In lighting, efficacy is a measure of how much light is produced per unit of energy used. It is typically measured in lumens (lm) per watt (W). The higher the efficacy, the more efficient the light source is and the lower energy costs it generates. LED lights are known for their high efficacy, with many models producing up to 120 lm/W. CFLs are also relatively efficient and have an efficacy rating of between 60-90 lm/W. Incandescent bulbs have a low efficacy rating at around 15 lm/W.