Supplemental lighting is responsible for a large portion of the power consumption in growth chambers and greenhouses. Lights not functioning at peak efficiency reduce growth and increase electric consumption. HID lights that appear to be working normally can have a significant reduction in light output without being visually apparent.
Nelson, Jacob; Hay, Alec; and Bugbee, Bruce, "Partial Capacitor Failure in HID Lamp Ballasts: An Insidious Problem" (2012). Controlled Environments. Paper 4.