The 50% Rule

By Matt Latchford

What is the 50% rule?

Well, I just made it up – just now. It’s not a new concept by any means; I’m applying it to lighting, although I suppose you could apply it to just about anything. It’s about efficiency and utility: how much of something do you get versus how much you lose, and is it worth it?

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More specifically, with lighting, how much light do you get out of a fixture from all the light generated inside the fixture? For example, let’s say that you have a downlight that has a lamp in it (it doesn’t matter what lamp for the sake of this argument, but let’s just say that it’s an efficient one). The fixture efficiency of that downlight is the ratio of how much light exits the fixture (ideally in the direction that you want it to) to how much light that lamp can produce overall. Most downlights have fixture efficiencies somewhere around 50%, meaning half of that light is lost inside the ceiling.

So how does this become a rule? The rule is that you should use more fixtures that have greater than 50% fixture efficiency than those that have less than 50% efficiency – otherwise you’re wasting more light than you’re using. This may seem fairly obvious when you consider it, but I doubt that you look at it this way when creating a design. I’m describing this theory as a rule of thumb – and giving it a name to remember it by – to put a subconscious bell in your mind, intended to start ringing when you’re designing.

Seems straightforward, right? Not exactly. There are a ton of light fixtures out there that piddle around in the 30% range, barely letting any light out at all. They’ve been evolving over the years and include a lot of very sleek and narrow fixtures that have high aesthetic appeal, or have a very specific purpose, like a tight beam of light from a tiny hole in the ceiling. Efficiency hasn’t always been a big part of that evolution, and while some of those fixtures do have a time and place, they are used all too frequently. Have you ever specified or used one of those two-inch-wide light fixtures in your designs? Lots of manufacturers have them – nothing new there. But have you looked at their efficiency? It’s often less than 50%. That means that more than half of that energy is wasted.

In the bigger building-wide picture too, any energy that doesn’t get out is not only wasted, but also turned into heat, which needs to be removed by air conditioning, which uses even more energy. So the wasted energy exceeds usable energy even further.

Of course, if a fixture passes 50% efficiency, that doesn’t mean you should stop there. If there is another fixture 20% more efficient, which produces quality light in the way you want it to, all the better. Every little watt counts.

How, then, do we measure worth? It’s subjective, and hard to precisely define. Ask yourself what you want to do and what tools you can use to do it. If your goal is to light a private office, and you have a choice between that two-inch-wide slot or a four-inch-wide version with twice the efficiency, I would argue that two out of three of your goals are met by using the wider version: lighting the space, energy efficiency, but maybe not as sexy. If you choose the narrower fixture, can you say the same? Maybe that wider fixture is starting to look sexier now?

There will always be some applications that really do have very specific solutions, and that’s why this isn’t a hard-and-fast rule. It’s merely a tipping of the scales – worse or better, overall.

Take this example: two identical private offices with the same design goals – illuminate to 30 footcandles average, and use less than one watt per square foot. One office is lit with 60% efficient fixtures; the other with 30% efficient fixtures. Both offices achieve almost the same light levels, and both meet the power code requirement. The one that uses the less efficient fixture uses twice as much power, and twice as many fixtures, to do the same thing. Then there’s also the added cost of lamps, ballasts, installation, and maintenance.

Now, one may ask: ‘who cares how efficient it is if it does the job and meets code?’ The U.S. Department of Energy does! Energy usage in buildings will only continue to be restricted, so we simply have to be more efficient in order to continue to meet code requirements. And, it’s the difference of just a few local kWh in your building; if you assume that every building in the country has some waste in them like yours, and add that all up, it could mean that whole power plants are operating just to satisfy that waste. It makes that much of a difference.

So, my plea to the design community is that if less than half the energy that goes into a fixture gets out of it in it’s intended form, then it behaves more like a trap than a delivery method – so use it sparingly. Some may argue that you can’t use many inefficient fixtures anyway because the codes are already too strict – otherwise you’d end up with under-illuminated spaces. But most of us are more reactionary, and only cut or change something if we find out that we’re over code-allowed watts, essentially using all the available energy we can. Try to be proactive and make the design more efficient than not from the outset. Let’s not build the least energy-efficient buildings the law allows – we can do better.

Photo Credit: Sleeping Sun

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