CUSUM (cumulative sum) is the running total of differences between expected and actual energy consumption over time. A rising curve means a lasting saving against the baseline, a falling curve means lasting deterioration, and a flat curve means no change.
How to read a CUSUM chart
For each day, the difference between actual consumption and the consumption expected from the baseline (EnB) is calculated, and these differences are summed. Individual fluctuations cancel out and the trend remains. A kink in the curve marks the moment when a measure or a fault took effect.
- Rising: consumption is consistently lower than expected (saving).
- Falling: consumption is consistently higher (deterioration).
- Flat: consumption matches expectations.
Example from manufacturing
In March, a plant adjusts its curing ovens. Until March the CUSUM curve is roughly flat; from March it rises. Steady growth means the saving is lasting rather than a one-off effect. If the curve later stopped rising, the kink would help trace what had changed.
What to watch out for
CUSUM is only as good as the baseline. An incorrect model or incomplete data produces false savings, so days with missing data and changes in operation must be recorded. The method is supported by measurement and verification of savings and EnPI indicators.
Frequently asked questions
What does a rising CUSUM curve mean?
Consumption is consistently lower than expected from the baseline. The steepness of the curve shows the size of the saving.
How does CUSUM differ from an ordinary consumption chart?
An ordinary chart shows instantaneous values, which fluctuate widely. CUSUM sums the deviations, so the long-term trend is visible even when consumption fluctuates.