Cannabis Lighting: Understanding Wattage, Spectrum, and PPFD

Of every input you control as an indoor grower, light matters more than nutrients, more than genetics expression, and arguably more than environment. Cannabis is a sun-loving plant, and the quantity and quality of photons hitting your canopy directly dictate how much biomass and resin you can produce. Yet most growers choose lights based on wattage and marketing rather than the two numbers that actually matter: PPFD, the intensity of light reaching the plant, and DLI, the total daily dose of photons. Add spectrum, light cycle, and hanging height, and you have the complete picture of how to drive a grow with light. This guide covers the technology, the measurements, and the practical setup decisions that separate a mediocre harvest from a heavy one.

Why light is the single biggest yield factor

Photosynthesis is the engine of growth, and light is its fuel. Below a certain intensity, cannabis simply cannot photosynthesize fast enough to grow vigorously, and no amount of nutrients or CO2 will compensate. Above that threshold, more light means more growth, up to a saturation point where the plant cannot use any additional photons and may even suffer photoinhibition. The goal of lighting is to deliver the maximum usable intensity without bleaching or stressing the plant, and to do so efficiently enough that the electricity bill does not eat your margins.

LED vs HPS vs CMH

The three dominant grow-light technologies each have real strengths.

High-Pressure Sodium (HPS) has been the commercial standard for decades. It produces intense, warm-red light that drives excellent flowering, and the upfront cost is low. Its weaknesses are heat output, energy efficiency, and a fixed spectrum heavy in red and yellow. A 1,000W HPS fixture radiates serious heat that must be managed with aggressive ventilation or air conditioning.

Ceramic Metal Halide (CMH or LEC) offers a fuller, whiter spectrum than HPS with better efficiency and less heat. CMH bulbs render trichome color and plant health accurately, which growers appreciate, but they still rely on fragile bulbs that degrade over 9 to 12 months.

Light-Emitting Diodes (LED) have become the dominant choice for new grows. Modern full-spectrum LED bars deliver 2.5 to 3.0 micromoles per joule, roughly double the efficiency of HPS, producing less heat per photon and allowing spectrum tuning. Their higher upfront cost is offset within a year or two by power savings, and the lack of bulbs to replace is a long-term win. The main caution with LEDs is that they can deliver deceptively intense light at the canopy, so hanging height and intensity ramp-up matter more than with HID.

Measuring intensity: PPFD and DLI

Wattage tells you how much power a light draws, not how much usable light the plant receives. The numbers that matter are PPFD (Photosynthetic Photon Flux Density), measured in micromoles per square meter per second, and DLI (Daily Light Integral), the total photons delivered over a day.

| Growth stage | Target PPFD | With CO2 enrichment | |---|---|---| | Seedling / clone | 150 to 400 | 400 to 600 | | Vegetative | 400 to 700 | 700 to 900 | | Early flower | 600 to 900 | 900 to 1,100 | | Late flower | 800 to 1,000 | 1,100 to 1,500 |

DLI combines intensity and duration: a plant under 900 PPFD for 12 hours receives a daily light integral of roughly 39 mol per square meter per day, which is solid for flowering. To measure these values, use a quantum PAR meter held at canopy height, not a phone app or lux meter, which read the wrong spectrum. Without a PAR meter you are guessing, and most growers who buy one immediately realize they were either under-lighting or bleaching their canopy.

Spectrum and how to steer growth

Spectrum influences morphology, resin production, and ultimately effect. Blue wavelengths, around 400 to 500 nanometers, encourage compact internodal spacing, tighter structure, and stockier vegetative growth. Red wavelengths, 620 to 730 nanometers, drive photosynthesis most efficiently and signal flowering, with far-red at 730 nm able to trigger the phytochrome red shift that can shorten the night and stretch growth.

Full-spectrum white LEDs, often described as 3000K or 3500K for flower and 4000K for veg, approximate the sun well enough for a full cycle. Many growers add UV-A and some deep blue in the final weeks to stress the plant into producing more trichomes as a defense response, similar to how high-altitude sativas coat themselves in resin under intense mountain sun.

Light schedules and photoperiod

The light cycle tells a photoperiod cannabis plant what season it is in. During vegetative growth, 18 hours on and 6 off, or a continuous 24 hours, keeps the plant in growth mode indefinitely. Switching to 12 hours on and 12 hours off mimics autumn and triggers flowering within one to two weeks. Autoflower strains carry Cannabis ruderalis genetics and flower based on age rather than light cycle, so they typically run 18/6 or 20/4 from seed to harvest.

Consistency matters: a single light leak during the dark period of a 12/12 flower can stress plants into hermaphroditism or re-vegging, so the dark cycle must be truly dark.

Hanging height and footprint

Too close and you bleach the top buds and stunt growth; too far and the lower canopy receives too little light to produce. LED manufacturers provide hanging-height charts, typically 18 to 30 inches above the canopy for high-wattage bars, but a PAR meter is the only reliable guide. The inverse square law means intensity drops off fast with distance, so even coverage across the canopy matters more than a bright hot spot in the center. Uniform, overlapping light from multiple bar fixtures beats one blinding central light every time.

Watch the plants: if upper leaves canoe upward and turn pale, raise the light. If stems stretch and internodes are long, intensity is too low.

Running costs and efficiency

A 1,000W HPS running 12 hours a day consumes 12 kWh daily, which at 15 cents per kWh is about 54 dollars per month for lighting alone, before fans and cooling. An equivalent-coverage LED drawing 600W would cost around 32 dollars monthly for the same light output, plus reduced air-conditioning load. Over a year or two, efficiency gains pay for the higher upfront LED cost, which is why nearly all new commercial installs are LED. Heat from lighting also drives cooling costs, so a cool-running fixture saves money twice.

Key takeaways

• Light is the biggest yield lever; measure it with a quantum PAR meter reading PPFD, not by wattage alone.
• Modern LEDs outperform HPS and CMH on efficiency and spectrum control, and pay back their higher cost through power savings.
• Target PPFD rises from 200 in seedling to 800 to 1,000 in late flower, with CO2 allowing higher intensities.
• Blue light drives compact vegetative structure; red and far-red drive photosynthesis and flowering.
• Photoperiod plants run 18/6 in veg and 12/12 in flower, while autoflowers can run 18/6 or 20/4 from seed to harvest.
• Hanging height and uniform coverage matter more than peak intensity in the center of the canopy.

Frequently asked questions