Retrofit Isolation Approach to M&V
Retrofit isolation (Options A and B) measures energy use of the affected system alone, using dedicated meters or sub-meters, before and after the retrofit, or by measuring the isolated system post-retrofit and comparing to a baseline model of that system. It is best when the measure is separable, savings are large relative to system load, and interactive effects are small or can be bounded.
In Option A, the M&V professional identifies which parameters drive savings and measures those; others are stipulated based on engineering judgment, manufacturer data, or historical records. The plan must justify each stipulation and state that stipulated values will not change. In Option B, all parameters that materially affect savings are measured - often with continuous logging - and savings are computed from measured data, typically with a model or engineering calculation.
Key concepts: measurement boundary, sub-metering, spot vs. continuous measurement, sampling, and uncertainty propagation. Traps: stipulating a parameter that is actually variable (e.g., operating hours) and thereby overstating savings; failing to account for interactive effects like reduced lighting heat increasing heating load; measuring only during a short period and extrapolating without justification; and confusing the measurement boundary - counting only the retrofit device while ignoring associated fans or pumps.
Worked example: A lighting retrofit in a warehouse. Option A measures lighting kW and stipulated operating hours (say 3,000 h/yr from a schedule survey). Baseline: 100 kW × 3,000 h = 300,000 kWh. Post: 40 kW × 3,000 h = 120,000 kWh. Savings = 180,000 kWh. If hours are uncertain, Option B with continuous logging of run hours reduces uncertainty. Interactive heating penalty (less lighting heat) should be estimated separately.