The Power of Lighting Power Density

When planning a new building or doing a major remodel, the lighting systems must meet local or state energy code. Primarily this means you are only allowed a certain amount of Lighting Power Density (LPD) — the lighting watts per square foot. Most LPD allowances are based on ASHRE 90.1 computer models for various spaces, so even though it may seem unduly rigorous, LPDs for all spaces or building types have been proven by creating a model that is typical of say a school, a parking garage, or even a corridor. The lighting that is used in the model are just basic modern fixtures that would be readily available. They don’t just pick LPD allowances out of thin air, and that explains why the numbers can be a little odd, e.g., 0.64 watts per square foot (w/sq.ft) for an automotive facility, or 0.94 w/sq.ft for a library, etc. If you use old, obsolete types of fixtures you may have trouble staying under the allowed LPD, while still meeting your light level goals.

That said, if using decent, modern fixtures and applying a careful lighting design, one can easily achieve lighting targets while remaining at or below what the code allows. The intent of this is to encourage the use of more efficient technologies in the built environment. A good example of this is how more stringent energy codes made the use of T12 fluorescent fixtures with magnetic ballasts much less practical than modern T8 fixtures with electronic ballasts. We are seeing the same transition from fluorescent fixtures to LED technology. 

Where there can be problems is when the architecture is unusual in some way, and the design team (owner, architect, etc.) decide to impose aggressive energy targets. For example, 40% below code. But, such a decision may not have been thoroughly thought out, as it could lead to a situation of having to use high-performance fixtures, which could be more expensive, devote additional time refining the design (again, more costly), or possibly compromising lighting targets. Or, all three.

It is always a good idea to have a thorough discussion with the entire design team in the early stages of the process to ensure that everyone is aware of the possible implications of aggressive energy goals.  #