Short answer: for a 4-foot shelf about 18 inches deep, use two 4-foot LED strips drawing a total of roughly 50 to 80 actual watts, on a timer for 14 to 16 hours a day. Hang them as the maker recommends and raise them as seedlings grow.
How much light seedlings need
Published targets for seedlings are lower than for mature crops, at about 6 to 12 mol/m²/day. Those figures come from secondary sources and vary. Over 16 hours, that works out to about 105 to 210 PPFD.
| Target PPFD | DLI at 16 hours | Approx. actual watts | Typical setup |
|---|---|---|---|
| 120 | 6.9 | About 50 W | Two modest LED strips |
| 160 | 9.2 | About 65 W | Two standard LED strips |
| 200 | 11.5 | About 80 W | Two stronger strips, or three standard ones |
How many strips?
Two strips spread across an 18-inch-deep shelf give more even light than one bright strip in the center. On a shelf 24 inches deep, use three. Strips should reach most of the way along the shelf, since light falls off at the ends. Put your most light-hungry trays in the middle.
How high should the light hang?
Many seed-starting guides recommend 2 to 4 inches above the seedlings. That advice was mostly written for fluorescent tubes, which are weak but cool. Stronger LEDs may need more distance, so the maker's instructions come first.
- Too far: stems stretch and lean toward the light.
- Too close: leaves turn pale or crisp at the tips.
- Use adjustable chains or hooks so you can raise the light as seedlings grow.
What it costs to run
Two 40-watt strips running 16 hours a day use about 38 kWh a month. At the EIA's forecast 2026 average of 18.2 cents per kWh, that's about $7 a month, and most seed-starting seasons last 6 to 10 weeks. Run your own numbers in the cost calculator.
What to look for
4-foot linkable LED strips
Look for actual watts, a plug-in link cable so one timer runs them all, and a daylight color of about 5000–6500K if you're using shop lights. See 4-foot LED strips on Amazon.
Plain LED shop lights
A common, lower-cost choice for seedlings. They rarely publish PPFD, so rely on actual watts and keep them close. See 4-foot shop lights on Amazon.
Hanging hardware and a timer
Adjustable hangers make raising lights easy. See adjustable hangers on Amazon.
Going further
If you're fitting out a whole rack, read our guide to a seed starting rack for a small apartment. To work out when to sow, use the seed starting calendar.
