The Short Answer (If You Only Read One Paragraph)
There is no universal "hours per day" number that works for every plant, and your light schedule should change by growth stage, not by what a Reddit comment told you. For most photoperiod plants, that means 16-18 hours of light during vegetative growth and 12 hours during flowering. For autoflowering or day-neutral varieties, 18/6 or 20/4 is usually a safer bet. If you're running a Spider Farmer G1500 Evo 150W or SE4500 320W, both are solid lights, but the G1500 Evo covers a true footprint of about 3x3 feet (not 4x4, despite what the marketing page suggests), and the SE4500 is closer to 5x5 feet for serious flowering. Wattage is not coverage. That distinction cost me $1,400 in 2019.
Here's the thing: most growers buying their first or second LED light are searching for the wrong number. They want to know "how many watts?" or "how many hours?" when the real question is "what PPFD does my canopy actually receive at the distance I plan to hang this thing?"
Why I Have Any Business Writing This
I've been handling plant lighting procurement for commercial greenhouse operations for 6 years. In that time, I've personally made three lighting mistakes that cost roughly $3,000 in wasted budget and more than a little embarrassment. I now maintain our team's lighting pre-check list—not because I'm smart, but because I got tired of explaining why our backup flower room was running 40% below target PPFD during week 6.
In 2019, I ordered 12 units of a 150W fixture for a 4x8-foot grow space. On paper, 1,800 total watts over 32 square feet looked fine. What I didn't calculate was the PPFD drop-off at the canopy edges—the fixtures were spaced too far apart, and the outer plants were getting maybe 450 μmol/m²/s instead of the 650+ I'd promised. I had to order 4 more units and eat the freight cost on the redo. That one was about $1,400 and a two-week delay.
In 2021, I ran a vegetative cycle on a 14/10 schedule because a grower I respected swore by it for his specific crop. It worked for his genetics. It did not work for ours. We ended up with leggy, stressed plants and a 15% yield drop on that harvest—roughly $900 down the drain. The lesson: schedules are not portable between operations.
The third one was simpler. I bought three "full spectrum" fixtures from a supplier who couldn't provide a spectrum chart. Big blue peak, almost nothing in red. Great for vegetative growth, useless for flowering. $700 wasted. I should have asked for the spectral distribution data before swiping the company card.
How Long Should You Actually Leave the Light On?
This is the question everyone asks, and most answers online treat it like a factory setting. It isn't.
For photoperiod crops: Vegetative stage runs 16-18 hours on, 6-8 hours off. Flowering stage switches to 12/12—no exceptions unless you're intentionally stress-testing. The 12-hour dark period isn't optional; it's the trigger. Interrupting it with even a few minutes of stray light can hermaphrodite some varieties.
For autoflowering or day-neutral crops: 18/6 is the common starting point. Some commercial growers push 20/4 for faster vegetative growth, but the returns diminish hard after 18 hours, and you're paying for electricity that isn't buying you much. I've run both. The difference in final dry weight was about 4% on a 20/4 vs. 18/6 side-by-side—not enough to justify the extra 14 hours of light per week across a 200-plant room.
For seedlings and clones: 18/6 or even 24/0 for the first week is common. But 24/0 isn't a magic growth hack—it mainly reduces stretching in weak-stemmed varieties. After the first true leaves appear, drop back to 18/6.
Here's the part most schedules don't tell you: your light distance changes the effective photoperiod requirement. A light hung 12 inches too high might need an extra hour of runtime to deliver the same DLI (daily light integral) to the canopy. I use a cheap quantum sensor to spot-check canopy PPFD every two weeks. It's not lab-grade, but it catches the "my lights drifted out of position" problem before it becomes a yield problem.
This is also where a smart controller like Spider Farmer's GGS system actually earns its keep. I was skeptical of app-based lighting control—it felt like a solution looking for a problem. But after the 2021 schedule mistake, I started using scheduled dimming and sunrise/sunset ramping. It doesn't fix a wrong schedule, but it does eliminate the "did I set the timer correctly after the power flickered?" uncertainty. If you're managing more than a handful of lights, that predictability is worth more than the controller costs.
Spider Farmer G1500 Evo and SE4500: The Honest Version
I've run both of these in our test room. Here's what I actually measured, not what the product page claims.
