Growing indoors comes with different challenges than growing outdoors. Light is likely the biggest differentiator. When growing outdoors, we rely on the sun. When growing indoors, we rely on artificial light, LED grow lights, for instance. There’s a vast number of different terms that are used to measure and describe LED grow lights, one of these terms is “efficacy”.
Efficacy is somewhat the same as efficiency.
Efficiency is a term we all understand and use. It’s used in all contexts to describe how effective something or someone is.
For example, the term can be used to describe how efficiently an ice hockey player is at scoring goals. If 1 of every 10 shots he takes results in a goal, his efficiency at scoring is 10%.
Another example would be a car’s range in gal/mile (or l/km). This describes how efficiently a car uses fuel.
Similarly, we measure a LED grow light’s efficiency at turning electricity into photos (light). But instead of the term “efficiency” we use the term “efficacy” and we measure it in unit umol/J or PPF/W, same thing, different names. Calculating a lamp’s efficacy is done by machines called “integrated sphere” that can detect exactly how many photos a lamp emits. The number of photons is measured in umol which is also called PPF or photosynthetic photon flux.
Once the output is measured, it’s then divided by the lamp’s power consumption which is measured in watts. Measuring power consumption is done with a power meter. The grow light’s power cord is connected to the power meter and then the power meter is connected to an electrical socket. The power meter then tells how many watts the lamp draws.
Dividing the light output with the number of watts tells us how efficiently a lamp runs, or rather, what it’s efficacy rating is.
Efficacy often shows quality. A poorly built lamp with poorly performing hardware will have low efficacy.
The biggest factor when determining efficacy is the efficacy rating of the diodes. Different diode brands have different rating. However, diodes that are run soft (low wattage) operate at higher efficacy than the same type of diode run at higher capacity (high wattage).
The temperature also plays into effect.
If a lamp has poor thermal management or operates in a warm environment, it will have a lower system (total) efficacy than a lamp with good thermal management or operated in a cool environment.
Other hardware such as the driver that converts the incoming AC electricity from the socket and transforms it into DC for the diodes also reduces the system efficacy. A very good driver, such the MeanWell brand, operates at 93% efficacy, i.e. 7% of the incoming electricity goes to waste.
If a lamp also has fans or other electrical components, then some incoming electricity will be consumed by these parts as well, resulting in less electricity reaching the diodes which means the system efficacy will be reduced further.
The length of various cables and junction points also cause a voltage drop which affects the output.
Generalising a bit, efficacy for LED grow lights can be broken down to a few categories:
0.7-1.0 umol/J: poor efficacy
1.0-1.3 umol/J: decent efficacy
1.3-1.6 umol/J: good efficacy
1.6-2.0 umol/J: very good efficacy
2.0+ umol/J: excellent efficacy
Some top of the line brands that manufacture minimalistic lamps, such as quantum boards or COBs, can reach around 2.5 umol/J system efficacy but these lamps are typically only used at large or commercial scale grow operations.
For beginner grow lights in the $100-200 price range covering 1-4 plants, the typical efficacy is around 1.0-1.5 umol/J.
The higher end a led grow light is, the better is usually the efficacy.
It’s usually a tradeoff between purchase price and efficacy. Cheap grow lights are not very efficient whereas efficient grow lights are usually expensive due to using premium parts to achieve high efficacy.