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Lumens vs Watts - What Is a Lumen and How Is It Different From a Watt?
Once upon a time, the wattage of a light bulb was used as a universal indicator of its brightness. A 60W bulb, for example, would always be brighter than a 40W bulb. This made it easy to find replacement bulbs of equal brightness.
For a long time, this was fine. But then, the emergence of energy-saving lighting - in particular, LED bulbs - threw a spanner in the works.
Well, the energy efficiency of these new bulbs meant that a higher wattage no longer meant a brighter light. A 10W LED bulb, for example, could be brighter than a traditional 40W bulb.
When LED lighting first emerged, this wasn't explained particularly well to consumers, so buying replacement light bulbs often became a frustrating, confusing affair.
The good news is that there is another easy way to tell if one bulb is brighter than another.
The answer lies in lumens.
What is a lumen?
It turns out that wattage was never the most accurate way to determine the brightness of a bulb. This is because it denotes the power input to a bulb, rather than its light output (which is what we're really looking for).
Instead, we need to pay attention to lumens.
Lumens (lm) are a measure of the amount of light which is visible to the human eye from a lamp or light source. The higher the lumen rating, the brighter the lamp will appear. A typical household lightbulb might emit anything between 300–1000 lumens, while a high-powered floodlight could emit in excess of 20,000 lumens.
The best thing about using lumens to determine brightness is that it works the same way no matter what type of light bulb you're looking at. When comparing an LED bulb with a halogen bulb, for example, the brighter bulb is always the one with the higher lumens count.
All light bulbs should have the lumens count (as well as their wattage) stated on their packaging, and on their online product listings, where applicable.
Do watts still matter?
Wattage alone may no longer tell you how bright a bulb is, but it is still an important measure in other respects.
Dimmer switches, for example, can only handle light bulbs up to a certain wattage (specified by the manufacturer), and it's important for safety reasons that this is not exceeded. It may not surprise you that the development of LED lighting has complicated this too (luckily, we also have a guide to help with that).
Using watts and lumens together, though, we can determine how energy efficient a light bulb is.
Calculating lumens per watt
So far, we've been talking as if a brighter bulb is a better bulb, but this is only half the story.
What we really want is a light bulb that has the right brightness, but which uses only a minimal amount of power. This is where we start talking about energy efficiency.
We already know that higher lumens equals a brighter bulb, and lower wattage equals less power, so it makes sense that a bulb with a high lumens count and a low wattage is going to be more energy efficient than another bulb with a low lumens count and a higher wattage.
To put it simply, to determine a bulb's energy efficiency, we need to know how many lumens it outputs for every watt.
This isn't difficult, as we already have all the information we need. As an easy example, we'll work this out using a 10-watt bulb that emits 1000 lumens.
All we need to do is divide the number of lumens (1000) by the number of watts (10):
1000 / 10 = 100 lumens per watt (lm/w).
Once we've worked this out, it's easy to compare the energy efficiency of different bulbs, irrespective of their type. It works whether you're comparing an LED bulb with an incandescent, or a halogen with a compact fluorescent, and so on. It means that we now have an easy way to tell which bulb is best.
As a side note, it's not worth us trying to say what a 'good' lumens-per-watt number is, as the development of lighting technology these days means that it's improving at a rapid rate. What seems like unbeatable energy efficiency right now might just be the standard months down the line.
Now that you know exactly what you're looking for, you might find that there can be quite a difference between bulbs that otherwise look the same. One 10W bulb might be much more efficient than another 10W bulb; a particular 8W bulb might not actually be as bright as a similar 6W bulb.
Use the simple calculation that we just learnt, however, and you will be well on the way to finding the best and brightest light bulbs.