Daylighting is recognized as best practice in energy codes and industry standards due to its documented positive effects on worker satisfaction and performance and potential to generate substantial energy savings. Daylighting and daylighting control are now encouraged or required by California’s Title 24 energy code, LEED-NC v.2.2, Northeast Collaborative for High Performance Schools, and ASHRAE Advanced Energy Design Guide for Small Office Buildings. At the time of writing, ASHRAE 90.1-2010 and 189.1 standards are expected to include requirements for daylighting controls.

While the benefits of daylighting and daylighting control are clear, demonstrated savings vary widely based on many factors.

The New Buildings Institute states daylight harvesting systems can generate maximum potential savings of 35-60%. The Lighting Design Lab states lighting energy savings can reach 60-80% in offices, classrooms and gymnasiums. According to the U.S. Department of Energy, daylight-response switching coupled with skylights has demonstrated energy savings in warehouses of 30-70%.

And that’s not counting HVAC impacts.

One of the challenges in estimating typical savings is it’s difficult to compare high-performance daylighting and glazing strategies against standard designs because of the numerous differences between buildings. How do we know realized energy savings are due to the daylighting strategy and not some other factor such as building orientation?

To address this question, the Energy Center of Wisconsin conducted a controlled experiment at the Energy Resource Station near Des Moines, Iowa. Two sets of four identical rooms provided the comparison testbed, with each supplied by independent lighting and HVAC systems. One set of rooms, the Test Rooms, were configured with high-performance glazing and direct/indirect light fixtures with daylighting dimming control. The other set of rooms, the Control Rooms, were configured with standard clear-glass glazing and recessed fluorescent fixtures with no photosensors or dimming control.

This allowed a direct comparison of lighting and HVAC energy consumption during three rounds of study conducted during the summer, fall and winter of 2003—or a total of 70 days of operation—based on three conditions: 1) the base case described above, 2) reduced fenestration (simulated by the use of exterior panels to partially cover the windows), and 3) adding an interior light shelf to enable deeper penetration of daylight into the room interior.

The Energy Center of Wisconsin measured lighting and HVAC energy savings exceeding 20% based on operating costs of abut $1.13/sq.ft.

Lighting energy savings were determined to be about one-third, or 32%, based on $0.15/sq.ft. annual operating costs for the Test Rooms compared to $0.22/sq.ft. for the Control Rooms, resulting in $0.07/sq.ft. savings per year. Cooling energy savings were measured at 25%, fan energy savings at 3%, and savings in demand charges at 24%, while heating energy increased marginally.

The Test Room fixtures frequently operated at some level of reduced output, around 50% on sunny days. Note that the building geometry offered a ratio of 75% perimeter area (friendly to sidelighting) to 25% core areas, so about a quarter of each room’s floor space did not receive daylight or daylighting dimming. (The windows also did not include blinds.)

The biggest operating cost savings, however—about two-thirds—resulted from lower cooling loads. About one-half were related to reduced demand charges. Interestingly, the three Test Room configurations (base case, etc.) showed very little change in lighting or HVAC energy use, although the reduced fenestration option did produce somewhat higher lighting energy consumption because of less daylight.

The researchers concluded: “The data from this experiment demonstrate clear and substantial reductions in lighting and HVAC energy consumption due to the lighting and window specifications.”

The “Energy Savings from Daylighting” report can be downloaded free here.