IPMVP Option C (Whole Facility) is recommended when the expected savings represent more than 10 % of the total building consumption and several ECMs coexist (making individual A/B isolation impractical).

Option C is holistic, captures ECM interactions and is simple to deploy because it relies on the main utility meter. The downsides are lower precision (consumption noise), and the need for sophisticated routine adjustments for weather, occupancy and production. A typical use case is a whole-building energy audit on a tertiary site with five or more concurrent renovations where isolation is not cost-effective.

Below the 10 % rule of thumb, savings get lost in measurement noise — use A or B isolation instead.