In daily and billing period methods, building-level savings are generated by summing the building’s estimated energy savings for each day or billing period of the reporting period. The portfolio savings are then calculated by aggregating total savings for each building in the portfolio.
For hourly models, portfolio uncertainty is difficult to calculate when savings are aggregated for each hour due to correlation in the error term.
To begin our discussion of hourly aggregation methods, consider two types of roll-ups:
In a vertical roll-up, hours within a day are grouped together for each building before aggregation. For example, one may choose to aggregate hourly energy savings in three-hour intervals throughout the day instead of each hour individually.
Although vertical roll-ups can reduce portfolio uncertainty, larger time intervals provide less information in portfolio loadshapes. Hourly methods are created to provide granular information about energy load impacts during each time-of-day. Less information is available if hours are “rolled-up” into larger time intervals.
Horizontal roll-ups aggregate each hourly estimate with estimates of the same hour across weeks. A horizontal roll-up can aggregate individual hours or time intervals, such as the three-hour interval discussed above.
There were a few additional suggestions from the working meeting (5/24) that could help create guidelines for aggregating portfolio loadshapes:
- Take 8760 (full year hourly load) building data and establishing criteria for “slicing and dicing” the data to identify patterns that can inform hourly aggregation guidelines.
- Leveraging measure specific load shapes, from existing technical reference manuals or deemed savings models, was yet another idea that was brought to the table.