Lumen vs. Lux vs. Watt – What is the difference?

A Lumen (or luminous flux, or Lm) is the total number of "packets of light" (or quantity of light output) produced by a light source.  For example, a 100-watt incandescent lamp emits about 1300 lumens. Read more here.
The lux (or lx) is a ratio of illumination (or lumens) over a distance: 1 lux = 1 lumen per square meter. 

For example, sunlight on average ranges from 40,000 to 120,000 lux, while an office ranges from 300-500 lux, with moonlight representing about 1 lux.

Simply put, lux is different from lumens because lux takes into account the actual area of which the lumens are spread, while lumens simply represent the total quantity of light produced by a light source. Read more here.

Wattage is a unit of power that represents one joule of energy per second, and is used to determine the energy consumption of a light (or electricity used).  The wattage of a light is broadly considered to correlate with the brightness or lumen output of a light; however, there is no linear defined correlation because every light type produces light at a different level of efficiency (or "efficacy"). 

For example, a typical LED light will produce 80-110 lumens of light per watt consumed, while a standard incandescent light will produce only 5-10 lumens of light per watt consumed.

This ratio of lumens per watt is also known as the light's efficacy, and is a commonly referred to measurement of light efficiency used to measure energy consumption of lighting. More on efficacy here.

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