How many watts of grow light do I need?
Size lights from the light the plants need, not from the watts on the box. Pick a PPFD target for the stage, multiply by the area, allow for light that misses the canopy, then divide by how efficient the fixture is. GenetixBay's advice: buy more than the minimum and dim it down.
Your result
How it works, in plain words
Light needed
Fixture light (µmol/s) = PPFD target × canopy m² ÷ 0.85. A 4 × 4 ft tent (1.49 m²) at 900 µmol/m²/s needs about 1,570 µmol/s.
Watts
Wall watts = that ÷ the fixture's µmol/J: about 925 W of double-ended HPS, 685 W of a budget LED, 605 W mid-tier or 525 W top-tier.
Oversize and dim
GenetixBay recommends about 50 W/ft² of dimmable capacity; the founder's rule of thumb is 800–900 W for a 4 × 4 (about 50–56 W/ft²). Dim to the stage's target and raise it toward saturation as the plants show they can take more. Often called overkill, but cannabis can use far more light than most guides assume: yield kept rising up to ~1,800 µmol/m²/s in trials (Rodriguez-Morrison 2021). Ramp up gradually, with the climate and CO₂ to match.
Watt for watt
Swapping HPS for LED? Keep the same watts unless the LED is genuinely top tier. You keep the heat load you already handle and turn it into more light.
Every number this calculator uses
The same constants feed the Calculate it properly guides, so the tools and the articles agree. Rules of thumb are labelled as such.
| Constant | Value | What it is | Evidence |
|---|---|---|---|
DLI_K | 0.0036 mol·s per µmol·h | DLI = PPFD × hours × 0.0036 | Established |
CANOPY_CAPTURE | 0.85 fraction | Share of the fixture's photons that land on the canopy in a reflective tent (assumption) | Estimate |
EFF_HPS | 1.7 µmol/J | Double-ended HPS at the fixture (independent tests 1.66–1.72) | Established |
EFF_BUDGET | 2.3 µmol/J | Budget LED panel at the wall (low-bin chips driven hard, budget driver) | Supported (arithmetic from datasheets) |
EFF_MID | 2.6 µmol/J | Mid-tier LED at the wall | Supported |
EFF_TOP | 3 µmol/J | Top-tier LED at the wall (about 6% of listed fixtures in 2022) | Established |
GB_W_PER_FT2 | 50 W/ft² of dimmable capacity | GenetixBay recommendation: oversize and dim down. Too much is better than not enough | House position (founder) |
Sources
- GenetixBay Knowledge Base. PPFD and DLI by stage, with the evidence (stage targets; DLI = PPFD × hours × 0.0036). https://genetixbay.com/knowledge/environment/ppfd-dli-by-stage.html
- GenetixBay Knowledge Base. LED vs HPS: the heat and power-bill hype, and an LED buyer-beware guide (fixture µmol/J tiers, 85% canopy capture, watt-for-watt house advice, 1 W = 3.412 BTU/h). https://genetixbay.com/knowledge/equipment/led-vs-hps-hype-and-buyer-beware.html
- Nelson JA, Bugbee B. 2014. Economic analysis of greenhouse lighting: light emitting diodes vs. high intensity discharge fixtures. PLoS ONE 9(6):e99010 (independent fixture efficacy tests). https://doi.org/10.1371/journal.pone.0099010
- Kusuma P, Pattison PM, Bugbee B. 2020. From physics to fixtures to food: current and potential LED efficacy. Horticulture Research 7:56. https://doi.org/10.1038/s41438-020-0283-7
- GenetixBay house position (founder's field experience, decades of indoor rooms): passive intake for most growers; oversize lights and dim down; replace HPS watt for watt unless the LED is top tier. https://genetixbay.com/editorial-policy
- Rodriguez-Morrison V, Llewellyn D, Zheng Y. 2021. Cannabis yield, potency, and leaf photosynthesis respond differently to increasing light levels in an indoor environment. Frontiers in Plant Science 12:646020. https://doi.org/10.3389/fpls.2021.646020
