# Grams per watt

> Grams per watt (g/W) is dry flower yield divided by the electrical wattage of the grow lights, a rough measure of how efficiently a grow turns power into product.

- Canonical: https://genetixbay.com/knowledge/glossary/grams-per-watt.html
- Also written: g/w, gpw, grams/watt, gram per watt, yield per watt
- Glossary category: Cultivation and training · Evidence: Supported
- Status: Draft: awaiting expert review · Updated: 2026-10-31
- Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay cannabis glossary"

Grams per watt is a yield efficiency figure: dry grams harvested divided by the wattage the lights draw. A 480 W light producing 480 g is "1 g/W". It is popular because it is easy to calculate, but it mixes two things: how efficient the light fixture is and how well the grower uses the light.

## In practice

Home growers commonly report around 0.5-1.0 g/W; efficient [LED](https://genetixbay.com/knowledge/glossary/led.md) rooms with good genetics and environment report more. Be careful comparing numbers: use wall wattage, not the "equivalent" wattage marketed on fixtures, and state the run time, since a longer flower adds weight.

A more physical measure is grams per mole of photons (g/mol), which removes [fixture efficiency](https://genetixbay.com/knowledge/glossary/photon-efficacy.md) from the equation. Research shows yield rises with photon delivery, and that fixture efficacy (µmol/J) matters more to cost than fine spectrum tweaks.

## Units and formula

g/W = dry flower (g) / fixture power draw (W). Example: 600 g / 720 W = 0.83 g/W.

## Don't confuse with

- **g/m²**: yield per area; the better comparison between grows.
- **Photon efficacy (µmol/J)**: a property of the light, not the grow.

## Related terms

[Yield](https://genetixbay.com/knowledge/glossary/yield.md), [PPFD](https://genetixbay.com/knowledge/glossary/ppfd.md), [Photon efficacy](https://genetixbay.com/knowledge/glossary/photon-efficacy.md), [DLI](https://genetixbay.com/knowledge/glossary/dli.md)

## Read more

- [PPFD and DLI by stage, with the evidence](https://genetixbay.com/knowledge/environment/ppfd-dli-by-stage.md)

## Sources

1. Westmoreland FM, Kusuma P, Bugbee B., 2021. Cannabis lighting: Decreasing blue photon fraction increases yield but efficacy is more important for cost effective production of cannabinoids. PLOS ONE 16(3):e0248988. https://doi.org/10.1371/journal.pone.0248988