Getting more sunlight per watt

Sunlight gives us light at no charge, which we can harness in our buildings to reduce our reliance on electrical lighting, while providing a more enjoyable indoor environment.

Leave it to an engineer to tell us how much that sunlight actually costs us. Lumens per watt (lpw) is the measure of lighting efficacy, telling us how much light (lumens) we get out for how much power (watts) we put in. The chart below shows typical efficacies of different lighting technologies, including incandescent (14), LED (30–50), T-5 fluorescent tube (95), and more.

The chart is courtesy of David Kaneda, AIA, PE, of Integrated Design Associates Inc. (acronym: IDeAs), who along with Peter Rumsey, PE, just wrapped up a great session here at the AIA convention called "The Paradox of Green Engineering."

Kaneda calculated the cost in electricity of using sunlighting, and found that it is 105 lpw. Most of that is accounted for by the cooling load from heat introduced by the light (Kaneda works out of California, by the way). Then, he adjusted the calculations to factor in high-performance glazing using low-emissivity coatings which allow transmission of visible light but reject heat. That boosts the performance of daylight to a whopping 175 lpw.

No doubt these are back of the envelope calculations that don't take into account all kinds of things, including location, season, shading devices, orientation, and tuning glazings of different sections by orientation (something we have written about in EBN, here. Also keep in mind that although it can be jarring to put any kind of cost on something like sunlight, these are very favorable numbers. Remember that LED lighting, currently hyped as the lighting of the future, is at about 30 lpw in many applications, and not more than 60.

If nothing else, it's another good reminder that more isn't always better with sunlight -- that sunlight can come with a tradeoff in cooling loads and discomfort. Remember the fundamentals of sustainable design and work with the sun's path, not against it.