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Energy Models: Getting Out What You Put In

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Today's tip is about the GIGO principle as it relates to energy modeling. GIGO or, garbage in, garbage out, basically means that the quality of the input data is ultimately what determines how useful the output data is. This is an especially cogent principle when it comes to an energy model, as the fewer assumptions the engineer doing the model must make, the more reliable the energy model will be. Common sense, right? But here are some things to consider.

Of course, there will always be some variables that are impossible to predict. The occupancy of the building may be greater than expected. The building's operating hours may be longer than expected. Equipment may not interact as expected. Even so, an energy model is still an essential (and if you're doing LEED, required) part of the construction process.

An honest and frank discussion between the facility manager and the design team at the very beginning is absolutely critical to making sure everything and everyone is on the same page. The information derived from this meeting — or more likely, these meetings...plural — will improve the quality of data input into the model. Inputs include window to wall ratio, tightness of air infiltration, thermal wall and roof insulation, performance criteria of glass, type and efficiency of mechanical system, lighting type and expected watts per square foot, plug loads watts per square foot, and occupancy.

If it's not completely understood what the owner is expecting in any of these areas, the engineer may add a "contingency factor" to the calculations, which can result in over-designed systems. This means that not just the energy model is wrong, but the facility manager will be spending more on energy than expected.

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