LED vs HPS for Cannabis: The Short Answer
Buy LED first. A current full-spectrum LED bar or board fixture beats a high pressure sodium lamp on nearly every metric that matters in a home or small commercial grow: photons per watt, heat load, spectrum control, and lifespan. HPS keeps two narrow advantages, and only two. It costs less to buy at the register, and it dumps enough radiant heat to keep a cold basement room in range without a heater.
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If you already own a ballast, a reflector, and a stash of bulbs, run the HPS until the bulbs expire, then move to LED. If you are starting from zero, buy LED.
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Head to Head
- Efficiency: Quality LED fixtures reach 2.5 to 3.0 micromoles per joule. Single-ended HPS runs about 1.0 to 1.4. You get the same light with roughly half the watts.
- Heat: HPS turns about 60 percent of its input into heat. LED moves most of its waste heat into the driver and heatsink, where a fan can pull it out of the tent.
- Spectrum: HPS is heavy in yellow and orange with little blue. LED lets you run more blue for stretch control in veg and more red in flower.
- Lifespan: HPS bulbs lose output after about 10,000 hours and need replacing. LED diodes hold most of their output past 50,000 hours.
- Upfront cost: HPS wins here. A complete 600W setup can cost less than a comparable LED bar.
- Cooling cost: LED wins. Smaller AC, smaller exhaust fan, lower electric bill.
Prerequisites Before You Buy
- A tent or room with a measured footprint in feet or meters.
- Ceiling height, since HPS needs more clearance than LED.
- A power budget for the circuit you plan to use.
- An exhaust fan and carbon filter sized to the space.
- A timer rated for the load, plus a dimmer if the fixture supports one.
How to Choose and Set Up Your Light
- Measure the canopy footprint you intend to light, not the tent floor. Write down length and width in feet.
- Set a target light intensity. Aim for 400 to 600 PPFD in vegetative growth and 600 to 900 PPFD in flower, with a daily light integral near 30 to 40 moles per square meter.
- Convert that target into fixture wattage. For LED, plan on roughly 30 to 40 watts per square foot of canopy. For HPS, plan on 50 to 60 watts per square foot to reach the same intensity.
- Compare fixtures at the same wattage and check the manufacturer output chart at 12 to 18 inches, not the marketing peak number.
- Mount the fixture and set hanging height. Start LED at 18 to 24 inches in veg, HPS at 24 to 30 inches, then adjust based on leaf response.
- Run the light on a timer set to 18 hours on for veg and 12 hours on for flower.
- Check canopy temperature at the top colas with an infrared thermometer. Hold it under 85 F.
- Adjust exhaust or add a small oscillating fan until the leaf surface stays in range and no hot spots form directly under the fixture.
When HPS Still Makes Sense
Choose HPS if your grow space runs cold, if you need maximum light output on the smallest possible fixture budget, or if you are running a sealed room and already have cooling capacity sized for the heat. In those cases a double-ended 1000W HPS with a quality reflector will produce solid flower.
Cannabis Grow Lights for Flowering: The Ultimate Buying Guide
Choose LED in every other case, including tents, closets, garages, bedrooms, and any space where you cannot vent heat outside. LED also suits multi-tier shelving, since the low profile of a bar fixture lets you stack two or three levels in the same vertical space.
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What to Check on the Label
- Efficacy in micromoles per joule, not "equivalent wattage."
- PPF output for the whole fixture.
- Dimmability and whether it runs on 120V or 240V.
- Driver brand and warranty length.
- Water resistance rating if humidity in the room runs high.
A 240W to 320W LED bar covers a 3x3 foot canopy well. A 480W to 640W unit fits a 4x4. For a 5x5, run two smaller fixtures instead of one large one so you can balance the canopy.