The Lure of the Time-Based Energy Code

By Matt Latchford

Energy codes got you down? Is squeezing wattage cramping your design? You’re not alone – a lot of designers feel this way, and for good reason. As the country demands more and more energy efficiency, we’re spending more and more time counting watts and squeezing every last drop from power allowances just to make our designs legal. Long gone are the days of halogen-lit everything, and decorative for the sake of decorative. We’re constantly compelled to use the most efficient light sources and fixtures, to put decorative lighting in the back seat, and to give functional lighting priority.

But is the current energy code the best way to save energy? Is lowering the allowable maximum connected load for lighting even enough to get us the savings we need to meet the national energy goals of 2030? Probably not.

Over the past decade, the allowable lighting power densities (LPD) have been lowered time and time again, sometimes logically, and other times less so. The mantra has been to increase energy savings by lowering the amount of connected electric lighting load – end users are then free to turn that connected load on and off at any time. The problem with this method is that it doesn’t account for real usage. How energy-efficient is a low-power lighting solution if it stays on all the time?

For example, take a typical ten-foot-square office space with 1.0 watt per square foot allowable LPD. You can use up to 100 watts in that particular office. Now, if you leave that office light on for 24 hours (i.e. you forgot to hit the switch on the way out), you’d have 240 watt-hours (that’s 0.24kW-h on your energy meter). But not everyone forgets to turn off their lights, so that scenario is the worst case.

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A lighting design can thus easily be checked against the code while still on paper, and this is pretty straightforward, but it doesn’t take into account how the end user will use that lighting. The lighting is designed for a maximum load at a single point in time (power), but is then measured as energy (power x time) – there’s a disconnect between the design and the application. The kicker is that there is no simple real-world method to check or enforce codes once a space is occupied. Owners are free to burn the midnight electrons and no one will say boo about it.

Now take that same office space but, instead of designing only for power allowances, you design it for power and time. What if you make an allowance for the lighting to be on for only 12 hours per day (a standard assumption for all but the craziest workaholic American). You could use the same 100 watts but the total energy used is now half that of the worst-case scenario. What if that same office has windows and daylight dimming, and the lighting is only on for 4 hours each day, just 40 watt-hours – we just went from half to one-sixth of the energy used!

So how do we predict how occupants will use lighting, and how can we make sure they then keep using it as intended? Mandates and accountability. As much as we’d like to assume that everyone will hit the light switch on the way out, that’s a bit too much wishful thinking. Cost is no deterrent, either – major corporations have money they seem happy to spend, and with the cost of energy artificially low in this country, there’s not much incentive.

There’s a growing movement in the code world to actually factor anticipated duration of use into the equation, measuring compliance in kilowatt-hours rather than just watts. We’ll always need to reference watts in our design process, but eventually we’ll have squeezed out all the watts we can, and it still won’t be enough. Adding time into the equation doesn’t immediately guarantee energy savings, but it does put it in terms that we can identify, relate to, track, and react to. It’s time to think more about energy, and less about power.

Photo Credit: Steve Ryan

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