Why Distance Matters More Than Wattage
The number on the box tells you how much electricity a light draws, not how much usable light actually reaches your plants. Move that same 150W panel from 75cm down to 30cm above the canopy and the light intensity at leaf level roughly doubles — with no change in running cost. Get the distance wrong in either direction and plants pay the price.
I run a Mars Hydro TS 1000 (150W) in a 60x60cm tent and a set of Barrina T5 strips above seedling trays, and I’ve made both mistakes: bleached leaves from hanging the panel too low early in a grow, and a tray of tomato seedlings so leggy they fell over because the T5s were too high. Getting the distance dialled in is the single most impactful adjustment you can make without spending any money.
Understanding PPFD: The Number Behind the Distance
PPFD (Photosynthetic Photon Flux Density) measures how many light particles useful for photosynthesis land on a square metre every second, expressed in µmol/m²/s. It is the metric that actually tells you whether your plants are getting the right dose of light — distance is just the lever you use to hit the right PPFD target.
Broad targets by growth stage:
- Seedlings: 100–300 µmol/m²/s — tender, easily stressed by strong light
- Vegetative growth: 400–600 µmol/m²/s — leaves are tougher and can absorb more
- Flowering / fruiting: 600–1,000 µmol/m²/s — highest demand, but push past 1,000 only with CO2 supplementation
You do not need a PAR meter to grow well. The chart below translates these targets into practical distances for the most common light types.
Grow Light Distance Chart by Type and Stage
Distances below are starting points. Your specific tent, reflectivity, and dimmer setting will all shift these figures slightly. Always verify against plant response over the first 48 hours.
| Light Type / Wattage | Seedling Distance | Vegetative Distance | Flower / Fruiting Distance |
|---|---|---|---|
| T5 strip (low wattage) | 5–10 cm | 10–20 cm | 15–25 cm |
| LED panel 100W | 45–60 cm | 35–45 cm | 25–35 cm |
| LED panel 150W | 60–75 cm | 40–50 cm | 30–40 cm |
| LED panel 200W | 70–85 cm | 45–60 cm | 35–50 cm |
| LED panel 300W+ | 80–100 cm | 55–70 cm | 40–60 cm |
These figures assume a modern full-spectrum LED with a quality driver. Blurple or older LED chips from a few years ago tend to be less efficient and may need to hang slightly lower to deliver equivalent intensity.
T5 Strips vs LED Panels: Different Rules Apply
T5 fluorescent and T5-style LED strip lights run cool and put out relatively low intensity. You can position them just 5–7cm above a seedling tray without any risk of heat damage or bleaching. This is how I use the Barrina strips — they sit just above the propagation dome, and seedlings emerge compact and dark green.
The trade-off is penetration. T5 output falls off quickly with distance, which makes them poor choices for mature plants taller than 30cm. For anything beyond the seedling stage, a proper LED panel — even a modest 100W — will cover the canopy far more evenly. For a deeper comparison of both types, see the best grow lights for indoor plants guide.
How to Set Your Mars Hydro TS 1000 at Each Stage
In my experience with the TS 1000, starting seedlings at around 60–75cm works well when the dimmer is set to 50% — higher than Mars Hydro’s conservative official chart suggests, but it compensates for the fact that most growers run a higher dimmer setting than the manual assumes. Once plants have four or five true leaves and I step the dimmer up to 70–75%, I drop to around 45cm for veg. For flowering at full power, 30–40cm gives consistent results across the canopy.
The TS 1000’s dimmer is genuinely useful here. Rather than always moving the light physically, you can lower intensity at the dimmer when plants are young and increase it as they mature. If your tent allows it, combining both adjustments — dimmer and height — gives you the most control over the PPFD your canopy receives.
If you are choosing between the TS 1000 and other panels in the same power bracket, the best LED grow lights comparison covers the key differences.
Signs Your Light Is Too Close
The damage appears on the leaves nearest the light source first. Watch for:
- Bleaching: Leaves or buds directly under the light turn pale yellow or white. This is photoinhibition — the plant’s light-harvesting system is overwhelmed and shuts down.
- Brown crispy tips: Leaf edges go brown and paper-dry, similar to nutrient burn but restricted to upper growth.
- Upward leaf curl (“tacos”): Leaves curl along their length to reduce the surface area exposed to the light.
- Praying leaves: Leaves pointing sharply upward is often the first sign, before any colour change appears.
