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Lumens per Watt vs. Micromoles per Joule: What These Measurements Mean, and How They Influence Your Grow Facility

When deciding what lights to purchase for your facility, it is imperative to understand the difference between efficiency and efficacy. While these two terms are often used interchangeably, they take on slightly different meanings when discussing LED lights. This distinction can make a huge impact on the growers’ choice, particularly after taking a few aspects into consideration. Some may argue the most important measurements to consider before purchasing LED lights are lumens per watt and micromoles per joule. These measurements speak to the quality of the light at hand.

Efficiency: Lumens per Watt

Understanding lumens per watt means understanding how efficient your lighting system is. Lumens, often abbreviated as lm, measure the overall light output. You have probably heard of watts before — this term describes the power consumption by a lamp. To put it simply, LPW determines how much light you’re getting from each unit of energy. The higher the LPW, the more efficient the bulb is.

Efficacy: Micromoles per Joule

Before dissecting what micromoles per joule represents, it could be helpful to understand what photosynthetically active radiation (PAR) and photosynthetic photon efficacy (PPE) mean. PAR is the type of light — not a measurement — that is absorbed by plants to be converted into growth. This type of light is represented by photons with 400 and 700 nm wavelengths. Furthermore, PPE represents the number of specifically PAR photons released by a particular light. You can read more about these terms in our recent post Invest in Uniformity: How LED Lights can Offer Superior Uniform Lighting for Your Grow.

This measurement reveals how effective the light is by determining how much useful light is absorbed, shown in micromoles per joule (μmol per joule). Essentially, this measures the efficacy of the light: how much usable light is released for the plants for photosynthesis.

Capitalizing on Efficiency, Efficacy

While LPW is very important to take into consideration, especially when determining brightness, micromoles per joule are particularly revealing when examining lights specifically designed for plant growth. Focusing on these qualities can save you money and time. According to Greenhouse Product News, “We do not recommend waiting for better technology. Many growers still operate old, dirty, 400-W HPS lamps, and upgrading to modern lighting technology can reduce electric cost by more than 50 percent. Growers who operate lamps for more than 4 months of the year (2,000 hours or more) and whose electricity prices are above average ($0.12 kWh) could realize payback in two to three years — especially if energy rebates are available.”

We agree with their final conclusion — don’t wait to take advantage of the improved efficiency and efficacy of LED lights.

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