Shortchanging Daylight

June 13, 2011 / no comments

The reason for daylighting in buildings is to save energy, and so the value (“payback”) of that daylighting can be calculated by predicting and pricing the amount of energy saved. That’s a common line of thought which is easy to slip into, but it’s dead wrong.

Let’s look at a simple example of a new office building. A typical office worker’s space, including adjoining corridor, is about 110 square feet. Under today’s codes, we’re allowed 100 watts maximum to light that space. If it’s lighted 10 hours per day, 5 days per week, 52 weeks per year, that lighting will use 260 kilowatt-hours per year. At a high-end cost of $0.20 per kWh, that’s 52 bucks for electricity to light that space for a year. Let’s add another 30% for extra cooling cost due to that electricity, and we’ve got almost 68 bucks.

If our worker is the median American clerical worker, according to the US Bureau of Labor Statistics, his salary rate is about $14.40 per hour. Throw in 25% for statutory fringe benefits, and he’s costing his employer 18 bucks per hour.

So let’s say we have a wonderful daylighting design which uses absolutely no electricity to light our worker’s space. That 68 bucks per year equals less than four hours of his salary. That’s right: four hours.

If we have a wonderful daylighting design which improves their productivity by 1%, that saves their employer 375 dollars per year. A productivity improvement of just 1% creates a “payback” five-and-a-half times greater than the value of saving all of their lighting electricity. Run those numbers for a more highly-paid professional, legal, or scientific worker, and the productivity value will be much higher still.

To put it another way, if we calculate the payback of daylighting based only on electricity, we’re grossly underestimating the real payback; we’re shortchanging the daylighting. And that will lead to incorrect design decisions.

Some sophisticated building owners and managers know this. Savvy retailers know that their sales will go up with daylighting. Knowledgeable educators realize that the performance of students in daylighted classrooms will improve. Daylighting produces known health benefits.

We tend to think of these benefits as intangible, but they actually represent large numbers of tangible dollars on the bottom line. Like productivity, these factors aren’t intangible, they’re just hard to quantify.

By the way, the same calculation applies to good electric lighting as well: it may save a few dollars in electricity each year, but its value is vastly greater than that.

Photo Credit: ©Anton Grassl/Esto