The number of LED grow lights you need depends on the size of your growing area, the crop and how intense the light has to be. Wattage alone is a poor guide. A better method is to choose a light intensity for your crop, then pick enough fixtures to deliver it evenly across your space.
Why watts alone do not answer the question
Two lights that both say 400 watts can grow very differently. Diode efficiency, the driver and the way the diodes are spread decide how much plant light actually reaches your plants. A light with efficient diodes and good optics at 300 watts may out-perform a poor 400-watt light. Several makers say the same thing: use measured light output at plant level, not nameplate wattage, as the test.
The three numbers that matter
- Growing area. The footprint you want to light, for example a 2 x 4 foot shelf or a 4 x 4 foot tent.
- Target PPFD. Photosynthetic photon flux density, in µmol/m²/s, is the amount of plant light landing on the canopy at one moment. Read what PPFD is.
- Photoperiod. How many hours a day the light runs, which together with PPFD sets your daily light integral. See daily light integral explained.
Step 1: Choose a target for your crop
Different plants want different light. These ranges are commonly used starting points. Published guidance varies, so treat them as a guide and confirm against your light maker's instructions:
| Plant stage or crop | Typical PPFD at the canopy (µmol/m²/s) |
|---|---|
| Seedlings and cuttings | About 200 to 400, often less at first |
| Leafy greens and herbs | About 150 to 300 (lettuce studies favor roughly 200 to 250) |
| Vegetative growth of fruiting plants | About 400 to 600 |
| Flowering and fruiting plants such as tomatoes and peppers | About 600 to 900 |
Step 2: Measure your space
Measure the length and width of the area you want to light, in feet. Light intensity drops toward the edges of a fixture's footprint, so the area that gets even light is smaller than the space under the fixture. Plan for the plants to sit inside the evenly lit part.
Step 3: Match a fixture to the footprint
Good makers publish a PPFD map: a grid showing how much light lands at different spots at a stated height. Compare the map with your target. For example, if a map shows 600 to 800 µmol/m²/s across a 3 x 3 foot square at 12 inches, it suits a 3 x 3 foot tent of fruiting plants. If your plants are leafy greens that want 200 to 250, the same light dimmed down, or hung higher, will cover a larger area.
Vendors suggest rough starting points for flowering plants: a 3 x 3 foot tent needs about 200 to 300 watts of true LED power, and a 4 x 4 foot tent somewhere between roughly 300 and 700 watts, depending on how efficient the light is. The wide range for the 4 x 4 foot tent shows why wattage is only a first guess. Always confirm with a PPFD map.
Step 4: One big light or several smaller ones?
For larger spaces several smaller fixtures usually spread light more evenly than one big one, and you do not lose everything if one fails. For small spaces one fixture is simplest. Whichever you choose, the aim is the same: even light across the whole canopy.
A worked example
Suppose you want to grow leafy greens on a 2 x 4 foot shelf (8 square feet).
- Target: about 200 µmol/m²/s for leafy greens.
- Photoperiod: 16 hours. Daily light integral = 200 × 16 × 3,600 ÷ 1,000,000 = 11.5 mol/m²/day, a level that research on hydroponic lettuce found worked well.
- Footprint: a fixture whose PPFD map shows about 200 across 2 x 4 feet at your hanging height, or a stronger light dimmed to match.
Now suppose you want tomatoes in a 4 x 4 foot tent. You need a much higher intensity, about 600 or more, and probably more than one fixture.
Check your results and adjust
- A light meter or PPFD app can verify intensity, although phone apps are rough estimates.
- Look at the plants: stretching and pale leaves mean too little light, while bleached or curled leaves close to the light mean too much.
- Use the dimmer to fine-tune. See how far to hang LED grow lights from plants.
Plan for electricity too
More lights mean more power. Check your outlet capacity: amps equal watts divided by volts, so 1,000 watts on 120 volts draws about 8.3 amps, and on 240 volts about 4.2 amps. See whether LED grow lights save electricity for the cost arithmetic.
Examples by space
These are illustrative, not prescriptions. Always confirm with the maker's PPFD map for your fixture.
| Space | Crop | What to aim for |
|---|---|---|
| A 2 x 4 foot shelf | Leafy greens or herbs | About 200 µmol/m²/s across the shelf for 14 to 16 hours |
| A 2 x 2 foot tent | Seedlings, then greens | Gentle at first, then about 200 to 400; a small dimmable light often covers it |
| A 3 x 3 foot tent | Tomatoes or peppers | About 600 or more at flowering across the whole canopy |
| A 4 x 4 foot tent | Several fruiting plants | About 600 or more across the canopy, which may need more than one fixture |
Natural light and supplemental lighting
If your plants also get window light, the grow light only has to supply the difference. A south-facing window adds a good share of light on sunny days, but only through the middle of the day, and it falls sharply in winter. Plan on the grow light doing most of the work in a tent and a smaller share on a windowsill.
Common sizing mistakes
- Buying by watts alone. Use the PPFD map.
- Ignoring the edges. The evenly lit area is smaller than the fixture's shadow.
- Sizing for the largest crop. A light that suits tomatoes can be dimmed for seedlings, so size for the hungriest crop and use the dimmer.
- Forgetting the walls. A tent with reflective walls wastes less light than an open shelf. See grow tent lighting setup.
- Underestimating growth. Plants grow toward the light and shrink the space between them, so plan for the mature plant, not the seedling.
Questions growers ask about sizing
Can one big light replace two small ones?
Sometimes, but two smaller lights usually spread light more evenly and give you a backup. For a large space the even spread matters more than the number.
Can I run a light dimmed instead of buying a smaller one?
Yes, and it is a good way to size. A light that is somewhat stronger than you need, dimmed, can cover the space evenly and then give you room to grow into fruiting plants.
Key takeaways
- Choose a target PPFD for the crop, then pick enough light to deliver it evenly across your space.
- Wattage is only a rough guide; use the maker's PPFD map and look at the lowest readings, not the peak.
- Seedlings and leafy greens want much less light than flowering and fruiting plants.
- Several smaller lights spread light more evenly than one large one in a big space.
- Check that your circuit can carry the load: amps equal watts divided by volts.
Where My LED grow lights fit
My LED Grow Lights run on 120v or 240v, detect the voltage automatically and include a dimmer switch, so one fixture can be set to the intensity that suits your plants. See all of our LED grow lights.