Wattage to Lumen Equivalents
Back in the days when most lights were of the incandescent filament types, wattage was a good way to describe light output. Everyone knew what a 40W or 60W bulb looked like. If you wanted a bright bulb for reading, you would select the 60W, if you just needed a light for a hallway, 40W was probably enough.
That changed with the introduction of fluorescent and compact fluorescent (CFL) bulbs. A 20W CFL bulb had a light output equal to a 40W incandescent bulb. Now, with the introduction of LEDs, a 10W LED is equivalent to that same 40W bulb. Clearly, wattages are no longer a good way to assess light output. Instead, we use lumens.
Efficacy is used to describe lumens per watt. In simple terms, efficacy describes the power needed (wattage) to produce the amount of light (lumens). For instance, a fixture that is 5 watts and produces 500 lumens has an efficacy of 100 lumens per watt. Likewise, a fixture that produces 300 lumens at 3 watts also has an efficacy of 100 lumens per watt. A fixture that produces 300 lumens at 2 watts has an efficacy of 150 lumens per watt. So the actual wattage used may not necessarily affect efficacy. Instead, efficacy will vary due to multiple factors related to the light source, such as how hard the LED is being driven and how well the heat it produces is dissipated.
Eventually, we will learn to assess light output with lumens – wattages will no longer interest us. In the meantime, the following table is a rough approximation of lumens and wattage equivalents. Keep in mind, however, that this table can be misleading because of individual lamp characteristics. It's common, for instance, that certain LED fixtures are far more efficient and produce much more lumens than their incandescent equivalents.
Lumens and watts conversion charts for your reference1:
|Lumens for Light Source Watts|
|Lumens||Incandescent Watts||LED Watts|
|Light Source||Luminous Efficacy |