Photon efficacy (photosynthetic photon efficacy, PPE) is a fixture's PAR 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 rangesbeing reviewed:
| Fixture class | µmol/J |
|---|---|
| T5 fluorescent | about 1.0–1.2 |
| Metal halide | about 1.2–1.5 |
| Ceramic metal halide | about 1.5–1.9 |
| Double-ended HPS | about 1.7–2.1 |
| Modern horticultural LED | 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 (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.
