The rule is a single line: required PPF = target PPFD × square metres. For flowering without CO₂ a sensible target is 800 µmol/m²/s, so a 1 × 1 m tent asks for about 800 µmol/s and a 2.4 × 2.4 m room for about 4,600.
With that number you can open any spec sheet and see whether the fixture gets there. Below is the table worked out for the most common sizes, with the model that fits and the monthly running cost.
The sum, before you open any catalogue
There are two ways to get a light purchase wrong: under-buying, and flowering never fills in; and over-buying, and you pay a bill that does not turn into harvest. The same multiplication avoids both.
PPF (µmol/s) = target PPFD (µmol/m²/s) × area (m²)
Starting targets: 200 µmol/m²/s in propagation, 450 in vegetative growth and 800 in flowering without added CO₂. If you want to know where those numbers come from, they are explained in the PPFD and DLI guide.
Two honest warnings before the table. First: PPFD is never uniform, the centre always reads higher than the corners, so these figures are averages. Second: mounting height changes the result as much as the model does; a bar fixture usually works well between 40 and 60 cm above the canopy in flowering.
Table by tent or room size
Worked out at an average of 800 µmol/m²/s, the flowering target without CO₂. If your room is vegetative only, divide the µmol/s by roughly 1.8.
Fixtures by room size
Flowering · 800 µmol/m²/s
| Tent or room | Area | Light required | Fixture that fits | Average PPFD | Power | Electricity/month |
|---|---|---|---|---|---|---|
| 0.6 × 0.6 m | 0.36 | 288 | 1 × 120 W · dim to 83 % | 967 | 120 W | 9 € |
| 0.8 × 0.8 m | 0.64 | 512 | 1 × 240 W · dim to 74 % | 1,087 | 240 W | 17 € |
| 1.0 × 1.0 m | 1.00 | 800 | 1 × 240 W | 696 | 240 W | 17 € |
| 1.2 × 1.2 m | 1.44 | 1,152 | 1 × 400 W | 806 | 400 W | 29 € |
| 1.5 × 1.5 m | 2.25 | 1,800 | 1 × 630 W Master Control | 812 | 630 W | 45 € |
| 2.4 × 1.2 m | 2.88 | 2,304 | 2 × 400 W | 806 | 800 W | 58 € |
| 2.0 × 2.0 m | 4.00 | 3,200 | 3 × 400 W | 870 | 1,200 W | 86 € |
| 2.4 × 2.4 m | 5.76 | 4,608 | 4 × 400 W | 806 | 1,600 W | 115 € |
| 3.0 × 3.0 m | 9.00 | 7,200 | 4 × 630 W Master Control | 812 | 2,520 W | 181 € |
Average PPFD across the whole area; the centre always reads above and the corners below. Running cost at 12 h/day, 30 days and €0.20/kWh. Where the fixture overshoots the target, the column shows the percentage to dim it to.
The running-cost column assumes 12 hours of light, 30 days and €0.20/kWh. Treat it as an order of magnitude and put your own tariff into the calculator below.
Work it out with your exact measurements
Commercial tents are rectangular and real rooms are almost never square. Put in the metres you actually have and the stage you will be running:
One big fixture or several small ones?
For the same total PPF, splitting into several units gives a flatter PPFD and less shadow in the corners — which is exactly where yield is lost. The trade-off is price per µmol, wiring, hanging points and more things that can fail.
- Up to 1.2 × 1.2 m: one properly sized unit. Adding a second one usually just adds cost.
- 1.2 to 2 m²: one large unit or two medium ones. Two spread better if the tent is long and narrow.
- Over 2 m²: several units, nearly always. This is where daisy-chaining and dimming from a single point start to matter.
Once you go past two or three fixtures the problem stops being light and becomes control: switching, dimming and running sunrise and sunset on all of them at once. That is what controllers are for, and the reason our panels carry an interconnection port.
Which fixture fits each case
These are the sizes that cover most installations. Price and availability come live from the shop:
The figure we compare with
Two panels at the same price can differ by 25 % in light output. To see it without argument we divide price by PPF and talk about euros per 100 µmol/s. The lower, the more photons per euro:
And after the purchase price comes the cost of running it, which over a 90-day cycle is usually the same order as part of the hardware:
Layout mistakes that cost yield
- A square panel in a long tent. Too much light in the middle, not enough at the ends. Two smaller units in a line spread far better.
- Hanging high “to cover more”. You cover more area with less PPFD everywhere; the whole crop drops a level.
- Forgetting the walls. A clean reflective wall sends light back into the corners and is the cheapest improvement there is.
- Mixing different models in one room. Different spectra and different optimal heights side by side: the crop grows unevenly and you will not know why.
Frequently asked questions
Real questions
-
It is the wrong question, even though the rough answer is useful: with 2.9 µmol/J fixtures you need about 280 W/m² for flowering at 800 µmol/m²/s. With a 2.0 µmol/J fixture you would need 400 W/m² for the same light. Watts depend on efficacy; µmol/s do not.
-
About 800 µmol/s for flowering without CO₂. A single mid-range unit covers it; two small units also work and spread slightly better, but cost more per µmol.
-
That is 5.76 m², so roughly 4,600 µmol/s at 800 µmol/m²/s. In practice it is solved with two large units or three medium ones, and at that point you want them to daisy-chain and run from a single controller.
-
It depends on the model and the stage. As an order of magnitude, a bar fixture works well between 40 and 60 cm above the canopy in flowering, and higher in the first weeks. The real reference is not the centimetre: it is the PPFD you measure at canopy level.
-
Yes, and it is the sensible way to grow, as long as you pick the same model with an interconnection port so they can later be dimmed together from one controller.
-
It is the meter figure: power × hours × days. No equivalences, no “peak power”. What it does not include is extraction, ventilation or dehumidification, which in summer can add up.
Where do the 800 µmol/m²/s come from?
Keep reading
If you want to understand the light targets per stage and how DLI is calculated, start here.
Go from the table to your actual room
The full calculator works by plant count or by floor area, tells you how many units of each model you need and what the whole installation draws.
