By Jim Terry
While street-lighting seems like an easy target for cost savings, that's not necessarily so. To decide whether LED is right for your operation, consider:
Energy usage. Replacing a 100 W high-pressure sodium (HPS) light with a 60 W LED produces the same basic light levels. That's a 40% energy savings, but is it enough to justify switching?
Cost-benefit analysis. Here's a basic method for determining a lighting system's operating costs.
First, calculate the energy cost using the following formula:
Then add maintenance costs.
For example, HPS requires a lamp replacement every five years and a new photocell every 10 years. With an average 20-year fixture life, that's five service calls: one for installation, three for lamp changes, and one for the photocell change. For new installations, include the cost of one new fixture. Divide all that by 20 for an annual cost.
Do the same thing for LEDs.
Lamps shouldn't need changing during the life of the fixture; just plan to clean the fixtures and change the photocells every 10 years. That's two service calls: one for installation and one for maintenance. Don't forget to add in the fixture cost because it can be pricey. This process lets you estimate your annual savings.
If initial cost isn't a factor because you're getting, say, an energy rebate or stimulus grant, consider just service calls and energy savings.
Photometrics and color. Manufacturers use different types of optics to project light, but in general you can expect better distribution from an LED fixture than from existing streetlights. While photometric layout will show how existing lighting stacks up against proposed new lighting, photometric software can't measure the color of the light.
HPS produces amber-colored light, while LEDs tend to be cooler, with more of a pure white or bluish cast. Research suggests that the latter enhances peripheral vision, and that it appears brighter than amber. While public reaction supports this view, the Illuminating Engineering Society of North America (IES) has yet to confirm this in its standards.
Compliance with accepted practices and standards. Manufacturer literature may refer to several types of lumens (see sidebar on page 36). Find out if the levels provided are given in mesopic lumens or actual lumens, a distinction that has greater ramifications than you'd think. If a municipal standard requires a certain footcandle level and the fixture cut sheet shows only mesopic lumens, for example, you may not be meeting your own lighting standards.
You want actual lumens. Ask the manufacturer how they measure the light, and ask for the nonmesopic photometry. If they can't provide this, it's a red flag.
Also important: When running a photometric calculation, make sure the .ies file was tested to IES LM-79-08 standards (it'll be noted in the text portion of the file). This refers to the accuracy of the testing and will ensure you get an accurate depiction of real-world, measurable light levels.
One word of warning: .ies files are mostly a jumble of numbers and are intended to be used with photometric calculation software. If you're unfamiliar with this type of software, check with a consultant or ask the manufacturer for assistance.
Electric utility practices. It feels good to be eco-friendly, but it feels even better when there's a financial reward. So make sure to explore the issue of tariffs if your streetlights aren't metered, which is the case more often than not.
Utilities establish a tariff rate to determine how much to bill based on the wattage and predicted maintenance costs of the light. Some utilities have a fixed tariff structure that starts at 100 or more watts, so they're going to bill for a 100 W fixture whether or not the fixture uses 100 W.
Tariffs are difficult and time-consuming to change, so it's crucial to know if your utility has a variable tariff for new technology that takes into account the lower energy usage and maintenance cost of LEDs.
Deciphering the jargon
The three levels of white light
Scotopic: no artificial light in a dark outdoor environment
Photopic: brightly lit environments, such as inside a building or outdoors during the day
Mesopic: in between; occurs when levels are very low, like in the typical street-lighting situation.
For a more detailed cost-benefit analysis of LED vs. high-pressure sodium, click here.