Is LED Lighting More Energy-Efficient Than Daylighting From Windows?

When reading the recent study, "Demand-side solutions to climate change mitigation consistent with high levels of well-being," a suggestion for reducing energy costs and carbon emissions caught my eye: "Replace artificial light with daylighting and use lighting sensors to avoid demand for lumens from artificial light." A citation in the study led to further research which claimed daylighting saved energy, but it was written in 2002—long before LEDs and solid-state lighting were invented.

I thought about it again when I saw the pitch for this new Solatube Tubular Daylighting Device (TDD) design that integrated LEDs. We wrote about Solatubes in the early days of Treehugger, touting it as a way of getting natural (and free) light into interior spaces. As Solatube International notes in its press release:

"Cost-effective, energy-efficient, and eco-friendly, a Solatube TDD harvests daylight at the rooftop, transfers it down a highly reflective tube, and distributes it evenly into an interior space through a diffuser at the ceiling—on both sunny and cloudy days—with virtually no maintenance."

The new design with the LEDs is pitched as the best of both worlds.

“We developed the commercial Integrated LED Light Kit in response to our customers’ desire for technology that would provide optimal lighting and energy savings,” said Robert E. Westfall Jr., President of Solatube International. “It’s an intelligent fusion of natural light and LED lighting that makes for a clean ceiling design.”

We have written many times on Treehugger about the importance of windows and how humans need access to natural light to keep our circadian rhythms in tune. We recently covered a Swedish study that concluded windows have important social and psychological functions.

But in this age of LEDs, I wondered, is it still even true that windows provide energy or carbon savings by reducing the need for artificial light? When that 2002 study was done, lighting was incandescent, which delivered 12 lumens per watt of electricity, or fluorescent tubes, which delivered about 60 lumens per watt. Now we have LED fixtures and bulbs that deliver over 200 lumens per watt.

Windows, on the other hand, are transparent to heat as well as light. Thermally, the best window doesn't perform as well as the worst wall. I wondered how many watts it takes to heat or cool a space that has windows sized for lighting.

The Solatube is an interesting example because it doesn't provide a view or ventilation but probably does allow some heat gain or loss. They publish the data but I haven't done a heat calculation in many years and can't answer the question: Does it take more watts to cool heat or cool the space because the Solatube or a window is there than it would to get the equivalent light out of LEDs?

Nick Grant, the founder of Elemental Solutions, has run the numbers on windows in Passivhaus designs; I quoted him in "Windows Are Hard," noting windows should have "size and position [that] are dictated by views and daylight" rather than energy gain or loss. I asked him what he thought about windows as light sources. He responded with a concern that architects might all turn into Charlie Munger, designing windowless buildings.

He also thought it would be a tough calculation. "Quite possible that modern LEDs could be more efficient than windows but I’d take that as a reason not to overdo the windows rather than to avoid windows! In terms of doing the sums you could prove anything depending on assumptions," said Grant.

He shares my view that windows should be designed to frame a view and for psychological purposes. "Despite my banging on about efficiency and sufficiency, I’m partial to the odd window placed for a Zen view or mid-winter sunbeam that does nothing for heating or daylight but lifts the spirits," said Grant.

Grant adds:

"Slate Cottage by Charles Grylls and built by Mike Whitfield had a lot of thought put into window placement and I think it works great. Budget was small but design aspirations high so every window had to work for its keep."

Architects previously designed windows as a source of heat, combined with thermal mass for storage. It was hard to get right. Writing in Green Building Advisor, Martin Holladay concluded windows “should be limited to that necessary to meet the functional and aesthetic needs of the building.”

The Turkish playwright Mehmet Murat Ildan wrote, “Your desire to be near to window is your desire to be close to life!” They are wonderful things that should be in every habitable room. But we should measure their impact on well-being and happiness—not watts or lumens.