The formal answer to this question is horrendous.
“The lumen is the standard unit for the luminous flux of a light source. It is an SI derived unit based on the candela. It can be defined as the luminous flux emitted into unit solid angle (1 sr) by an isotropic point source having a luminous intensity of 1 candela. The unit lumen is then equal to cd x sr. The abbreviation is lm and the symbol is Φv. In terms of radiant power (also called radiant flux) it can be expressed as:
Luminous flux in lumens = Radiant power (watts) x 683 lumens/watt x luminous efficacy” according to HyperPhysics, a publication from Georgia State University.
That means nothing to anyone but top-tier nerds.
A lumen is the amount of light seen by the eye coming from a single source, like a lightbulb, according to a publication from the National Optical Astronomy Observatory (NOAO).
So What Exactly Is A Lumen
Think of a lumen like the amount of water coming out of a hose. Sometimes it’s only a few drops at a time, other times it’s a few gallons. In the case of commercial LED light strings, you’d generally want the latter.
Many of us may confuse watts with lumens, which seems logical, but they’re not the same. A watt is the energy per second required by a bulb. Going back to our metaphor, think of it like how much pressure is needed to push a few drops out of the hose or a constant stream.
In other words, that’s not the same thing.
A lumen is also not the amount of light on a surface. That’s luminance, which is measured in lux. That’s the intensity of light within a specific area. Returning to our metaphor, it’s like the amount of water from your hose in a particular spot in your yard. (It’s worth mentioning this isn’t a perfect metaphor.)
“One lux is equal to one lumen per square meter (lux = lumens/m2). Essentially, as light travels from the emitter, it will disperse throughout an area. The further the light has to travel the more it will be dispersed,” according to Banner Engineering. “Therefore, the amount of lux in an area or on a surface can vary depending upon the distance the light travels and the angle at which it is dispersed.”
So, to reiterate:
Lumens are the amount of light from the light bulb.
Watts are the amount of power needed for the light bulb.
Lux is the amount of light on a surface in front of the light bulb.
If you’re used to looking for watts, don’t stress about swapping over to lumens.
What Lumen Value Should I Be Looking For In My Lights?
Generally speaking, look for a bulb that provides about 1600 lumens if you’re used to buying 100 watt bulbs. Similarly, go for 1100 lumens to replace a 75 watt bulb. A 60 watt bulb will equal about 800 lumens. A 40 watt bulb will go for about 450 lumens.
This relates to “efficacy” in light bulbs.
If you want to know how efficient a bulb is, you can calculate its efficacy, which is the output in lumens of light divided by the input in power in watts. Once you divide those numbers, you see how energy efficient your light is. Performing this bit of math could help you spend less money per light bulb per year.
This is where LED lights come into play. They’re far more efficient than, for example, their incandescent counterparts, which lose 90 percent of their energy. Switching to an LED saves considerable money for the average consumer, relative to their energy usage.
By and large, if we all did it, there’d be significant impact.
“By 2027, widespread use of LEDs could save about 348 TWh (compared to no LED use) of electricity: This is the equivalent annual electrical output of 44 large electric power plants (1000 megawatts each), and a total savings of more than $30 billion at today’s electricity prices,” according to the U.S. Department of Energy.
However you choose to light your home, which is completely up to you, you’re now armed with the rudimentary building blocks of what lumens are and why they’re a more advantageous way to measure home, business, or recreational lighting. Knowledge is power, or in this case, lower power bills. Use it wisely.