A: Efficiency generally relates to one of two ratios in regards to lighting fixtures. It can refer to the ratio input power to output power of the LED driver (driver efficiency) or it can refer to the ratio of lumens exiting the bare lamp and the light exiting the total fixture (exiting light can be reduced due to lamp shades, reflectors etc.)
Efficacy is a term that is generally used when the input units and output units are different. For lighting, we are concerned with the conversion of electrical energy (watts) into light (lumens). The efficacy of a lighting fixture is a measure of the amount of light produced per unit of electricity and will take account of energy lost by the power supply (AC/DC conversion), energy lost converting the DC energy into light (LED), and the energy lost in directing the light to where it is needed. It is measured in lumens per watt.