Illuminating the Debate on Energy Efficient Site Lighting
When looking at electricity use in developments made up of several buildings, it is not uncommon to see extreme variation in the common area electricity costs from development to development. Diving into the data, it is obvious that exterior lighting makes up a large percentage of common area bills. Site lighting for pedestrian pathways, building entrances and parking lots present a great opportunity for implementing greener options and realizing substantial savings on electricity bills.
In the green building world there is currently a debate about what the best type of fixtures are for large scale outdoor lighting. A question we hear all the time is: how do LEDs compare to other higher efficiency exterior lighting options such as metal halide lamps, which is a type of high-intensity discharge (HID) lighting.
How can we compare site lighting options?
The simplest way to compare the relative efficiency of lighting is to look at the “source efficiency” of the lighting source (the “lamp”). Lamps are rated in lumens per watt. This metric essentially defines how much light is generated for every unit of energy (each watt) put in. A higher lumens/watt means more light is generated for each unit of energy—it is more efficient.
If it were only so simple. Source efficiency is proving to be a problematic metric to compare lighting for a number of reasons. Once a lamp is in a fixture, it performs differently than the lamp does in a lab setting. For example:
- A substantial percentage of light emitted from the lamp is not emitted directly to the place where the light is needed. It is essentially reflected back and lost inside the fixture. Therefore the number of lumens actually lighting a particular surface (the luminare efficiency) is different than the source efficiency. Unfortunately, most manufacturers only publish the source efficiency.
- Heat is a by-product of operating the light and tends to build up over time while the light is turned on. Overall lumens produced tend to diminish as the fixture heats up, which adds an element of variability that is often not accounted for.
- Exterior lighting fixtures almost always include a covering over the lamp for protection against weather and for durability. Light produced by the lamp itself is not 100% transmitted through this cover which further reduces the amount of lumens reaching the surface.
- The lumen output deteriorates over the life of the fixture, and some types of fixtures will depreciate faster than others.
Below is a graph showing how lumen output depreciates over time for a fixture:
LED vs. Metal Halide
LED and Metal Halide lighting tend to have very similar source efficiency numbers, meaning the lamps convert input power (wattage) into light (lumens) at almost equivalent efficiencies (usually around 100 lumens/watt). But given all the variability in performance and efficiency that can occur when a blub is deployed in a fixture, it is very difficult to compare fixtures to determine which is the most efficient. There are no industry practices or regulations that require testing of luminaire efficiency.
So what can be said about LED and metal halide site lighting to come to some understanding of what is the best option for a project?
A look at some more qualitative considerations for LED and metal halide lighting can help guide a choice.
LED lights tend to:
- Provide uniform, directed lighting
- Minimal color distortion
- Fixtures for LED lamps tend to accumulate less heat
- Have a longer life (30,000 hours on average as compared to 10,000 for MH)
- Cost considerably more
Therefore, it may be the case that to light an entire site, fewer LED lamps are needed to get the same job done due to the directed, uniform and high quality light the fixture produces. And if color rendition is an important consideration, LED may be a more favorable choice as well. Additionally, if the lighting is in an area that will be difficult to access, LED lights provide a potential benefit in that they need to be replaced less frequently. So the cost of replacement (e.g. renting a lift) is an important consideration.
In summary, as the measurement, testing and reporting standards currently stand, it is not clear which option will use less energy. HID lighting and LED lighting are both great improvements over incandescent lighting. But site-specific considerations and what your priorities are for your exterior lighting strategy can be the more useful guide in selecting your next fixture. The cost of a lighting designer for a one-off project may tip the scales in favor of the more traditional fixtures.