What's a Watt?
A watt is a measurement of how much power an electronic device uses. In this case, how much energy a light bulb uses.
Technical definition: the SI unit of power, equivalent to one joule per second, corresponding to the power in an electric circuit in which the potential difference is one volt and the current one ampere.
What you need to know: The higher the wattage of the light source the more energy it uses.
What's a Lumen?
A Lumen is a measurement of brightness. They account for how much visible light is being produced and also how large of an area the projection of the light covers. One lumen is equivalent to one candle’s worth of light.
Technical definition: the SI unit of luminous flux, equal to the amount of light emitted per second in a unit solid angle of one steradian from a uniform source of one candela.
What you need to know: The higher the lumen count, the brighter the light.
What's a Kelvin?
A kelvin is a measurement of light color.
Technical definition: the SI base unit of thermodynamic temperature, equal in magnitude to the degree Celsius.
What you need to know: The higher the Kelvin count the “cooler”, or more blue, the light color. The lower the Kelvin count the “warmer”, or more red, the light color.
You can see the different light colors from warm to cool , with their Kelvin counts, in the picture below
Figure 1. Color of Lighting with Corresponding Kelvin Count. Any-Image.com. www.any-lamp.com/lumen-to-watt.com
What's the Difference? Why Buy Based Off Lumens?
Essentially, watts tell you how much energy a light consumes, lumens tell how bright a light is, and Kelvins tell the light color.
In the past you would use watts as an indicator of how bright your light is. However, lights have gotten much more energy efficient with the introduction of LEDs and CFL’s, so now you cannot tell how bright a light is based on watts alone. If you are buying a light based off of brightness, use Lumens to help guide your decision.
You can see how Watts are not a reliable indicator of brightness below:
How Can I Convert Watts to Lumens?
The U.S. Department of Energy has a useful converter for Lumens and Watts here
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