Watts and lumens — they both describe light bulbs, but do you know what they actually mean? Did you know that wattage is no longer a reliable way to tell how bright a bulb is? Do you know what makes lumens a worthwhile successor? Let’s talk about the two most famous light specs, and how they affect your search for the best bulbs on the market.
Watts are a measurement of energy, specifically the energy needed to power electronic devices like light bulbs. Watts are all about real power, which means they measure the rate at which energy is consumed. One watt equals one joule per second (you can go down that physics rabbit hole if you want to). So, a 100-watt bulb that you turn on for 10 hours uses around 100 watts constantly, or 1,000 “watt hours.” This is also known as a kilowatt, which is a common measurement for the expenditure of energy in your house.
Interestingly, watts do not measure brightness, nor does the number of watts say anything about brightness at all. However, for a long time dimmer bulbs required a lower number of watts than brighter bulbs, so we got used to using watts as a sort of shortcut to describe brightness. This is quickly changing in today’s market, however.
Lumens are a measurement of brightness. They tell you the visible light intensity that is produced by a bulb or other lighting device. Lumens also take into account how the light is projected. A bulb with a hood that only shines light in a single direction will technically produce far fewer lumens than that same bulb shining light in every direction at once. In science-speak, the luminous flux produced by one candela across a solid angle of one steradian equals one lumen. There are many other ways to measure brightness — lux, nits, etc. — but lumens are particularly useful for measuring light in common products like bulbs.
Technology aside, lumens have a close-to-linear relationship with watts. In other words, a certain amount of watts will produce a certain amount of lumens in brightness for a given bulb. However, bulbs and diodes don’t stay the same forever: Some — particularly the brightest bulbs — will decrease in lumens over time as they wear out, no matter how many watts of energy they’re using. This wear and tear affects projector bulbs in particular, and we’ll talk a little more about that below.