can I provide more light for my herbs indoors?, Aaron Steil, Iowa State University Extension, 2024.
• A Closer Look at LED Efficacy, Erik Runkle, Michigan State University, GPN, 2022.
• New horticultural lighting technical requirements launched, HortiDaily, 2022.
• Horticultural Lighting Testing and Certification, UL Solutions, accessed October 21, 2026.
• SANSI 15-Watt PAR25 Full Spectrum LED Grow Light Bulb (product page), The Home Depot, accessed October 21, 2026.
• GT-Lite 9-Watt LED Grow Light Bulb (product page), The Home Depot, accessed October 21, 2026.
• SunBlaster Prism Lens LED Strip Light, 24", 6400K 24W (product page), Hydrofarm, accessed October 21, 2026.
• Barre LED SunBlaster 24 po, 24 W, 6400 K (product page, in French), Pépinière.ca, accessed October 21, 2026.