LED technology has been nothing short of revolutionary for the lighting industry. Compared to traditional lamp types like incandescent and fluorescent, LEDs offer more energy efficiency, better light quality, and a longer lifespan—heavy-hitting benefits that have helped drive their rapid adoption across sectors. According to the U.S. Department of Energy, LEDs accounted for 48% of installed lighting units in 2020, up from 8% in 2015, and 1% in 2010.
But as their popularity grows, so too does the competition among manufacturers to deliver the brightest and most efficient LED fixture. In the lighting world, these characteristics are commonly communicated using ‘luminous efficacy,’ a measure of how effective (or we should say, efficacious) a light source is at converting electrical power into visible light.
While efficacy is an important metric for understanding a fixture’s performance, it’s become somewhat embattled recently, as manufacturers struggle to constantly one-up each other. One of the easiest ways is to measure the output of the bare LED chip rather than the complete fixture (including secondary optics like lenses) during testing. In doing so, manufacturers achieve impressive results, but they also make it very difficult for specifiers and end-users to meaningfully compare specs—or to know the true output of a system.
In this article, we’ll dive into what efficacy is (and it isn’t), why it’s relevant to solar lighting, how manufacturers use it to misrepresent their products’ capabilities, and some tips for separating trustworthy, efficacious products from those just claiming to be.
The Illuminating Engineering Society (IES) defines efficacy as “the total emitted luminous flux divided by the total source electrical input power.” It’s typically expressed in lumens per watt (Lm/W or LPW) and it measures how well a light source converts electrical power into visible light. For example, if you have a fixture that emits 1500 lumens with 20W of power, the efficacy would be 75 LPW. If you had a fixture that emits 2500 lumens at 20W, its efficacy would be 125 LPW—significantly higher because it generates more light with the same power.
Even though efficacy is essentially a measure of how efficient a light source is, it is not the same as “efficiency.” They are two distinct (though commonly confused) lighting concepts. Whereas efficacy represents the amount of emitted visible light in relation to the power used, efficiency represents the amount of light produced by the light source in relation to the amount of light emitted by the fixture (i.e. what it’s capable of versus what it does in reality). Efficacy is the more useful metric for comparing fixtures, just be aware that the terms are often used interchangeably, but they are not one in the same!
Considering that most lighting infrastructure in the U.S. is connected to the grid and metered (i.e., you pay for what you use), it’s easy to see why a higher-efficacy fixture would be attractive. More lumens per watt essentially means getting the same light for less money, plus it puts less strain on the grid, frees up energy for other uses, and reduces the risk of blackouts, brownouts, and power surges.
These are all good reasons to use high-efficacy fixtures—if your lights are connected to the grid. But what if they’re powered by the sun, and therefore free to operate? Does reducing energy consumption even matter when energy is free and plentiful?
The answer is unequivocally yes. As we’ve explained, a higher-efficacy fixture means more light with fewer watts. Fewer watts mean less energy is needed to power it throughout the night. And a lower power requirement means fewer and smaller solar panels and batteries, resulting in more affordable products. (It also means manufacturers can reduce the Effective Projected Area (EPA) of their systems, recommend cheaper poles, and make solar lighting a sustainable solution for high-latitude locations.)
In a quest to maximize efficacy and draw customers’ attention, some manufacturers have begun misrepresenting the real-world performance of their fixtures. The most common tactic is to remove secondary optics like lenses during testing, measuring the unfiltered output of the LED chip rather than the complete lighting unit or fixture.
Removing the lens allows them to achieve very high LPW values because lenses, like drivers, temper the chip’s output. The results might look good—200 LPW good—but they’re produced under lab conditions and are all but impossible to replicate once the chip is integrated into the fixture and installed at the project site. (It’s also worth noting that lenses play a critical role in distributing light and ensuring uniformity, so it’s not like you can just forgo them.)
This wouldn’t be such an issue if manufacturers were upfront about how they measure efficacy. However, most don’t specify whether the efficacy reported on their technical sheets is for the LED chip alone or the compete fixture, including the driver and lens. This makes it very difficult for prospective buyers to do their due diligence and compare specs—but it doesn’t make it impossible.
The importance of LM-79 testing
We’re going to let you in on an industry secret we wish wasn’t a secret: LM-79 testing. LM-79 is an IES-approved method for measuring the optical and electrical characteristics of solid-state lighting (i.e. LED) products, including total luminous flux, intensity, chromaticity, and yes, efficacy.
The most important thing to know about LM-79 is that it is only performed on complete luminaires and cannot be used for “components of SSL product, such as LED packages or LED arrays.” It uses highly specialized equipment like integrating spheres and goniophotometers to capture a snapshot of the system’s capabilities at a certain point in time at a single temperature, providing data that can be directly compared between manufacturers and models. This, it probably goes without saying, is what you want!
The second most important thing to know about LM-79 is that it is the only test that provides reliable data on lumen output and efficacy. There are other IES tests—LM-80, TM-21—but these are concerned with lumen depreciation, color shift, and lifespan. They’re also performed on components (those LED chips you shouldn’t pay attention to!). As NVC Lighting put it, “For an end user, specifier, or installer to judge the relative merits of an LED lamp or lighting fixture, an LM-79 test report is recommended.”
How do you know whether a product has undergone LM-79 testing? Spec sheets and IES files are good places to start. These are generally available to download from manufacturers’ websites, and a quick Control +F for ‘LM-79’ should yield an answer. We scanned three Tier 1 street lighting products and were able to easily locate it on all three. (Tier 1 manufacturers like Acuity and Cree generally have more rigorous testing standards than smaller, lesser-known companies.)
Other red flags to watch for
Other than not having LM-79, there are a few other things customers can look for to identify implausible efficacy claims. The first is very simple: efficacy ratings that fall outside the range of reputable manufacturers. Looking at the examples we referenced above, you’ll find they span from about 120 to 160 LPW depending on the performance package. If the system you’re eyeing claims significantly more—or less—than this, that should give you pause. Ask the manufacturer how they arrived at this number, whether it pertains to the fixture or the chip, and whether it has been independently verified.
Another warning sign is efficacy that doesn’t drop with higher lumen packages. For example, if a manufacturer claims a fixture has an efficacy of 150 LPW when powered at 20W and can supposedly maintain that same efficacy when powered at 100W—that’s a problem. Lower watts should result in higher efficacy; higher watts should result in lower efficacy. Manufacturers that claim the same efficacy at all outputs is a red flag.
Making informed purchase decisions
Understanding efficacy and using it as a point of comparison is key to making an informed lighting purchase. However, not all manufacturers measure efficacy in the same way—or even call it the same name. Fortunately, independent tests like LM-79 standardize the way efficacy (and other characteristics) are measured, making it much easier to draw equal comparisons between models and brands, and ultimately, to select a system with the right capabilities for your project.
If you can’t find the information you need on a manufacturer’s website or technical documentation, here are some questions you can ask:
Have a question for us—about efficacy or anything else? Our team of solar experts are standing by and would love to hear from you.
