Under the Hood of the WegoScore (Part I)
Last week we launched the WegoScore for multifamily buildings, the simplest useful metric for monitoring multifamily building utility performance.
I want to address one subtle, but important aspect of the WegoScore: the score maps directly to the efficiency of a building, not just how it stacks up to other buildings. We think this is the best way to score buildings.
Given a set of measurements — in our case, usage intensities for energy (EUI) or water (WUI) — there are two ways to assign scores:
- Performance-based: the score is directly proportional to the measurement. In a classroom setting, this score would be your grade on an exam.
- Ranking-based: the score is proportional to the percent of peers you have surpassed. In the classroom, this score would tell you how many others stand between you and the head of the class. While a good metric for bragging, there is no way to tell how much this kind of score will improve by studying more for your Building Management 101 final.
Take a look at these two typical distributions of utility usage we come across. The left graph shows the characteristically long-tailed nature (the most inefficient buildings use several times the water of efficient ones), while the right shows a multi-peaked situation (due, for instance, to electricity using much less site energy than gas).
The two types of scores are overlaid to show the difference visually. (The scores are bracketed between top and bottom 5% of the data to mitigate the effects of outliers.)
The performance-based score is evenly measured and maps simply to the actual usage. For the ranking-based score, however, EUI and WUI intervals grow and shrink arbitrarily as scores move from 1 to 100.
Performance-based scores have the attractive feature that a certain difference in points (say 10 points) always corresponds to the same difference in usage (say 12 kBTU/sqft/year). Building managers can use this feature to their advantage: they can tell how much usage reduction is necessary to bring their less efficient buildings in line with their efficient ones, with just a glance at the scores.
Stay tuned for more WegoScore analysis...
Why do the buildings with the lowest usage get the highest score? And why 1-to-100?
We think of it this way: the most efficient buildings get the best score. And, thanks to the barrage of 100-point exams throughout school, we thought 100 was most easily identified with a 'best' score. (And that giving 0's would be too mean.)
The 5% and 95% cutoffs seem arbitrary. Why that level?
They are arbitrary, but some parameters have to be set using intuition alone. Using the full range of data (0% to 100%) would give the most inefficient building undue influence over its peers' scores.
So, those cutoffs imply that a merely great building would receive the same score as an awesome net zero building, right?
Indeed, both buildings will get WegoScores of 100, even though the net zero building owner invested more money and shows more environmental stewardship. It is important to remember, though that the WegoScore does not grade on a curve, so there are plenty of buildings with low scores. These buildings represent the greatest energy-saving potential in the built environment and should be prioritized for upgrades.