The Space-by-Space method lists approximately 100 space types (Open office, Enclosed office, Classroom, Restroom, Corridor, etc.) and provides an LPD for each space of that type in a building. The Space-by-Space Method generally takes a similar approach but considers each room individually. In my experience, the LPD for each building type is roughly equivalent to the LPD that would result if each space in a typical example of that building type was calculated individually using the Space-by-Space Method and then added together. This method is easy to implement and is hard to argue with – the energy consumption of all fixtures in the building are added up and then divided by the overall area of the building. The Building Area Method lists approximately 30 different building types (Office, School, Hospital, etc.) and provides an LPD for the entire square footage of the building. That said, LPD is a useful metric for making basic comparisons and is quick and easy to calculate.ĪSHRAE 90.1 offers two approaches to calculating LPD – the Building Area Method and the Space-by-Space Method. It's an interesting topic that lighting professionals can argue about at length. There are currently several proposals to revise LPD to take into consideration the length of time that a fixture is actually turned on and therefore using energy, and to also take into consideration the effect that controls have on energy use. LPD is an admittedly inexact measure – wattage is a measure of energy use over time, but LPD ignores this and does not have a time component. LPD is measured in watts per square foot, or energy consumed divided by the size of a space.
The metric that the above codes and standards use to measure lighting energy use is Lighting Power Density, or LPD (it's no accident that my firm is named LPD – I am passionate about energy efficiency). The focus of this post is on ASHRAE 90.1, but as the above list should make clear, can be generally assumed to apply to most other codes and standards as well. Others, like the BCGBS, are based closely on LEED, and in essence mandate compliance with LEED standards. These codes are typically based very closely on ASHRAE 90.1 or the IECC, with specific or across-the-board percentage energy use reductions or added requirements for control systems. New York City ( NYCEEC) and California ( Title 24) have also adopted such codes. For example, in Baltimore we have the Baltimore City Green Building Standard (BCGBS).
This post is intended to serve as a guide for lighting design clients to understand what issues are involved in energy-efficient lighting design and how to make sure that you hire a lighting designer who can deliver a system that meets your goals. Some embrace this change, others have fought to have it reversed.
Energy efficiency has always been an integral part of lighting design, but it has quickly become one of the top issues that lighting designers and electrical engineers deal with on a daily basis.