The best LED grow light for indoor growing has a full spectrum, an efficiency of at least 2.5 µmol/J and a dimmer. For flowering, allow around 300–400 real watts (drawn from the socket) per m² of grow tent: 100–150 W for a 60x60, 180–250 W for an 80x80 and 400–600 W for a 120x120. Ignore the "equivalent watts" printed on the box and compare PPF, efficiency and the PPFD map instead.
In brief
- Only the real power draw counts: a "1000 W" light that draws 100 W is a 100 W light.
- Efficiency = PPF ÷ real watts; below 2.2 µmol/J a light is inefficient, from 2.5 µmol/J upwards it is good.
- One full-spectrum LED with a dimmer covers seedlings, the vegetative stage and flowering.
- For the same amount of light, an LED uses roughly 30–50% less electricity than HPS and puts less heat on the buds.
- Quality diodes are rated L90 at 50,000 hours or more (they keep 90% of their output); a serious warranty runs from 3 to 5 years.
How many watts: real power and watts per m²
Many budget lights carry an inflated figure ("600 W", "1000 W") that is only the theoretical sum of the diodes or a comparison with old lamps. Look on the spec sheet for "power consumption" or "actual power draw": that is what you pay for on your bill and what produces the light. The table applies to flowering with an LED of about 2.7 µmol/J; in the vegetative stage about 50–75% of the power is enough, set with the dimmer.
| Grow tent | Area | Real LED watts (flowering) | Indicative PPF |
|---|---|---|---|
| 60x60 cm | 0.36 m² | 100–150 W | 270–400 µmol/s |
| 80x80 cm | 0.64 m² | 180–250 W | 480–680 µmol/s |
| 100x100 cm | 1.0 m² | 300–400 W | 810–1,080 µmol/s |
| 120x120 cm | 1.44 m² | 400–600 W | 1,080–1,620 µmol/s |
With these figures, in a tent with reflective walls, the canopy receives an average of about 600–900 µmol/m²/s, the recommended range for flowering without added CO2. A less efficient light needs more watts for the same light; a more efficient one needs fewer. For tent size, see the guide on choosing a grow tent.
PPF, PPFD and efficiency: reading the spec sheet
- PPF (µmol/s): the total amount of plant-usable light the fixture emits.
- PPFD (µmol/m²/s): how much light reaches one point of the canopy; it depends on hanging height and area.
- Efficiency (µmol/J): PPF divided by real watts. Budget LEDs 1.8–2.2; good ones 2.5–2.8; the best exceed 3.0.
- Find the real watts and the stated PPF, then work out the efficiency: 650 µmol/s ÷ 240 W = 2.7 µmol/J.
- Check the PPFD map: it must state the measured area (e.g. 80x80 cm) and the height (e.g. 30 or 45 cm).
- Compare the average and the minimum, not just the peak in the centre: the closer they are, the more even the light.
- Check the driver brand, the protection rating (IP65 suits the humidity in a tent), the CE mark and a Swiss plug (type J) or a suitable adapter.
Distances and PPFD values for each stage are in the guide on LED grow light distance. Always follow the manufacturer's instructions and, where possible, check with a PAR meter or a calibrated app.
Full spectrum, deep red and dimmer
Full-spectrum grow LEDs generally use white diodes (around 3,000–4,000 K) plus deep red at 660 nm, which is highly efficient for photosynthesis. White light helps you too: you see the true leaf colours and spot deficiencies and pests earlier, which was hard under the old purple lamps. Some models add far red (730 nm) or UV on a separate channel: these are options, not requirements, and UV calls for protective glasses.
A dimmer is essential: it lets you use the same light from seedlings (25–40% power) to flowering (80–100%) and turn the intensity down if the tent gets too warm. Some lights connect to a controller that handles switching, intensity and a gradual sunrise and sunset; the hours of light for each stage are explained in the guide to the cannabis light schedule.
LED bars or panel: evenness of light
- Bars: 4–8 strips spread over the area. More even light right into the corners and well-spread heat; they cost more and are the choice for tents of 100x100 and larger.
- Panel (quantum board): a single board with many diodes. Cheaper and compact, ideal for 60x60 and 80x80; the light is stronger in the centre than at the edges.
- Compact or "UFO" models: light concentrated on one spot, suitable for seedlings, cuttings or very small spaces. Tents that come with a light already fitted are among the compact systems.
LED or HPS: heat, consumption and costs
The comparison uses a 240 W LED and a 400 W HPS, which deliver a similar amount of light. Calculated over a cycle of 4 weeks at 18 hours and 9 weeks at 12 hours, both at full power, with an indicative price of 0.30 CHF/kWh.
| Item | LED 240 W | HPS 400 W |
|---|---|---|
| Efficiency | 2.5–2.8 µmol/J | 1.3–1.8 µmol/J |
| Consumption per cycle | 302 kWh | 504 kWh (plus ballast losses) |
| Electricity cost per cycle | about 91 CHF | about 151 CHF |
| Heat on the buds | low, distance 30–45 cm | high, distance 40–60 cm plus cooling |
| Spare parts | none for years | bulb roughly every 1–2 years |
LED calculation: 0.24 kW × 18 h × 28 days = 121 kWh; 0.24 kW × 12 h × 63 days = 181 kWh; total 302 kWh × 0.30 CHF = 90.60 CHF. HPS: (0.4 × 18 × 28) + (0.4 × 12 × 63) = 504 kWh, i.e. 151.20 CHF. All the electricity ends up as heat in the tent anyway, but HPS radiates far more of it onto the buds and needs a stronger extractor fan in summer.
HPS still makes sense if you already own one, if you grow in a cold room (a cellar below 15 °C in winter) where its heat is useful, or if your starting budget is very tight. Replacement bulbs and ballasts are in the HPS and bulbs sections.
Lifespan, warranty and passive cooling
Quality diodes are rated L90 at 50,000 hours or more (they keep 90% of their output): at 18 hours a day that is over seven years. The first part to fail is usually the driver, which is why a recognised brand and a 3–5 year warranty matter. Choose passive cooling with an aluminium heat sink: no fans, no noise, nothing to wear out. Keep the heat sink clean and leave room for air above the light.
Frequently asked questions
How many LED watts does an 80x80 grow tent need?
For flowering, about 180–250 real watts from an LED of at least 2.5 µmol/J. In the vegetative stage about 50–75% of that is enough, set with the dimmer.
Is one LED light suitable for both veg and flowering?
Yes: a full-spectrum LED with a dimmer covers every stage. Between stages you change the intensity and hours of light, not the light itself.
LED or HPS: which is better value?
In a grow tent for personal use, LED is better value: for the same light it uses roughly 30–50% less electricity, runs cooler and has no bulbs to replace. HPS only makes sense if you already have one or grow in a cold room.
How long does an LED grow light last?
A quality light is rated L90 at 50,000 hours or more: it keeps 90% of its output for over seven years at 18 hours a day. The driver is the weak point: choose a known brand with at least a 3-year warranty.
LED panels and bars are available in the LED section at Gro.w.LaB.
Updated on 13 October 2026.