How many watts of LED do I need per plant?

Bad question, but here's the workable answer — 20–30W of efficient LED (2.5+ µmol/J) per square foot of canopy for leafy greens; 35–50W/ft² for fruiting crops. Plant count matters less than canopy area.

Is 1000W HPS the same as 1000W LED?

In wattage, yes. In usable photons, no. A modern LED at 2.7 µmol/J delivers ~50% more PAR than a 1000W HPS at ~1.7 µmol/J. The same wattage of LED replaces 1.5x the HPS.

Do I need full-spectrum or can I just use shop lights?

Cheap 5000K shop LEDs work for lettuce and herbs up to ~12 inches if you cluster enough of them. Below ~150 PPFD nothing finishes properly. Use the watts/area math, not the bulb count.

My lamp says 300W on the box but draws 150W from the wall. Which is right?

150W. Most LED lamps are labeled with "equivalent" or peak diode wattage, not actual draw. Use a Kill-A-Watt meter or check the spec sheet for "input power" — that's the real number.

DOC №182SEC: LIGHTINGREV: 2026-05-19AI ASSISTED

How Many Watts of Grow Light Per Plant? A Sizing Guide That Uses Real Numbers

Wattage is the wrong unit, but everyone shops by it. Here's how to translate watts into PPFD, DLI, and actual photons that grow plants — for lettuce, basil, tomato, and more.

BY ROOTLESS FARM

Quick answer

Forget the wattage on the box. The number that grows plants is µmol/J (efficacy) — how many usable photons a lamp emits per watt drawn. A 100W lamp at 2.7 µmol/J delivers 270 µmol/s of PAR; a 100W lamp at 1.2 µmol/J delivers 120 µmol/s. Same wattage, more than 2× the photosynthesis.

The right sizing rule, in plain English: 20–30W of LED at ≥ 2.5 µmol/J per square foot of canopy covers leafy greens. 35–50W/ft² covers fruiting crops. For DLI and spectrum, see the linked pillars.

Why "watts per plant" is the wrong unit

A 12-inch lettuce head and a 5-foot tomato plant both count as "one plant" but need 5× different total photons. Hydroponic lighting scales with canopy area, not headcount. A 2×4 ft tent fits 8 lettuces or 4 basil or 2 tomatoes — same area, same lamp.

So the operational question is: how many watts of efficient LED do I need per square foot of canopy?

The math, briefly

The chain runs: watts → µmol/s of PAR → PPFD over canopy area → DLI for the photoperiod.

Watts (input power): what your wall meter reads. The number on the box is often "equivalent wattage." Use the spec sheet.
Efficacy (µmol/J): lamp photons emitted per joule of electricity. Modern LEDs: 2.3–3.0 µmol/J. Old HPS: 1.5–1.8. Cheap blurple LEDs: 1.0–1.5.
PPF (µmol/s): watts × efficacy. A 100W lamp at 2.7 µmol/J = 270 µmol/s.
PPFD (µmol/m²/s): PPF divided by the lit canopy area in m². A 270 µmol/s lamp over 0.4 m² (about 4 ft²) ≈ 675 µmol/m²/s before losses.
DLI (mol/m²/day): PPFD × seconds of photoperiod ÷ 1,000,000.

The DLI calculator does the arithmetic. Below is the shortcut for shoppers.

Wattage targets by crop (using ≥ 2.5 µmol/J LEDs)

Crop	Target PPFD (µmol/m²/s)	Photoperiod	DLI (mol/m²/day)	Watts per ft² (LED)
Microgreens	150–250	16h	9–14	12–18
Lettuce, leafy greens	200–300	16h	12–17	20–25
Basil, herbs	300–400	16h	17–23	25–32
Strawberry	350–500	14h	18–25	30–38
Cucumber	400–600	14h	20–30	35–45
Tomato (veg)	400–500	16h	23–29	32–40
Tomato (fruiting)	600–900	12h	26–39	45–55
Pepper	500–700	14h	25–35	40–50

These assume:

Modern LED at 2.5–2.7 µmol/J efficacy.
~20% loss from canopy edges and reflectance.
Canopy 18–24 inches from the lamp.

For old HPS or low-efficacy LEDs, multiply watts/ft² by 1.5–1.8.

Worked examples

Single butterhead lettuce on a kitchen counter.

