Grow Lights for Herbs: How Much Light, How Close and What It Costs
Even a sunny window in winter can give herbs only about half the daily light they need, and a small white LED fills the gap cheaply.
Herbs run on the total light they get in a day, and a winter window often doesn’t supply enough of it. When researchers in South Korea measured a south-facing window through a full year, it delivered 7 to 8 units of daily light from September to December. Iowa State University puts basil and other leafy herbs at 12 to 16.
The fix is simple to buy. For a small tray of pots, a white LED of about 20 W, hung about 30 cm (12 inches) above the leaves on a 12 to 14 hour timer, covers the shortfall for under 10 kWh a month. If your window is dim, plan on the lamp doing all the work, which takes about 30 W for the same tray. The numbers behind that are below. Our windowsill herbs guide covers pots, watering and which herbs to start with.
What does a herb actually need?
A daily total of light, which growers call the daily light integral, or DLI. Iowa State’s horticulturists say it “basically tells you how much photosynthesis can occur.”
DLI is measured in moles of usable light per square metre per day (mol/m²/day). It comes from two numbers, the light hitting the leaves each second (PPFD, in micromoles per square metre per second) and the hours it’s on. The formula is PPFD × hours × 0.0036.
The same guide sorts indoor plants into light bands. Herbs and leafy vegetables such as basil and lettuce sit in its high band, 12 to 16. It also lists herbs among the plants in its very high band, 18 to 30, so 12 to 16 works as a sensible minimum and more light still helps. A 2025 Virginia Tech extension guide giving general recommendations breaks it down by crop.
| Crop | DLI (mol/m²/day), Virginia Tech |
|---|---|
| Parsley | 10 to 15 |
| Cilantro | 15 to 20 |
| Basil | 15 to 25 |
Why does a window fall short in winter?
Short days, a low sun and cloud. Iowa State lists all three as the reasons indoor light is “particularly low in the winter.” Even outdoors in open ground, Virginia Tech gives Virginia’s average DLI as 40 to 45 in July and only 15 to 20 in December.
The 2017 peer-reviewed study from South Korea shows what reaches the inside of a window. Researchers at a national horticulture institute logged light 10 cm inside balcony windows facing east, south and west through 2016. Outdoors averaged 13 in December, while the south-facing balcony got 7 to 8 from September to December. The east and west balconies got less again, 10 to 20% of outdoor light.
That’s one test building in Jeonju, so treat it as a guide to scale. In the Southern Hemisphere, read north-facing for south-facing and swap the seasons.
How much light should a lamp add?
Enough to cover the gap. If the window gives about 7 and the herbs want 13 to 15, the middle of Iowa State’s 12 to 16, the lamp needs to add 6 to 8 mol/m²/day. Spread over 14 hours, the formula turns that into a PPFD at the leaves of about 120 to 160. A lamp working alone needs about 240 to 320 to reach 12 to 16 in the same hours.
Iowa State’s best run is 12 to 14 hours a day, with a minimum of 10 and a maximum of 16. A dimmer light run longer gives the same daily total as a brighter one run for less time.
The plants tell you whether it’s working. The same team’s adjustment table reads stretched, spindly or yellowing growth as too little light, so move the lamp closer. Scorched or bleached leaves and extremely compact growth mean too much, so raise it. Steady, even growth in between is the target.
Are white LEDs good enough for herbs?
Yes. Many LED grow lights glow pink because they emit mostly red and blue, the colours plants use most. A 2020 peer-reviewed review of LED lighting adds that “photobiology is thus driven by the number of photons, not energy or lumens,” and that green photons are “up to 10% less photosynthetically efficient” than the most common red LED.
White LEDs still deliver blue and red along with green, and the review notes they now make up more than 60% of some horticultural fixtures. For ordinary fixtures, Iowa State recommends full-spectrum, sometimes called “daylight”.
Why LEDs beat fluorescent tubes
They turn more electricity into usable light, measured in micromoles per joule (µmol/J). In a 2014 peer-reviewed test, the best fluorescent fixture reached 0.95 µmol/J and the best LEDs about 1.7. The 2020 review puts typical horticultural LED fixtures at 2 to 3 µmol/J as of 2020. Cheap home lamps may fall below that range, so their real draw could be higher than our example below.
Distance follows from the technology. The same extension’s lighting series puts fluorescent tubes within 15 to 30 cm, because their light “drops off sharply” with distance and fades as tubes age, while most LEDs can sit 30 to 60 cm away. If the box lists PPF in µmol/s, divide it by the lit area in square metres for a rough upper estimate of PPFD, since some light always misses the plants.
What does it cost to run?
For one tray, not much. The energy formula is watts × hours a day × days ÷ 1,000 = kilowatt hours (kWh).
To size the lamp, Iowa State’s fixture formula allows for about 20% of the light missing the plants. By that formula, a 50 × 50 cm tray topped up by 160 PPFD needs a fixture putting out about 50 µmol/s.
- LED at 2.5 µmol/J, the middle of the 2020 range, draws about 20 W. Run 14 hours a day for 30 days, that’s 20 × 14 × 30 ÷ 1,000 = 8.4 kWh a month.
- Fluorescent at 0.95 µmol/J, the best in the 2014 test, needs about 53 W for the same light, or about 22 kWh a month.
- LED working alone at 240 PPFD draws about 30 W, or 12.6 kWh a month.
Electricity prices vary by country and plan, so use your own rate. At an illustrative 30 cents per kWh, the LED top-up costs about 2.50 a month and the lamp-alone setup about 3.80.
What this means for you
- Start with the simple spec. A white LED of about 20 W, about 30 cm above the leaves, on a 12 to 14 hour timer, for a small tray.
- Size up for a dim window. Plan on about 30 W and let the lamp do all the work.
- Skip fluorescent tubes. An LED uses about half the power for the same light, or less with a modern fixture.
- Watch the stems. Stretching means add light, bleached leaves mean cut it back.
- Work out your cost as watts × hours × days ÷ 1,000, times your rate.
Which herb struggles most for you in winter, and is it sitting on a windowsill or under a lamp? Hit reply and tell us.
How we made this: AI tools helped us research and draft this post. We checked every figure, quote and link against its original source before publishing. Figures are as of 8 November 2026.
Grown with intention by Dynamic Nexus Garden.