Verifying Rooftop Unit Savings After Economizer Repair
An energy services company is verifying savings after repairing the economizer on a 20-ton rooftop unit. The unit serves a retail space and has a supply-fan VFD on a separate circuit. At the unit disconnect, baseline measurements were 460 V, 24.0 A per phase, 0.88 power factor, and 3,200 annual run hours. After repair, measurements are 458 V, 21.5 A per phase, 0.91 power factor, and 2,900 run hours. The weather was milder in the post period: 2,400 cooling degree days (CDD) in the baseline and 2,100 CDD in the post period. The utility meter is the billing boundary. Current clamps are rated ±2% of reading, voltage ±1%, and power factor ±0.02. The customer wants a single percent savings number for a brochure and asks whether the savings are guaranteed. The economizer now opens fully and the outside-air damper linkage was repaired; no other equipment was changed.
1. Calculate raw and weather-normalized annual energy savings for the unit disconnect, showing formulas.
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Use three-phase power: P = √3 × V × A × PF. Baseline: 1.732 × 460 × 24.0 × 0.88 = 16.83 kW. Baseline energy = 16.83 × 3,200 = 53,856 kWh. Post: 1.732 × 458 × 21.5 × 0.91 = 15.52 kW. Post energy = 15.52 × 2,900 = 45,008 kWh. Raw savings = 53,856 − 45,008 = 8,848 kWh, or 16.4%. For simple weather normalization, baseline kWh per CDD = 53,856 ÷ 2,400 = 22.44. At the post-period 2,100 CDD, normalized baseline = 22.44 × 2,100 = 47,124 kWh. Normalized savings = 47,124 − 45,008 = 2,116 kWh, or about 4.5%. The raw number overstates savings because the post period was milder; the normalized estimate is more defensible but still depends on the normalization model.
2. Identify measurement uncertainty and boundary issues that affect the claim.
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The power measurement uncertainty is roughly the root-sum-square of current (±2%), voltage (±1%), and power factor (±0.02, about ±2.3% at 0.88 PF), or about 3-4% before run-hour and weather effects. Run hours and the CDD normalization add larger uncertainty than the electrical measurement. The boundary is the unit disconnect, which includes the compressor and condenser fan but excludes the supply-fan VFD on a separate circuit. The billing meter includes the VFD, so the verified savings at the disconnect are not the same as whole-unit or billing savings. Also, economizer savings depend on weather, damper operation, and building schedule. A brochure should not present the raw 16.4% as a guaranteed result; it should state the measurement boundary, normalization method, and uncertainty.
3. Draft the key messages for the customer report, including how to present savings without overstating.
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Report the measured baseline and post conditions, the calculation method, and both raw and weather-normalized results. Lead with the normalized savings of about 2,100 kWh per year for the measured loads, and show the raw 8,800 kWh figure only with the explanation that weather and runtime differed. State that the supply-fan VFD is outside the measurement boundary and that billing savings may differ. Present a range rather than a single brochure number, for example approximately 2,100 kWh/year with measurement and normalization uncertainty. Avoid the word guaranteed; use estimated savings based on measured data and stated assumptions. Include the economizer repair scope, the fact that no other equipment changed, and a recommendation to verify persistence with post-repair trend data.