Yield by Light Wattage

What 'yield by wattage' actually means

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

Two things to nail down before the number means anything:

• Wall watts, not 'equivalent' watts. Many LED fixtures are marketed with inflated 'HPS equivalent' numbers. Measure actual draw with a wattmeter at the outlet Strong evidence.
• Dried and trimmed weight. Wet weight roughly quadruples dry weight and is meaningless for comparison.

The more physically meaningful metric is grams per mole of photons (g/mol), because plants respond to photons (PPFD and DLI), not watts ^[1][2]. Two fixtures at the same wattage can deliver very different photon output depending on efficacy (μmol/J). A modern LED at 2.7 μmol/J delivers roughly 50% more usable light than an HPS at ~1.7 μmol/J for the same wall watts ^[3].

Why growers use it

Grams per watt is popular because it's simple, comparable across grows, and ties directly to running cost. At $0.15/kWh and a 12-hour flower photoperiod over 63 days, a 600 W fixture uses about 227 kWh, or ~$34 in electricity. Divide by yield to get cost per gram from lighting alone.

It's also a diagnostic. If you're stuck at 0.4 g/W, the problem usually isn't the light — it's canopy coverage, PPFD, environment, or genetics. If you're at 1.2 g/W, further gains come from CO2, defoliation timing, and cultivar selection, not more watts.

What g/W is not good for:

• Comparing LED to HPS fairly (LED wins on μmol/J, so g/W is unfair to HPS unless normalized) Strong evidence.
• Marketing claims of '2+ g/W' without disclosed methodology Disputed.

Realistic yield ranges

Based on hobbyist and commercial reports, and normalized to wall watts of the light only (not HVAC):

| Skill / setup | Expected g/W | |---|---| | First-time grower, mid-tier LED | 0.3–0.6 | | Competent home grower, tuned environment | 0.8–1.2 | | Experienced grower, high-efficacy LED, CO2 enrichment | 1.2–1.8 | | Commercial cultivation, optimized | 1.5–2.5+ Weak / limited |

The '1 gram per watt' benchmark was popularized in the HPS era on forums like Overgrow and ICMag in the early 2000s. It was aspirational then and remains a reasonable target for a competent grower today Anecdote. Peer-reviewed cannabis yield-per-watt data is sparse; most figures come from horticulture research on PPFD and DLI response curves, which show cannabis yield increasing roughly linearly with light intensity up to ~1500 μmol/m²/s under CO2 enrichment ^[1][2].

How to calculate and improve your g/W (step-by-step)

Step 1: Measure actual wattage. Plug your light into a Kill A Watt or similar meter. Record the draw at the dimmer setting you'll use. Ignore the box rating.

Step 2: Track your harvest. Dry to 62% RH stem-snap (roughly 10–12 days depending on environment), trim, then weigh. Only count flower, not trim or stems.

Step 3: Do the math. Dried flower grams ÷ measured watts = g/W.

Step 4: Diagnose the bottleneck.

• If g/W < 0.6: light is probably fine; problem is canopy coverage, low PPFD at canopy (target 600–900 μmol/m²/s in flower without CO2), or environment (VPD outside 1.0–1.5 kPa in flower) ^[1].
• If g/W is 0.6–1.0: work on training — ScrOG, topping, and defoliation to flatten and fill the canopy.
• If g/W is 1.0–1.5: consider CO2 enrichment (1000–1200 ppm) and higher PPFD (1000–1500 μmol/m²/s). CO2 without enough light is wasted ^[2].
• If g/W > 1.5: gains come from cultivar selection and dialing runs, not equipment.

Step 5: Recalculate every harvest. Change one variable at a time so you know what moved the number.

Common mistakes

• Using the box wattage. A '1000 W LED' often draws 100–150 W from the wall. Your g/W will look absurdly high and mean nothing.
• Counting wet weight. Adds 3–4× phantom yield.
• Ignoring efficacy (μmol/J). A cheap LED at 1.8 μmol/J and a quality board at 2.8 μmol/J at the same wattage produce very different yields. This is why g/W favors better fixtures ^[3].
• Chasing g/W past the point of quality. Cramming more bud mass per watt often means larfy, airy flower or overdriven plants with reduced terpene content Weak / limited.
• Comparing across cultivars. A high-yield cultivar like Critical Mass will beat a low-yield boutique cultivar on g/W regardless of skill. Compare to yourself, not to strangers.
• Not including all lighting. If you run supplemental UV or far-red bars, count those watts too.

Related techniques and concepts

• PPFD and DLI — the photon-based metrics that actually drive yield.
• LED vs HPS — why g/W comparisons need normalization.
• ScrOG and Topping — canopy techniques that raise g/W without buying more light.
• VPD — environment tuning that unlocks the yield the light can support.
• CO2 enrichment — the last big lever once light and environment are dialed.

Sources

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