Efficacy is the ratio of light output from a lamp to the electric power it consumes and is measured in lumens per watt (LPW).
Below is a list of different types of light bulbs and the light output of each (measured in lumens per watt):
Conventional incandescent lamps in a single four-way traffic light consume roughly 85 kWh of electricity per day and cost about $1,600 per year to operate. LED lights use just 10 percent of the electricity that incandescent lamps use, so the opportunity for savings is enormous.