Spider Farmer G1500 Evo 150W
The G1500 Evo is a 150W bar-style LED that punches above its price class for small-space growing. At 18 inches above canopy, I measured roughly 780 μmol/m²/s at center and about 520 at the edges of a 3x3-foot footprint. That's a usable flowering intensity for a 3x3 space if you're running CO2 or growing a variety that doesn't demand max PPFD. For a 4x4, it's underpowered—you'll see stretching at the corners and uneven ripening.
The build quality is decent for the price. Passive cooling is adequate at 150W, the daisy-chain port is convenient, and the dimming knob is responsive. No fan means no fan noise, which matters if this is in a living space or a lightly insulated outbuilding. The spectrum is a reasonably balanced full-spectrum curve with enough red to flower decently and enough blue to keep stretch reasonable in veg.
What I don't love: the hanging hardware is basic. The included ratchet straps are functional but not confidence-inspiring for overhead mounting in a commercial setting. I swapped ours for heavier cables. And the claimed 4x4 veg footprint is optimistic—3.5x3.5 is more realistic.
Spider Farmer SE4500 320W
The SE4500 is the more interesting fixture for commercial or serious hobby operations. At 24 inches, I measured around 950 μmol/m²/s at center and 680 at the edges of a 5x5 footprint. That's genuine flowering intensity for a 5x5 space, and it competes well against fixtures that cost 20-30% more.
The bar design spreads heat better than quantum-board style fixtures, which matters in sealed rooms with CO2 supplementation. The dimming range goes down to 10% or so, which is useful for propagation or for managing intensity as plants acclimate after transplant. The spectrum on the SE series leans slightly more red than the G series, which I prefer for flowering crops.
The honest downside: 320W over a 5x5 means you're at about 12.8W per square foot, which is on the low side for high-demand flowering crops under CO2. If you're pushing for maximum yield per square foot, you'll want to run this closer to 20-24 inches and accept a smaller effective footprint, or pair two units for a 4x8. One SE4500 is not a "replace a 600W HPS" fixture—that's marketing language. It's a solid 320W LED that does exactly what a 320W LED should do, and it does it efficiently.
The Specification Sheet Trap Nobody Warns You About
Here's a pattern I've seen repeatedly: growers compare lights by wattage and price per watt, then get surprised when the $0.85/W fixture outperforms the $1.20/W fixture in one room but not another. The variable they're missing is PPFD distribution uniformity.
A light that delivers 900 μmol/m²/s dead-center but 400 at the corners is not the same as a light that delivers 700 across the whole footprint—even if the total wattage is identical. The first light will give you a great crop in the middle of the room and larfy, underdeveloped plants at the edges. The second will give you consistent quality everywhere. For commercial operations selling by weight and grade, uniformity matters more than peak intensity.
This is also where the "spotlight LED" and "downlight lighting" search results get confusing. If you search those terms hoping to find supplemental grow lighting, you'll mostly get architectural fixtures designed for human-occupied spaces—wrong spectrum, wrong beam angle, inadequate PPFD, and often no IP rating for humid environments. I've seen growers try to repurpose PAR38 spotlight LEDs as supplemental lighting. The results were predictable: stretched plants leaning toward the nearest decent light source, and one fixture that corroded after two months in a humid greenhouse because it wasn't rated for the environment.
If you need supplemental or interlighting, buy fixtures designed for plant use. The upfront cost is higher, but the alternative is buying twice.
When This Advice Doesn't Apply
I should be upfront about the limits here.
If you're growing low-light houseplants or shade-tolerant ornamentals, none of this matters. A 150W fixture over a 4x4 monstera collection is overkill, and your "how long should the light be on" question has a different answer: 10-12 hours is plenty for most tropical houseplants that evolved under canopy. The PPFD targets I use for flowering crops would burn or stress them.
If you're running a commercial operation with CO2 supplementation, my PPFD numbers are starting points, not targets. You'll want to push 20-30% higher intensity and maintain tighter environmental control. And if you're in a region with high electricity costs, the math on light hours changes—18/6 vs. 20/4 might not pencil out when you're paying €0.35/kWh.
Finally: I'm writing from experience with a specific set of crops and a specific set of rooms. Your genetics, your climate, and your ventilation will change the numbers. The best thing I ever did was buy a $150 quantum sensor and start measuring instead of guessing. The second-best thing was writing down every mistake so I wouldn't repeat it.
That checklist still lives on our shared drive. It's got 47 items now. I've probably caught 30 potential errors with it in the past 18 months. Not bad for a guy who once ordered 12 lights for a 32-square-foot room and still got the coverage wrong.