Efficacy

Photosynthetic Photon Efficacy is the amount of light emitted for a given amount of power used. In horticulture, efficacy is measured taking the PPF of a light source’s output divided by the input power (see equation below). The higher this number, the more efficient the fixture is producing light, however it does not provide how well that light will produce plants. It also may be used by regulatory committees to evaluate rebates. Keep in mind, however, that any energy outside of PAR will reduce the efficacy number making the fixture appear less efficient even though the spectrum may be better for growing.

Efficiency

Fixture efficiency is a calculation to determine how much light exits a fixture. If a fixture has a 90% efficiency rating, that means 90% of the light produced by the lamp is leaving the fixture. The higher this number is, the better your plants will grow. However, this calculation does not represent the uniformity of light hitting the canopy of your plants.

Between the two, efficacy is the most commonly used metric in grow lighting, especially for LED lighting.