The short answer
Grow lights do one job: deliver enough usable light, in the right spectrum, for enough hours a day. Everything on the box comes back to two numbers. Intensity at the canopy, usually written as PPFD, and total light delivered over a day, called DLI. Get those right and your plants grow. Get them wrong and you can spend $600 and still end up with stretchy, airy buds.
If you are standing in a cannabis store looking at a display shelf or asking a budtender what people buy, here is the honest version. A modern LED bar fixture pulling 200 to 300 watts from the wall will cover a 2x4 foot tent through flower. The same money spent on a blurple panel advertising "1000W" will not, because that 1000W number is a marketing equivalent, not a measurement.
What the numbers on the box mean
- PPF (micromoles per second): total light the fixture puts out. This is the number you compare between lights.
- PPFD (micromoles per square meter per second): how much of that light lands on a given square meter of canopy. It drops fast with distance and at the edges of the tent.
- Efficacy (PPF per watt, or micromoles per joule): how efficiently the fixture turns electricity into light. Cheap lights sit around 1.5. Good LED bars hit 2.5 to 3.0.
- Coverage area: the footprint where PPFD stays reasonably even. A fixture bright in the center and dim at the corners will still disappoint you.
- Color temperature: 3000K and 3500K white diodes are the common pick for a full cycle. Some fixtures add deep red for flowering, a little blue for veg, and small amounts of UV or far-red.
Watts: ignore the big sticker
The real question is how many watts the light draws from your wall outlet, which you will find on the spec sheet or the driver label, not in the product name. Two fixtures can both claim "1000W equivalent" while one pulls 100 watts and the other pulls 480. Only the second one is going to flower out a 4x4 tent.
Dimming matters more than most beginners expect. Running a 300 watt fixture at 60 percent in veg, then turning it up as the canopy fills in, is easier on your plants and your power bill than blasting full output from day one.
LED, HID, and the heat question
Double-ended HPS still works, and some growers swear by the spectrum and the radiant heat in a cold basement. It also runs a hot bulb, needs a ballast, needs air cooling, and burns out. Ceramic metal halide sits in the middle. LED bar fixtures are what most people buy now, mainly because the efficiency gap is real and the heat goes up into the fixture instead of onto the plants, so you need less ducting and less air conditioning.
None of that removes the need for airflow. LED grows run cooler at the leaf surface, which can hide how much water the plants are drinking, and dense canopies still need air moving through them.
Photoperiod, stages, and light leaks
Standard photoperiod cannabis is a short-day plant. Vegetative growth happens on long days, typically 18 hours on and 6 off. Flowering is triggered when the dark period gets long enough, usually 12 hours of uninterrupted darkness. That dark period has to be truly dark. A zipper gap, a power strip light, or a hallway glow can interrupt it and stall flowering.
Autoflowering plants do not care about the dark cycle and will run on 18/6, 20/4, or even 24/0 from start to finish. That flexibility is why a lot of store staff recommend them to first-time growers, even though yield per plant tends to be lower.
Distance, footprint, and how many plants
There is no universal hang height. Seedlings want the light far away and turned down. Once the canopy fills the tent, most LED bars sit 12 to 24 inches above the top colas, and you check with the manufacturer chart or a cheap PAR meter. If you see bleached tips and tacoing leaves, you are too close or too bright. If stems stretch and nodes space out, you are too far or too dim.
Coverage follows the fixture shape. A long bar designed for a 2x4 tent wastes its output in a 4x4, and a square fixture in a long tent leaves dead corners. Buy the light for your tent, not the other way around.
What to ask and what staff cannot tell you
Budtenders and shop staff can usually tell you which fixtures customers come back for, how loud the driver fan is, and whether a brand honors warranty claims. What they cannot do is tell you how many plants you can legally grow. Cultivation limits, plant counts, and whether home growing is allowed at all vary by state, and sometimes by county or lease. Read your state's rules directly.
Electrical safety is worth a minute too. Run high-wattage lighting on a circuit that can handle it, use grounded outlets, and look for a recognized safety listing rather than a bare CE sticker. Cheap timers and daisy-chained power strips are a common cause of grow room fires.
Quick checklist before you buy
- Measure your tent or room and note the square footage.
- Find the wall wattage and the efficacy number, not the product name.
- Check the PPFD map, not just the peak center reading.
- Confirm the fixture dims, and whether the driver is remote or built in.
- Budget for ventilation and a timer, not just the light.
- Check your state's cultivation rules before you buy anything at all.