Luminous efficacy is the amount of visible light produced per unit of electrical power and is expressed in lumens per watt (lm/W). For a complete work light, use operational lumens divided by power consumption when comparing product-level efficacy. Source-level LED efficacy and complete-lamp efficacy are not directly comparable because the finished product includes driver, optical, lens, and thermal losses. Efficacy also does not describe light distribution or durability. A slightly lower lm/W lamp can provide better task visibility if its optics deliver more of the available light to the required area.