A comprehensive restoration of Detroit’s street lighting system is poised to save the beleaguered city nearly $3 million a year – and the project comes with an estimated payback of just two years. Could an outdoor lighting retrofit at your facility slash your electricity expenses in the same way? An in-depth study by the DOE examines how the city maximized savings from a much-needed lighting update.
Before the retrofit, only about half of the city’s 88,000 high-pressure sodium street lights worked reliably and many existing units were removed from residential areas to save money, resulting in intermittent stretches with no illumination on many blocks and leading to dissatisfaction for residents.
To make matters worse, like many municipalities, Detroit’s street lights are unmetered – the utility calculates a monthly charge based on the size of every luminaire and ballast, the average monthly hours of use, the price charged for street electricity, and the monthly tariffs for each size of luminaire. This means that Detroit pays for every light regardless of whether it’s working – even when 50% or more of them are dark.
The largest share of savings comes from the choice of LED lighting to replace the high-pressure sodium system in place. The new system distributes 65,000 LED lights in 88.2, 106 and 262.5W, which will save an anticipated 46 million kWh and $2.9 million annually compared to the 70, 150, 250 and 400W units used in the old system. If the new system is instead compared to the savings from replacing the old units with 65,000 modern high-pressure sodium lights at wattages of 150, 250 and 400W, the LEDs save roughly 34 million kWh and $2.4 million.
The city also found savings through these strategies.
- Reduced maintenance costs: “Maintenance savings are more difficult to quantify than energy savings given the extraordinary circumstances surrounding the failed condition of the previous system,” the study authors write. However, conventional relamping generally takes place every four to five years, the authors note, whereas the replacement cycle for the new LEDs would be closer to 15-20 years.
- Eliminating copper wiring: The LEDs’ reduced power demands allowed the city to use aluminum distribution wire, which both reduces the upfront cost and deters copper thieves thanks to the lower inherent value of aluminum. The lack of damage from theft helps lower maintenance costs even further.
- Increasing distance between lights: The original system placed lights in alleyways and multiple poles on every residential block, but the new design reduces the number of luminaires for residential streets to lower both first costs and future O&M costs. A typical 600-foot block might feature a light pole on each corner and one in the center for a 300-foot spacing, with all other street lights removed, the study says. However, 300 feet “is a very long throw for any type of shielded luminaire, and consequently there are significant areas of darkness and related glare issues in the spaces between poles, sometimes yielding a negative response,” the authors say.
Because the lighting system needed to be repaired and replaced no matter what type of lamp was used, the study authors examined the cost of the LEDs compared to new high pressure sodium lamps when calculating simple payback. Assuming a cost of $100 for a conventional cobrahead luminaire and ballast, the additional investment required for LEDs will be paid back from energy savings alone in 2.3 years. Maintenance savings will shorten the payback period further.