If you see bleaching on buds during flowering, raise the light immediately. Unlike leaf bleaching, bleached buds do not recover and the affected tissue is lost for good.
Signs Your Light Is Too Far Away
Under-lit plants enter a survival response called etiolation. Signs include:
- Stretching and leggy stems: Long gaps between leaf nodes as the plant reaches upward toward a light source it cannot find.
- Pale, washed-out colour: Less chlorophyll is produced because there is less light to power photosynthesis, turning leaves a faded yellow-green.
- Small, underdeveloped leaves: New growth comes in undersized and weak.
- Slow growth overall: Plants appear to stall — fewer new leaves, no upward progress despite healthy watering and feeding.
Etiolated seedlings are a common problem when T5 strips are mounted too high or angled badly. I learned this the hard way with a batch of chilli seedlings: they were 15cm tall with thread-thin stems before I realised the strips had been nudged up during a tray rearrangement.
For dedicated seedling lighting advice, including how to avoid exactly this problem, see the guide to grow lights for seedlings.
How to Adjust: The 48-Hour Method
Rather than chasing a perfect number from day one, set the light at the recommended starting height and give plants 48 hours to respond before adjusting. This is enough time for stress signs to appear if something is wrong, but not long enough to cause lasting damage.
Move in 5cm increments, not large jumps. The difference between 30cm and 50cm above a 150W panel is significant — if you skip straight from one end to the other you will overshoot in the other direction. Small moves, one at a time, produce much more consistent results.
When plants hold their shape, show no curling, maintain a healthy dark green, and grow at a steady pace, you have found the right distance. Record it so you do not need to repeat the process next grow.
Running Costs at Common Wattages
At the Ofgem Q2 2026 unit rate of approximately 24.7p/kWh, here is what different setups cost to run at typical daily light periods:
- T5 strips (around 30W total), 16 hours/day: approximately 12p per day — under £1 per week
- 100W LED panel, 18 hours/day: approximately 44p per day — around £3.10 per week
- 150W LED panel, 18 hours/day: approximately 66p per day — around £4.60 per week
- 300W LED panel, 12 hours/day: approximately 88p per day — around £6.20 per week
Running a mid-range LED tent setup through a full 10-week grow (seedling through flower) costs in the region of £35–£50 at current rates. That is a fraction of what a comparable outdoor growing season costs once you factor in heated propagation and supplemental lighting, and it is not something worth cutting corners on by buying a cheap, inefficient light. The best grow lights overview covers which options give the best output per watt at each price point.
Quick Reference: How to Set Your Distance in Five Steps
- Identify your light type and wattage — T5 strip, 100W, 150W, 200W, or 300W+ panel.
- Determine the growth stage — seedling, vegetative, or flowering.
- Set the initial distance using the chart above.
- Watch for stress signals over 48 hours — bleaching and curl mean too close; stretching and pale colour mean too far.
- Adjust in 5cm steps and wait 24 hours before moving again until plants look settled.
Frequently Asked Questions
How Far Should a 150W LED Grow Light Be from Plants?
Start at 60–75cm above seedlings, drop to 40–50cm for vegetative growth, and finish at 30–40cm during flowering. The Mars Hydro TS 1000 follows this pattern well. Always monitor the top leaves for yellowing or bleaching and adjust up if stress signs appear.
What Happens If a Grow Light Is Too Close to Plants?
Too much light causes photoinhibition and heat stress. The most visible symptoms are bleached white or yellow patches on the leaves and buds closest to the light, crispy brown tips, and leaves curling upward. Move the light 10–15cm higher and the damage should stop spreading within a day or two. Bleached leaves will not recover, but new growth will come in healthy once the distance is corrected.
Do Grow Lights Use a Lot of Electricity?
For home tent growers, no. At the Q2 2026 Ofgem rate of around 24.7p/kWh, a 150W LED panel running 18 hours a day costs roughly 66p per day. A full 10-week grow cycle on that setup comes to approximately £46. T5 strip setups for seedlings cost well under 20p per day. Running costs are low enough that choosing a quality, efficient light over a cheap power-hungry one saves money over time even if the upfront cost is higher.
How Do I Know If My Plants Are Getting Enough Light?
Well-lit plants show compact internode spacing, a deep rich green, and leaves that hold broadly horizontal. If plants are stretching toward the light, turning pale or yellow-green, or producing noticeably small leaves, the light is either too far away, too dim, or running for too few hours each day. Increasing light hours is often the easiest first fix — most seedlings and vegetative plants do well on 18 hours of light per day.