The efficiency of any device, machine or process is how much work gets done for each measure of energy it takes.
In other words, to determine a light bulb’s efficiency you need to know how much light it produces (the ‘work) and how much electricity it consumes (the ‘energy’). To do this, you need to establish two things which can be found on the box your light bulb came in.
Both the wattage and the lumens should be clearly displayed. To establish the efficiency of the bulb you need to divide the number of lumens by the number of watts. This gives the standard measure of light bulb efficiency, the lumens per watt.
Simply put, a higher number of lumens per watt means more light produced for less power, so more efficiency. A lower number of lumens per watt means less efficiency as less light is produced for more power.
To take an example an average 40 watt light bulb uses 40 watts of electricity to produce 490 lumens. That light bulb’s efficiency is 12.75 lumens per watt.
A typical 40 watt LED light bulb uses just 6 watts of electricity to produce 450 lumens. That’s nearly 75 lumens per watt, almost a 33% increase in efficiency.
So it really does make sense to switch to LED lighting wherever possible.