Understanding Energy Performance Metrics

Hypothetical situation: You’re looking to purchase a new car. With rising gas prices, fuel efficiency is an important consideration. But would you overlook all other aspects of a car’s performance to get the best fuel efficiency possible? Probably not. “No one would buy a car that does not go very fast and takes a long time to get to maximum speed”. Why then would you do that with your lighting system? If the energy efficiency of a lighting system overshadows the quality of the lighting, the system is inadequate.

Engineers, Lighting Designers and Architects have to practice a similar balancing act to achieve an energy efficient lighting system that also satisfies the needs of building occupants. If you approach it from a pure energy standpoint you might be far from happy with the results. Simply changing light bulbs may be a low-hanging fruit option, but if you don’t look at it holistically, the lighting quality suffers.” Looking only at energy efficiency may initially help the bottom line, but in the long term, looking at lighting systems both from energy-related metrics as well as quality-of-light metrics will save money and keep the building occupants happy.

Energy Performance Metrics

The baseline standard to which efficient lighting systems adhere is ASHRAE 90.1, Energy Standard for Buildings Except Low-Rise Residential Buildings. The standard, continually updated as new lighting technology is developed, sets lighting power density limits for different types of facilities. Municipalities often use ASHRAE 90.1 as a standard for local building codes. Because the standard is being constantly updated, different versions of the standard are used by different municipalities, so it’s important to research local codes. To determine if a lighting system meets or beats code, engineers and lighting designers can use a variety of quantitative measurements.

Lighting power density is measured in watts per square foot. To determine a lighting system’s watts per square foot, multiply the number of lighting fixtures by the watts used per fixture. Then, divide that number by the total number of square feet being lit. For example, a 5,000-square-foot office space with 50 lighting fixtures, each using 120 watts uses 1.2 watts per square foot. Compare the watts per square foot number with the most current ASHRAE  standard to determine where a lighting system falls on the scale of efficiency. With the target now being set at 0.6 watts per Sqft it would be a long way adrift.

Lumens are a measurement of the volume of light a single lamp emits. Factors such as the fixture, age of the lamp and voltage of the circuit all affect the lumen output. Lumens are used to determine a measurement called efficacy, or lumens per watt. This measurement compares light output to energy consumption and indicates how efficient the lamp or the lamp-and-ballast system is at producing raw light output. The higher the lumens-per-watt number, the more efficient the lamp.

Lumens are also used to measure footcandles. Footcandles is a measurement of how much light actually hits a surface. Using general guidelines engineers  can determine if enough light is reaching the work surface of a desk, for instance. This measurement has often been used as the main factor in determining appropriate lighting for a space, but only looking at footcandles to determine lighting quality is backwards thinking. If you address all the lighting quality issues first, footcandles often take care of themselves. By Increasing the quality of light, footcandles can sometimes be reduced significantly to save energy, while users can still see better than before so why would you worry about footcandles first, again, old school thinking.

“Lighting quality is a result of addressing a full complement of criteria — a holistic approach. There are several common quality-of-light metrics you should consider when installing a sustainable lighting system.

The color temperature of a lamp, measured in degrees Kelvin (K), can largely influence the ambiance of a space. People often associate types of lighting with different settings and moods — very cool fluorescent lighting with a doctor’s office, for example. Using cool fluorescent lamps in other spaces will bring the same mood to the space. For example, residential incandescent lamps have a color temperature of 2,700K, or very warm. (Warm refers to the reddish light from a lamp; cool refers to a bluer tone.) Typical fluorescent lighting ranges between 3,500K and 4,100K, much cooler on the spectrum. That’s why occupants generally perceive fluorescent lighting in offices to be cold, clean or harsh. The light from incandescent lamps is thought to be much more intimate and personal.

The engineering and design community can use these colour temperature perceptions to their advantage. In areas where an intimate setting is desired, such as a small café or lounge area, use warm color temperature lamps. Retail spaces, areas where graphics work is done or any other space that requires perceived sharp lighting can benefit from a cooler temperature lamp.

Another aspect related to lighting color is the color rendering index (CRI). CRI is a measure of a lamp’s ability to render colors accurately. The scale ranges from 1 to 100, with 100 representing the same rendering ability as an incandescent lamp.

