How Many Watts of LED Grow Light Do Desktop Plants Need?
Desktop growing is the most common question we get from home growers: “How many watts do I actually need on my desk?” It is also the question most often answered badly, because wattage is a measure of electricity, not of light.
This page gives you the wattage shortcut so you can size a fixture in about thirty seconds. It also explains when the shortcut breaks down, and what to measure instead.
The quick answer
| Plant type | Watts per square foot (LED) |
|---|---|
| Low-light houseplants (pothos, snake plant) | 10–15W |
| Herbs (basil, mint, parsley) | 15–20W |
| Leafy greens, microgreens | 20–30W |
| Compact fruiting (peppers, cherry tomato) | 30–40W |
For a typical 2 ft × 1.5 ft desktop shelf (3 sq ft) of herbs, you want roughly 45–60W of efficient LED light.
Notice that the table is expressed per square foot of canopy, not per plant and not per pot. That distinction is where most sizing errors come from, and it is covered below.
Why watts alone is not the whole story
Efficiency matters more than raw watts. A 20W fixture at 2.2 µmol/J delivers more usable photons than an old 30W fixture at 1.0 µmol/J. Check the µmol/J (efficacy) figure, not just the wattage.
This matters more than it did five years ago. The cheapest thing on the market is almost always the least efficient thing on the market, so a naive “buy the biggest wattage you can afford” strategy now buys the fewest photons per dollar. Within the modern range, fixtures cluster roughly between 1.8 and 2.8 µmol/J — and across that spread, the same watts can differ by more than 50% in delivered light.
The wattage table above assumes a modern, efficient panel. If you are comparing a new fixture against a bulb you already own, do the maths in photons, not watts.
Measure your actual growing area
Before you use the table, measure the space your leaves will occupy when the plants are established — not the space your pots occupy today.
- Single pot on a desk: roughly 0.5–1 sq ft of canopy once a bushy plant is mature. At 15–20W per sq ft for herbs, that is 10–20W.
- Shelf or windowsill tray: measure the tray. A standard 10 × 20 inch propagation tray is 1.4 sq ft, which lands in the microgreens band.
- Shelf with several pots: add the gaps. Two pots 12 inches apart cover more floor than their pot footprints suggest, because light spills sideways and a plant with a bare lower stem is a plant you paid for twice.
If you prefer metric, 1 sq ft is 0.093 m². A 2 ft × 1.5 ft shelf is 0.28 m². Our desk and small-space setup guide works through the same decisions with diagrams rather than tables.
Distance matters
Desktop lights are usually close to the canopy:
- 10–20W fixtures: keep 6–12 inches from leaves
- 20–40W fixtures: keep 12–18 inches
If leaves curl or bleach, raise the light or turn it down — a dimmer saves you both.
Height is the free variable most growers ignore. A fixture that is technically undersized but mounted 18 inches above a small canopy will out-deliver an oversized fixture mounted 4 inches from the leaves, because the close one is cooking the top growth while the lower leaves starve. Fix the distance before you decide to buy something bigger.
Five sizing mistakes that cost people money
Sizing to the pot instead of the canopy. A 3-inch nursery pot is about 0.05 sq ft. A mature basil in that pot spreads to roughly 1 sq ft of leaf. Size for the second number.
Ignoring the gap between plants. Three pots on a 2 ft shelf are not 3 × pot width. Light falls off toward the edges of a fixture, so the plant in the middle gets more than the plant at the end.
Buying a fixed-output light for a mixed shelf. A seedling stage and a fruiting stage do not want the same intensity. If one fixture has to serve both, buy dimmable — this is the single most useful feature on a small domestic setup.
Running the light all day. Plants need a dark period, and past roughly 16–18 hours you are adding electricity without adding yield. This is worth understanding properly rather than by habit; the PPFD and DLI calculator shows how intensity and hours trade off against each other for a given daily total.
Trusting the wattage sticker on an old fixture. A five-year-old LED bar at 1.0 µmol/J delivering 30W is worth less light than a new 20W panel. Efficiency moved a lot; the sticker did not.
What it actually costs to run
Electricity is rarely the deciding factor, but it is worth knowing, and it changes behaviour: growers who know the running cost dim sensibly instead of leaving the fixture pinned at maximum all day. The figures below assume $0.15 per kWh and a 16-hour photoperiod.
| Fixture | Daily use at 16h | Yearly use | Yearly cost |
|---|---|---|---|
| 20W panel | 0.32 kWh | 117 kWh | ~$18 |
| 45W panel | 0.72 kWh | 263 kWh | ~$39 |
| 60W panel | 0.96 kWh | 350 kWh | ~$53 |
The spread between a small and a large desktop fixture is a few tens of dollars a year. Compared to the plant’s own value, that is rarely the reason to choose. Choose for the canopy instead.
Our recommendation
For most desks, a 20–30W dimmable full-spectrum panel is the sweet spot: bright enough for herbs, dim enough for low-light plants.
If you need more reach, our 45W–50W panel covers a 2 ft × 1.5 ft shelf, and the dimmable spider bar suits a wider tray where you want the light spread rather than the intensity. The quantum board is the option when you move past a shelf into a small cabinet or closet.
If you are still torn between two sizes, send us the shelf dimensions and what you want to grow and we will tell you which one — including when the answer is the smaller one. Ask us to size a light for your shelf
Related reading
- Desk and small-space grow light setup — the physical setup, mounting, and timer choices
- PPFD and DLI calculator — work out intensity and photoperiod instead of guessing at watts
- How blue light affects indoor planting — what to look for on a spectrum label
- The role of UV in grow lights — whether the UV diode is worth paying for
FAQ
How many watts for a single herb pot?
A single pot of basil, mint, or parsley needs roughly 10–20W of efficient LED light once the plant is established. Below about 10W, expect a thin, stretched plant that leans hard toward the window.
Is 20W enough to grow basil indoors?
Yes, for one or two pots. A 20W panel at 12 inches above the leaves will sustain a small basil plant indefinitely. For a full 2 ft × 1.5 ft shelf of herbs you want 45–60W. Our indoor basil guide covers the longer-term setup in detail.
Do grow lights need to be on 24 hours a day?
No. 16–18 hours of light followed by a genuine dark period is the productive range for most indoor plants. Running 24 hours adds electricity without adding yield, and continuous light interrupts the flowering signal that red and far-red content control.
How far above my plants should the light be?
12–18 inches for a 20–40W fixture, 6–12 inches for a 10–20W fixture. Raise it if leaves curl or bleach at the edges, which is light stress rather than a sign you need more light.
Is a higher wattage always better?
No, and often it is worse. An oversized fixture is harder to dim to the right level, wastes electricity, and is more likely to cause heat stress. For a mixed shelf, a smaller dimmable fixture usually beats a larger fixed-output one.