Efficacy is a term used when the input and output units of measurement differ. In lighting, the light produced in lumens is compared to the electricity input in watts. The resulting measure of performance is expressed in Lumens per Watt. Efficiency is a dimensionless term used when input and output units of measure are the same. Lighting fixture optical efficiency is the ratio of the total lumens exiting the fixture to the total lumens produced by the light source. Power supply efficiency is the ratio of power delivered to the load to the power taken from the electricity source.