Light & power
Get the light right and you set the ceiling for everything else in the tent.
Light is the one input with a direct line to yield — double the usable light and you roughly double the growth. Buy honest watts, hang it at the right height, and the plant does the rest.
What good looks like
What good lighting looks like
- Pick it
Buy on wall draw, not marketing
A reputable full-spectrum white LED — 200W for an 80×80cm tent, 300-320W for a 120×120cm — judged on actual wall draw and a published PPFD map. One fixture covers veg and flower.
- Height
Measure at the canopy
Hung at 30-45cm in flower so centre and corners land in the target PPFD band. The Photone app or a PAR meter confirms it — not 800 in the middle and 200 in the corners.
- Intensity
Right PPFD for the stage
Seedlings ~300-400 PPFD, veg 300-600, flower 600-900. Dimmable lights run low early and full for flower, so nothing gets starved or scorched.
- Schedule
18/6 then 12/12
18 hours on in veg reads as summer; 12 hours of uninterrupted dark triggers flower. Autos ignore the flip and run 18/6 from seed to harvest.
- Power
Safe and earthed
Total tent draw added up and kept comfortably under the 13A socket limit, on a thick-cable lead, with RCD protection, three-pin earthed plugs, and a drip loop on every cable near water.
The numbers that matter
The numbers that actually matter
Do this
The light & power checklist
- Buy on actual wall draw, not the inflated marketing wattage
- Check the listing has a real PPFD map before you spend a cent
- Match the light to the tent — 200W for 80×80cm, 300-320W for 120×120cm
- Hang it at 30-45cm in flower and measure PPFD at the canopy with the Photone app
- Dim it for seedlings and veg, run full power for flower
- Before flower, get in the tent, close it, wait two minutes and seal any light you can see
- Add up the amps of everything plugged in and stay under the 13A socket limit
- Confirm RCD protection, earthed three-pin plugs, and a drip loop on cables near water
Watch for
Catch these early
The early sign, what it means, and the fix. The full stories are in the book.
Long spindly internodes, pale leaves, stems reaching upward
The light is too weak or hung too high, so PPFD at the canopy is starvation-level.
Fix: Check the actual wall draw and PPFD, lower the light or replace an underpowered fixture.
📖 The sixty-euro blurple that drew 130W and cost three months of stretch. The full story →
Top leaves bleaching white at the tips and curling away from the light
PPFD at those leaves is more than the plant can process — the light is too close.
Fix: Raise the light 10-15cm and check again the next day.
📖 The UV-B grower who cooked the canopy at point-blank range. The full story →
Banana-shaped pollen sacs appearing between the calyxes in flower
A light leak in the dark period is confusing a photoperiod plant into turning hermaphrodite.
Fix: Do the two-minute dark-tent check and seal every leak with tape before flipping.
📖 The Insomniac who left the zip cracked and found nanners at week seven. The full story →
Extension lead warm to the touch near the plug, a smell of hot plastic
You’re pulling more current through the cable than it was built for.
Fix: Add up the amps, drop below the 13A limit, and move to a thick-cable lead.
📖 The Octopus — seven things stacked on one lead, an hour from a fire. The full story →
Questions
Light & power FAQ
LED or HPS for a first grow?
LED in 2026. HPS still grows heavy buds but runs hotter and costs roughly double to run — about €76 a month in flower against €40 for the LED doing the same job. If someone hands you a free HPS kit, use it — otherwise spend on LED.
Why is a “1000W” light only sixty euro?
Because the 1000W is fiction — what the diodes could theoretically handle, not what the light pulls from the wall. The real draw is often around 130W. The only wattage that matters is the actual wall draw.
Do I need separate lights for veg and flower?
No. A full-spectrum white LED covers both from one fixture. The old metal-halide-for-veg, HPS-for-flower advice died when LEDs started covering the full range.
How close should the light be?
For a 200-320W LED, 30-45cm in flower is a good starting band. Then measure PPFD at the canopy and adjust — the inverse square law means every centimetre you raise it costs intensity.
Can I check the plants during the dark period?
Use a low-wattage green LED. Plants barely respond to green wavelengths, so a brief look won’t disrupt the flower trigger. Keep it short and dim.
The whole story is in the book
Grow Good Bud walks through the spectrum, the inverse square law, and the electrical maths in full — so you only pay for the light once.
The web gives you the lesson; Grow Good Bud keeps the scars. The kit to grow it is at Dublin Indoor Gardening.