The Lumen Illusion: How System Efficacy Helps Us Look Beyond Lumens When Choosing a Light

PLB & SCL2 Series – Port Lavaca, TX

If you’ve been shopping for lighting, it’s a guarantee that you’ve come across marketing materials stating a light’s lumen performance. Although this is one of the most popular industry-standard terms for highlighting a light’s performance, it’s only part of the story.

What is System Efficacy?

Efficacy (measured in lumens per watt) is a measure of how efficiently a light turns the power (watts) into light (lumens). To better understand a light fixture’s true performance, we can use efficacy’s simple calculation.

Efficacy = Lumens ÷ Watts

(It’s important to use the lumens produced and the watts consumed of the system instead of the LED alone)

System Efficacy takes into account both the LEDs’ efficacy and the system’s inefficiencies. 

Additional factors considered in calculating system efficacy are:

• Thermal Performance
• Optical Efficiency
• Driver Efficiency
• Shielding and Structure

By leveraging this formula, a light fixture’s real-world performance becomes clearer. Comparing the system efficacy number against the chart below will give you a good idea of the performance of a light, or if more information is required.

(The above table is for system efficacy. It is not uncommon for LED efficacy to be above 160 lm/W)

The above table shows that as you increase the system efficacy, the performance of the light increases to a point. Once the light reaches a sufficiently high system efficacy, the only way to further improve it is by sacrificing other areas of performance: such as uplight, glare, distribution, color temperature, etc. This threshold where a higher system efficacy doesn’t translate to higher performance is typically around 140 lm/W but varies from product to product.

Informed Buying

A system efficacy rating is one of a host of great metrics like BUG ratings, color temperatures, and optical distributions to measure a light’s performance. The best metric to use is called application efficacy, which focuses on putting light into a useful place. Application efficacy can be shown using lighting layouts where system efficacy along with third-party qualified photometrics (which include all of the above metrics) are considered.

At First Light, we use all of these tools to make sure you are purchasing the right light for your application and to help you wade through the underperforming products and unsubstantiated claims of some manufacturers.