In a study recently published by “Physical Review Letters,” a pair of MIT researchers demonstrate that an LED can actually put out more optical power than the electrical power fed into it. Although this might seem implausible, there is a phenomenon taking place which allows the LED to actually use the heat energy created by the LED to add to its photon production. In the study, researchers were able to definitively demonstrate the remarkable energy efficiency of LED lights. And while their conclusions did not expressly correlate to efficiency with general illumination, it did demonstrate the potential of operating LED lights.
While commercial grade LED lighting isn’t quite as efficient as the MIT researchers portrayed in their study, it is still noticeably more efficient than other forms of light. In terms of general energy conversion to visible light output, today’s LEDs are capable of reaching up to 80% efficiency under perfect conditions, but for the most part average around 60-70%. However, this is a significant improvement over incandescent bulbs which waste up to 90% of their energy as heat. LEDs even surpass metal halide for actual energy to light conversion, with metal halide converting about 50% of the electrical energy supplied into visible light.
Of course, other factors come into play as well when considering real world applications. Incandescent, fluorescent, and HID light sources for example radiate light in all directions. This means that in order to project the maximum amount of their light output onto targeted areas, the fixture housing them must incorporate a reflector in order to redirect much of their output. The problem here however, is that even with an effective reflector, a lot of light is lost to fixture absorption, diffusion, and spillage. The end result is less light reaching the targeted area, adversely affecting how efficiently the fixture can illuminate a given amount of area.
LEDs on the other hand are directional light sources, meaning they only radiate light over a limited portion of their surfaces. The result is less light lost to absorption and more light reaching the targeted area. Because of this, it is possible to use an LED fixture producing less actual lumens to direct the same amount of light onto a target as a more powerful HID or fluorescent. Regardless, the most common way different light sources are compared efficiency wise is to look at their actual lumen per watt ratios, which isn’t a very reliable way to measure actual efficiency. For example outside of the laboratory, the incandescent bulb produces about 14-17 lumens per watt, the metal halide around 65 to 100 lumens per watt, and the LED around 60 to 100 lumens per watt. As we can see, metal halide and LED light sources provide similar lumen output. This does not mean though that they will provide the same efficiency performance in real world applications.
Consider for example an LED and metal halide fixture side by side. Each lamp produces 100 lumens per watt, and each produces a total of 1,000 lumens. However, we would still have two very different lamps, each using 10 watts to produce 1,000 lumens. Now, if we had the metal halide bulb suspended naked over a surface, about only ¼ of those 1,000 lumens would be directed towards that surface, the rest would radiate upwards and off to the sides providing no useful illumination (Fig B). In order to get that lost light onto the target, the metal halide would have to be fit with a reflector. Even with the reflector in place to redirect the lost light downwards only 60% of the lamps output reaches the target surface due to lumen losses from diffusion and absorption. (Fig A) The result is that of that initial 1,000 lumens, only 600 lumens are actually directed downwards towards our targeted surface.
LED on the other hand produces directional light, typically radiating its output over a 120-150 degree area. This means LED fixtures can be hung over target surfaces and receive nearly all of the initial 1,000 lumens without the need for a reflector. (Fig C) Therefore, a LED fixture producing 600 lumens and using only 6 watts could direct the same amount of light to a targeted surface as a 10 watt metal halide producing 1,000 lumens.
In the end, what really counts are the results found through real world application of LEDs and examples of the savings generated by switching to LEDs.
In 2011 the city of Sacramento California replaced all the metal halide lighting within an 180,000 square foot parking garage with LEDs coupled to occupancy sensors. After a short evaluation period, the city determined that an 88% reduction in energy use was achieved, with an annual savings of $34,500 expected as a result.
Advocate BroMenn Medical Center located in Bloomington-Normal, Ill replaced their metal halide parking lot lighting with LED fixtures. Their annual savings amount to $33,000, with another $13,000 in savings expected from reduced maintenance costs.
The Lewis Bear Company in Pensacola, FL, a regional franchise distribution point for Anheuser Busch, performed a full upgrade of its facilities with LEDs. The result was an annual savings of $54,498.
These are only a few examples of just how effectively LEDs can improve efficiency and reduce costs. In each of these cases, and many more like them, not only was efficiency and reliability improved, but light levels and quality were either maintained or improved over the metal halide systems being replaced. So, to answer the question of whether or not LEDs really are that efficient, and can they really save that much money, the answer is a resounding yes.