# Photon efficacy

> Photon efficacy is how many micromoles of PAR photons a fixture produces per joule of electricity (µmol/J); higher means more light for the same power bill and heat.

- Canonical: https://genetixbay.com/knowledge/glossary/photon-efficacy.html
- Also written: PPE, photosynthetic photon efficacy, µmol/J, umol/J, fixture efficiency, efficacy
- Glossary category: Environment and light · Evidence: Established
- 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"

Photon efficacy (photosynthetic photon efficacy, PPE) is a fixture's [PAR](https://genetixbay.com/knowledge/glossary/par.md) output divided by its electrical input, in micromoles per joule (µmol/J). One watt is one joule per second, so it is also µmol/s per watt. It is the best single number for comparing running cost, because every watt not turned into light becomes heat you also pay to remove.

## In practice

Approximate fixture-level ranges:

| Fixture class | µmol/J |
|---|---|
| [T5 fluorescent](https://genetixbay.com/knowledge/glossary/t5-fluorescent.md) | about 1.0–1.2 |
| [Metal halide](https://genetixbay.com/knowledge/glossary/metal-halide.md) | about 1.2–1.5 |
| [Ceramic metal halide](https://genetixbay.com/knowledge/glossary/cmh.md) | about 1.5–1.9 |
| Double-ended [HPS](https://genetixbay.com/knowledge/glossary/hps.md) | about 1.7–2.1 |
| Modern horticultural [LED](https://genetixbay.com/knowledge/glossary/led.md) | about 2.5–3.2 |

Kusuma, Pattison and Bugbee (2020) reviewed the physics and put the best LEDs near 3 µmol/J, with a practical ceiling around 4. Westmoreland and colleagues (2021) concluded that efficacy matters more than fine spectrum tuning for cost-effective cannabis production. *Established*.

A grower switching from a 1,000 W HPS at 1.9 µmol/J to a 700 W LED at 2.7 µmol/J gets about the same photons (1,900 vs 1,890 µmol/s) for 30% less power and much less heat.

## Units and formula

Efficacy (µmol/J) = [PPF](https://genetixbay.com/knowledge/glossary/ppf.md) (µmol/s) ÷ wall power (W). Use measured wall power, not the "equivalent" wattage on the box.

## Don't confuse with

- **Luminous efficacy (lm/W)**: eye-weighted; irrelevant to plants.
- **PPFD**: light at the canopy, which also depends on height and reflectors.

## Related terms

[PPF](https://genetixbay.com/knowledge/glossary/ppf.md), [LED](https://genetixbay.com/knowledge/glossary/led.md), [HPS](https://genetixbay.com/knowledge/glossary/hps.md), [LED vs HPS](https://genetixbay.com/knowledge/glossary/led-vs-hps.md)

## Read more

- [Light spectrum: blue, red, far-red and UV, what's real](https://genetixbay.com/knowledge/environment/light-spectrum.md)
- [PPFD and DLI by stage, with the evidence](https://genetixbay.com/knowledge/environment/ppfd-dli-by-stage.md)

## Sources

1. Kusuma P, Pattison PM, Bugbee B, 2020. From physics to fixtures to food: current and potential LED efficacy. Horticulture Research 7:56. https://doi.org/10.1038/s41438-020-0283-7
2. 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