There are many measures used for building energy efficiency: total kBtu, kBtu/sf/yr, kWh/yr, therms/yr, kWh/sq.m/yr, $/yr or kBtu/person, and more. But determining when to use which metric, and even more important, how to make sense of a comparison of the energy efficiency of two different buildings, is no easy task.
Let’s use two glasses of water as an example. Some comparisons seem black and white – such as the question of which of the two cups of water sitting in front of us is hotter. Obviously the one that burns your finger is hotter than the one with the ice cubes floating around the rim. But the comparison becomes much more difficult when we start looking at which building performs better. Not only is there much more information required in making this decision, there is also the question of how you look at the numbers.
Let’s return to the example of the two cups of water. Our first question was pretty easy, but now let us look at the question of which cup of water requires less energy to maintain a temperature of 90 degrees. At this point quite a few more variables come into play. First, how much water is in the cup? If one cup has more water, it will require much more energy to maintain its temperature. Second, what type of cup is the water held in: glass, plastic, ceramic, or maybe an covered, insulated coffee mug? Third, how are the cups being heated — maybe one is being heated efficiently in the microwave, while another is being held over an open campfire. The cup over the open campfire will require much more energy to be used to keep it at temperature because the heating source is extremely inefficient. Another consideration might be where are the cups sitting while not being actively heated? If one cup is sitting on a sunny window ledge while another sits in a dark closet, it would make a dramatic difference. These are simple examples of the types of questions that have to be dealt with when measuring buildings and their energy performance, though building variables are even more varied and complex.
Making Sense of the Numbers
Now let us look at the second challenge – how to make sense of the numbers. The best way to look at energy utilization is to have an actual measured number for the amount of energy used during a defined period of time. For a building, this would be the total btus or kWhs used by all energy sources for a year, though in our example of the two cups of water this would likely be in btus per hour. For the sake of example, let’s say that cup 1 required 100 btus per hour to maintain temperature, while cup 2 used 150 btus per hour to maintain the same temperature. At first glance, you would say that cup 1 was more energy efficient. But here is where it gets complicated. What if cup 1 contained only 10 ounces of water while cup 2 contained 30 ounces of water. Therefore cup 1 requires 10 btus/hr/ounce while cup 2 requires 5 btus/hr/ounce. Looking at the numbers this way, it seems as if cup 2 is more energy efficient. So what really is the standard that we should be using? When looking at buildings what really makes sense?
I don’t have a definitive answer. The purpose of this discussion is not to decide on an unit of energy measurement that should always be used, but to outline the inherent challenges in comparing these metrics. Trying to understand how one building compares to anther in terms of energy usage is an enormous task, and different answers emerge based on how the numbers are manipulated and reported. Making it easier to make sense of building energy statistics is a challenge that needs to be addressed if we want to change the status quo and move toward increasing the stock of net-zero energy buildings. Do you have a suggestion? Please comment below.