Balancing Warehouse Light Levels with Energy Efficiency

Calculating appropriate light levels for a warehouse is made challenging by the fact that all different types of tasks may take place under one roof – and often within one area. Picking, packaging, shipping, receiving and office work all require different quantities of light in order to assure worker safety and performance. But the real trick is making sure that light levels are high enough to satisfy all these needs while still meeting energy efficiency requirements.

Measuring Light Levels: Lumens and Footcandles

 A lumen is a metric unit measuring the total amount of visible light emitted by a source – its “illumenance.” For example, an 11-watt LED PAR 30 emits between 530 and 560 lumens. When you’re trying to set appropriate light levels in a warehouse, however, what’s important is not so much the illumenance of the light source itself, but how much light from that source reaches the nearby work surfaces. That measurement is taken in “footcandles.” A footcandle is one lumen of light density per square foot. In counties that use the metric system, this measurement would be taken in “lux,” which equals one lumen per square meter.

The Illuminating Engineers Society of North America (IESNA) has published recommended footcandle levels for warehouses based on the activities that take place in various parts of the facility.  For example, in areas of low activity, like cold storage, you might not need more than 5 FC’s. In areas where large items with large labels are being moved around, 10 FC’s might suffice. If the items and print were small, you might need up to 30.

Keep in mind that the work surface in a warehouse includes vertical as well as horizontal planes. Sufficient vertical light levels are needed to read labels on cartons in the stacks and to see what’s ahead when driving a forklift; sufficient horizontal illumination is needed for picking and paperwork.

In addition to the tasks taking place, lighting design should also take into account the age of the workers. In fact, it might be useful to conduct interviews to determine the existence of a safety or health problem caused by insufficient lighting. Symptoms would include eye fatigue, eye strain, headaches, or tripping and falling.

Another factor in setting light levels is the color of the walls and ceilings. Generally, dark colored walls and ceilings require more illumination than light-colored surfaces. Light-colored, highly-reflective ceilings are better able to re-direct the percentage of light that is directed upwards by industrial fixtures. Also bear in mind that metal racks will reflect light while brown cardboard cartons will absorb it.

Energy Efficiency Comes into Play

Yet another factor to take into consideration when setting light levels in a warehouse is the amount of energy the lights will consume.  Generally speaking, you want to keep the wattage down as far as possible to conserve energy while still meeting light level requirements.  

When you assess how much energy is being used to power electric lights throughout an entire building, what you’re measuring is the “lighting load.” It is often referred to in terms of “lighting power density” and it’s measured in watts per square foot. To calculate the lighting load for a commercial building, you would need the following numbers:

• Square footage of area to be lit
• Number of fixtures
• Watts used per fixture

For example, if a warehouse has 80 metal halide fixtures and each fixture uses 400 watts, you would multiply 80 by 400 to get the total amount of energy the lights are consuming - 32,000 watts or 32 kW. If the warehouse is 50,000 square feet, you would divide 32,000 watts by 50,000 square feet to get the watts per square foot - .64. So the lighting load for that building would be 0.64.

If you replaced those fixtures with 150-watt LED high bays, the total energy consumed would be 12,000 watts or 12 kW, and the watts per square foot would be .24. If you take into account the fact that reducing the lighting load also lowers the need for cooling, it’s likely that retrofitting that warehouse with new energy efficient lights will cut the energy load by nearly two-thirds. Yet light levels will not be adversely affected. In fact, those LED high bays will produce brighter, whiter, more uniform lighting with better color rendering and fewer shadows than the metal halides.

Control Systems

Of course, the way to save the most energy is by shutting the lights off entirely – and in certain circumstances, this too can be done without adversely affecting light levels. For example, large skylights or windows may provide sufficient natural light during certain parts of the day to allow the lights to be dimmed or switched off. This is generally accomplished via photosensors attached to the light fixtures. Know as “daylight harvesting,” this not only reduces utility costs, but there’s also evidence that natural light make people more comfortable and productive.

Another place where lights can be switched off or dimmed is in unoccupied areas of the warehouse. This can be done manually or it can be automated through the use of sensors. Occupancy sensors turn lights on automatically when they detect motion, and then turn them off automatically soon after the area is vacated. Vacancy sensors require occupants to manually turn on the lights, but the sensors then turn the lights off automatically when the area is vacated. Either type of control will significantly reduce utility bills and cut carbon emissions while assuring that the moment someone steps into the area, their way will be well lit.