Yield by Light Wattage

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What 'yield by wattage' actually means

Grams per watt (g/W) is dried, trimmed flower weight divided by the electrical wattage the grow light draws from the wall. If a fixture drawing 300 W produces 300 g of dried flower, that's 1.0 g/W.

Two critical rules:

Wall watts, not marketing equivalents. Many LED fixtures are labeled with inflated 'HPS equivalent' numbers (e.g., '600 W equivalent' while drawing 200 W from the wall). Always measure actual consumption with a wattmeter at the outlet, not the box rating Strong evidence ^[1].

Dried and trimmed weight only. Use flower that has been dried to 62% relative humidity and hand-trimmed. Wet weight is 3–4 times heavier than dry and ruins comparisons across growers.

Why photons matter more than watts

Plants respond to photons (measured as PPFD: photosynthetic photon flux density), not watts. A more physically meaningful benchmark is grams per mole of photons delivered, because two fixtures at the same wattage can deliver very different photon output depending on efficacy (measured in μmol per joule, μmol/J). A modern LED at ~2.7 μmol/J delivers roughly 50% more usable light energy than an HPS at ~1.7 μmol/J for the same wall watts Strong evidence ^[1].

This is why g/W alone can mislead: it favors efficient fixtures but doesn't account for the quality of light delivered. Use g/W as a tracking tool, not as a ranking.

Why growers use it

Grams per watt is popular because it is simple to calculate, directly comparable across your own harvests, and ties yield to running cost. At typical electricity rates ($0.12–0.18/kWh) and a 12-hour flowering photoperiod over 63 days, a 600 W fixture uses approximately 227 kWh, or $27–41 in electricity. Dividing yield by that cost gives you the real-world cost per gram from lighting energy alone.

G/W is also a diagnostic tool. If you're consistently at 0.4 g/W, the problem usually isn't the light—it's canopy coverage, low PPFD at the plant surface, poor environment (temperature, humidity, CO2), or unsuitable cultivar. If you're at 1.2 g/W and want to go higher, buying more light rarely helps; the bottleneck is usually genetics, CO2, or training technique.

What g/W is NOT good for

• Comparing LED to HPS directly. LED wins on efficacy (μmol/J), so a g/W comparison will always favor LED even if both setups are equally well-tuned. Normalize to photon output (g/mol) if you want a fair cross-technology comparison Disputed ^[1].
• Marketing claims. Claims of '2+ g/W' often omit methodology, use "marketing watts" instead of wall watts, count wet weight, or exclude environmental costs. Look for disclosure of all three: wall wattage, dried weight, and harvest duration.

Realistic yield ranges

The following ranges are based on published horticultural research, cannabis-specific peer-reviewed studies, and documented grower reports. All figures assume wall watts of the light fixture only (HVAC and other infrastructure excluded) and dried, trimmed weight.

The '1 gram per watt' benchmark originated in the HPS era (early 2000s on forums like Overgrow and ICMag) as an aspirational target for a well-run indoor room. It remains a reasonable target for a competent home grower today Anecdote.

Peer-reviewed cannabis yield-per-watt data is limited, but cannabis light-response research shows yield increasing roughly linearly with PPFD up to 1200–1500 μmol/m²/s under CO2 enrichment, with diminishing returns at higher intensities Strong evidence ^[2][4].

How to measure and improve your g/W

Step 1: Measure actual wall wattage

Plug your light fixture into a Kill A Watt meter (or equivalent power meter). Record the draw at the dimmer level you plan to use during flowering. Do this for a full 10-second average to account for ballast ripple. Ignore the rated wattage on the box.

Step 2: Standardize drying and trimming

Harvest, hang-dry in darkness at 60–65°F and 45–55% RH for 7–12 days until stems snap cleanly. Trim away fan leaves and sugar leaves by hand or machine. Weigh only the finished flower buds, not trim or stems. Record the weight to 0.1 g precision.

Step 3: Calculate g/W

Dried flower grams ÷ measured wall watts = g/W. Example: 250 g harvest ÷ 300 W = 0.83 g/W.

