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Getting more sunlight per watt

Posted May 16, 2008 09:54 AM by Tristan Roberts
Related Categories: Science & Tech
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.

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1 Thanks for the great little g posted by Anita Wells on 03/26/2010 at 01:11 pm

Thanks for the great little graph.All I wanted to know was if 2 ,12 watt florescent standard T-5 tubes be the equilivent to 1, 24 watt hp T-5 tube. Your chart tells me yes, all other things being equal. simple question hard to find answer. Must have visited 20 different sites before I got here. So thanks again.

2 That's a thought-provoking pe posted by Ethan Goldman on 05/19/2008 at 09:23 am

That's a thought-provoking perspective on the costs and benefits of natural lighting, and I'd like to see a little more details on the calculations behind it. For example, does it count the lumens of sunlight provided, or the desired/required lumens of artificial light that it would offset? Are the numbers all in site energy or in primary energy? The point of this comparison seems to be as part of the economic calculation of optimal glazing area, so I presume the units are "lumens offset per watt expended".

However, I have to take issue with the subtitle "plus it's free!" The very point of the calculation is to show how much additional energy must be expended on space conditioning (which is not free) when glazing area is expanded. In fact, not only is the energy for daylighting not free, but since you can't turn it off (the energy is expended whether or not the light is needed), it's "free" like the included minutes of your cell phone plan: you are going to pay the same amount per month regardless, so you might as well use as much as you can.

3 The issue of turning off dayl posted by Susan Aiello on 05/11/2009 at 08:08 am

The issue of turning off daylight when it is not being used is a good one. I have been an interior designer for many more years than I have been a LEED AP, and have been turning off excess daylight for a number of years by using window treatments.

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