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Lumens per watt (lm/W) is the key efficiency metric for lighting systems, quantifying the amount of visible light output per unit of power consumed. This efficacy rating allows energy managers to compare different lighting technologies, such as fluorescent, HID, or LED systems. CRI measures the accuracy of color rendering relative to natural light, while CCT describes the warmth or coolness of light, both important for quality but not efficiency. Power factor relates to the electrical characteristics of a device, not directly to light production efficiency. For Certified Energy Managers, lighting is often a high-impact area for energy savings, as retrofits can offer fast paybacks. Understanding efficacy helps in evaluating replacement options, selecting controls, and meeting code requirements such as ASHRAE 90.1 or IECC standards. The shift from incandescent (10–17 lm/W) to modern LEDs (100–150+ lm/W) demonstrates the dramatic improvements available, underscoring why efficacy is a critical metric in lighting design and management.