Everyone has a pair of pants or a tie that looks one color when you put it on at home under incandescent lamps or daylight,” he says. “But when you get to the office with fluorescent lamps, it looks like a completely different color. The goal should be to get the best colour rendering lamps possible. “When you select a light source, especially fluorescent, you have two choices: good or better CRI, The better CRI lamp costs a little more, but studies show that the better the color spectrum, the better the visibility.” Occupants in spaces with higher CRI lamps will perceive the light as crisper, because colors and shades will be rendered more clearly.

Other lighting quality issues can’t be measured or obtained from the manufacturer in a spec sheet. But they are just as important to consider in a sustainable lighting system design.

Glare — both directly from a light source to the eye and indirectly off a computer monitor or other surface — is a major issue, especially in office environments. Taking steps to reduce glare from both electric lighting and daylight sources will reduce strain on occupants’ eyes and garner a more favorable view of the lighting system.

Another simple way to help ensure occupants are happy with the lighting system is illuminance contrast. It is often overlooked because many engineers have never heard the term. Again, it’s best explained anecdotally: How do you feel outside on a sunny day vs. a cloudy day? “You always get the same answer. People prefer sunny days. Recent research has shown that the contrast between sunny and shaded areas on a sunny day stimulates the pleasure center of the brain through the optic nerve. It’s a simple concept that can be used in facilities to give more energy, interest and life to a space. Designers can use properly placed accent lighting to add contrast and shadows in some areas.

Striking a balance between energy efficiency and lighting quality is what will produce the best lighting system possible, but efficiency is in the eye of the beholder. Although engineers can implement very austere sustainability practices and achieve low energy bills, if occupants are dissatisfied or unable to work optimally, the savings and sustainable prestige are worth nothing.

A popular step in saving energy costs on lighting is lighting controls. However, the controls, especially motion sensors and automatic shut-offs, have sometimes frustrated occupants. This is generally because occupants have not been properly educated on how the controls work, what to expect and how to operate them properly. “The most important thing you have to do is educate the user”. The first thing they’re going to do is complain that the lights are off.” An easy way to prevent this is to post signs around the facility or near the lighting controls explaining the need for them, what to expect and how they benefit the facility.

Another way to give occupants more control over the lighting to accommodate different needs is to implement a lighting strategy incorporating layers of light. A space like an office is usually divided into three layers: ambient light, or the light that illuminates the bulk of the space, generally by overhead fixtures; task lighting, which is stationed in specific work areas; and accent lighting, which adds interest to the space. By keeping ambient light levels relatively low, engineers allow occupants to decide if ambient light is enough or if they’d like to turn on task lighting. That way, each occupant uses only the amount of light necessary, keeping energy costs lower.

Perhaps the biggest asset to both sustainable lighting systems and occupant satisfaction is daylight. “Nothing is more energy efficient than free”, therefore, take advantage of every possible source of daylight available. “We have to start utilizing the free natural resource of daylight in good lighting design”.

That doesn’t mean however that daylight should just be allowed to stream in through windows and skylights without carefully considering it as part of the entire lighting design. Daylight should be integrated with electric lights so they work in tandem to keep the lighting levels appropriate in the space. Shading systems, louvers, light shelves and photo-sensor lighting controls that automatically dim the lights can all be elements of a successful daylighting strategy.

Daylight and artificial light can work together quite well, providing that glare is satisfactorily addressed and automated systems are installed. Everyone has had the experience of walking through an airport, mall or other public building and noticing the lights are on in the middle of the afternoon while daylight is streaming in through the windows or skylights. That’s exactly what we should all try to avoid by installing dimming controls.

To avoid complaints or confusion, it’s important — as with occupancy sensors — to educate occupants about how to operate the dimming controls and what to expect when they’re operating properly. When operating as designed and commissioned, daylight dimming systems are one of the greatest assets to energy efficiency in a building.

Integrated daylight dimming systems may not be as easy to implement as a simple across-the-board relamping project, but the benefits to the facility’s bottom line and the occupants of the space far outweigh the time and effort required. When thinking about implementing an energy efficient lighting system, Architects and engineers need to lay out a strategy that is appropriate for the space and go for the investments that make sense for the long term, not just the low-hanging fruit which may turn out to be rotten.”

By Jon Lewis