Canopy area: ~0.5 ft².
Target: 250 PPFD × 16h = 14 DLI.
LED needed: 0.5 ft² × 22 W/ft² = 11 W.
Buy: a 20W desk grow light at 2.5 µmol/J. Done. Total cost $25.

2×2 ft tent of mixed lettuce + herbs.

Canopy area: 4 ft².
Target: 250–350 PPFD × 16h = 14–20 DLI.
LED needed: 4 ft² × 25 W/ft² = 100 W actual draw.
Buy: a 100W quantum-board LED at 2.7 µmol/J (Mars Hydro TS1000, Spider Farmer SF1000 class).

2×4 ft tent for 2 tomato plants (fruiting).

Canopy area: 8 ft².
Target: 700 PPFD × 12h = 30 DLI.
LED needed: 8 ft² × 50 W/ft² = 400 W actual draw.
Buy: a 400W LED bar fixture at 2.7+ µmol/J (Spider Farmer SF4000, ViparSpectra XS4000 class).

What efficacy actually buys you

Two lamps draw 200W. Lamp A is 1.5 µmol/J. Lamp B is 2.7 µmol/J.

Lamp A: 300 µmol/s. Covers ~3 ft² of leafy greens.
Lamp B: 540 µmol/s. Covers ~6 ft² of leafy greens, or 3 ft² of fruiting crops.

Same wattage, same heat, same electricity bill. The cheaper lamp is the more expensive lamp once you account for missed harvests.

A practical efficacy floor: don't buy below 2.3 µmol/J in 2026. Anything below is either old stock or fraudulent labeling.

How to verify efficacy

Spec sheet first. Reputable brands publish PPF (µmol/s), input wattage, and µmol/J. If the brand only shows "watts equivalent" or "lumens," it's a consumer lamp, not a grow lamp.
Third-party tests. Migro, GrowLightMeter.com, and PARsource publish independent measurements. Cross-reference before purchase.
Suspicious gaps. A 400W lamp listed at 4 µmol/J would be world-record efficacy and is almost certainly fake.
PAR map. A good brand publishes a PPFD heatmap over the intended coverage area at multiple hanging heights. No heatmap = no data.

Why HPS still works (and when LED is wrong)

HPS at ~1.7 µmol/J underperforms modern LED per watt. But:

HPS pumps more far-red and IR — useful for late-flower fruiting crops.
HPS heats the canopy, which can be a feature in cold rooms.
The capital cost per watt is lower (an old 600W HPS + ballast is $80 used).

For a winter tomato grow in an unheated garage, a 600W HPS may still be the right call. For everything else in 2026, LED wins.

Common mistakes

Hanging the lamp too high to "spread the light." PPFD drops with the square of distance. A lamp at 36" delivers ~half the PPFD of the same lamp at 24". Measure before assuming.
Buying total wattage but no PPFD map. "500W of LED" over 16 ft² can mean 400 PPFD or 250 PPFD depending on optics. Demand the PAR map.
Confusing "equivalent wattage" with actual draw. A "1000W LED" that pulls 200W from the wall is a 200W lamp. Period.
Underbuying for fruiting crops. Tomatoes and peppers eat photons. A lamp sized for lettuce gives you tomatoes the size of marbles.

Where to go next

If you're picking a specific lamp, see how to choose a grow light. For the dose math, read PPFD and DLI explained, then plug your numbers into the DLI calculator. For spectrum, see light spectrum for hydroponics.

FAQ

4 entries

Q01How many watts of LED do I need per plant?: Bad question, but here's the workable answer — 20–30W of efficient LED (2.5+ µmol/J) per square foot of canopy for leafy greens; 35–50W/ft² for fruiting crops. Plant count matters less than canopy area.
Q02Is 1000W HPS the same as 1000W LED?: In wattage, yes. In usable photons, no. A modern LED at 2.7 µmol/J delivers ~50% more PAR than a 1000W HPS at ~1.7 µmol/J. The same wattage of LED replaces 1.5x the HPS.
Q03Do I need full-spectrum or can I just use shop lights?: Cheap 5000K shop LEDs work for lettuce and herbs up to ~12 inches if you cluster enough of them. Below ~150 PPFD nothing finishes properly. Use the watts/area math, not the bulb count.
Q04My lamp says 300W on the box but draws 150W from the wall. Which is right?: 150W. Most LED lamps are labeled with "equivalent" or peak diode wattage, not actual draw. Use a Kill-A-Watt meter or check the spec sheet for "input power" — that's the real number.

FAQ

Read next