Step 4: Diagnose your bottleneck

If g/W < 0.6: Light intensity or spectrum is probably not the limiting factor. Check:

• Canopy coverage. Are you filling the light footprint? Target 100% horizontal coverage at canopy height before flip to flower.
• PPFD at canopy. Measure or estimate; aim for 600–900 μmol/m²/s in flower without CO2, using a PPFD meter or light spec from the manufacturer Strong evidence ^[2].
• Environment. Verify vapor pressure deficit (VPD) is 1.0–1.5 kPa during lights-on in flower. Outside this range, stomata close and photosynthesis crashes Strong evidence ^[3].
• Genetics. Is the cultivar known as a heavy yielder? Boutique or heirloom strains often yield less per watt.

If g/W is 0.6–1.0: You have light but aren't filling it efficiently. Improve canopy coverage:

• Use ScrOG (screen of green) or mainlining to flatten the plant and expose more sites to light.
• Increase days of vegetative growth to let plants reach the light spread.
• Top or fim-prune to branch.
• Remove lower leaves judiciously to open air gaps (mild defoliation).

If g/W is 1.0–1.5: Equipment is tuned; gains come from biochemistry. Consider:

• CO2 enrichment at 1000–1200 ppm, but only if PPFD is already at 800+ μmol/m²/s. CO2 without enough light is wasted Strong evidence ^[2].
• Cultivar selection. Switch to a known high-yield strain if your current choice is moderate-yield.
• Nutrient dialing. Minor tweaks to NPK and micronutrients; run side-by-side tests.

If g/W > 1.5: You have found the practical ceiling for your cultivar and environment. Further gains are minimal and come from incremental cultivar selection or marginal environment tuning.

Step 5: Change one variable per run

After each harvest, adjust only one factor: light height, PPFD, defoliation intensity, CO2 level, or cultivar. Recalculate g/W and compare. This isolates what actually moved the number.

Common mistakes

• Using the manufacturer's rated wattage instead of wall draw. A fixture marked '1000 W LED' often draws 100–150 W. This inflates your g/W by 7–10×, making progress invisible and comparisons meaningless. Always use a meter.

• Weighing wet or partially dried flower. Wet flower is 3–4 times heavier than fully dried. Premature weighing phantom-inflates yield by 200–300%.

• Ignoring fixture efficacy (μmol/J). Two LEDs at the same wattage can differ by 30–50% in photon output. A cheap board at 1.8 μmol/J and a quality one at 2.8 μmol/J will produce very different yields at the same wattage. This is why g/W favors better-engineered fixtures Strong evidence ^[1].

• Comparing across different cultivars. A natural high-yielder like Critical Mass will out-produce a boutique low-yield cultivar on g/W no matter your skill. Compare your own harvests, not strangers' results.

• Forgetting supplemental lighting watts. If you run supplemental UV, far-red, or side-light bars, add their wattage to your total. Otherwise, g/W will underestimate your true light investment.

• Expecting linear gains from more light alone. Once PPFD is adequate (800+ μmol/m²/s without CO2), yield plateaus. Adding 100 W more light without improving environment or canopy coverage yields little Weak / limited ^[2].

Related techniques and concepts

• VPD (Vapor Pressure Deficit) Basics — Environment control that unlocks the photosynthesis your light provides.
• Light Height by Stage — Positioning to optimize PPFD without bleaching.
• More Light Is Always Better — Myth debunked: diminishing returns and when to stop adding light.
• Defoliation Always Boosts Yield — Strategic leaf removal to improve light penetration.
• Topping Always Increases Yield — Branch training to fill the canopy.
• Light Bleaching — What happens when PPFD exceeds plant tolerance.
• Bar-Style LED Grow Lights — Efficient fixture design and why efficacy matters.
• HID Lights — Traditional high-intensity discharge technology and g/W comparisons.

Sources

How this page was made

Related

• Bar-Style LED Grow Lights — Distributed LED fixtures designed for uniform light coverage and efficient photon delivery...
• Light Bleaching — Pale, white-tipped buds caused by excessive light intensity; does not increase potency.
• Defoliation Always Boosts Yield — The claim is popular but unsupported. Selective defoliation can help airflow; aggressive s...
• HID Lights — Gas-discharge lamps that produce high light intensity; standard in older indoor grows, now...
• Light Height by Stage — Vertical light distance control for stage-appropriate intensity.
• More Light Is Always Better — A half-truth from the LED transition that ignores photosynthetic saturation and economics.