When to use: Use to benchmark the efficiency of a chilled-water plant. Sum the electrical demand of the chiller, cooling tower fans, condenser water pump, and chilled water pump, then divide by the cooling load in tons to get kW/ton (lower is better). A well-tuned all-variable-speed water-cooled plant targets ≈ 0.5–0.7 kW/ton. Because efficiency degrades at low load, set the part-load ratio (PLR) to estimate kW/ton at the operating point.
This tool benchmarks chiller plant efficiency in kW/ton by summing chiller, cooling tower, condenser pump, and chilled water pump electrical demand against the cooling load, then comparing results against ASHRAE 90.1 efficiency targets. Engineers use it to identify whether a chiller plant is operating efficiently and to estimate part-load performance.
System kW/ton = Total plant kW / cooling load in tons, where total plant kW includes the chiller compressor, cooling tower fans, condenser water pump, and chilled water pump. Lower kW/ton means better efficiency. The coefficient of performance (COP) is the inverse normalized to standard units: COP = 3.517 / (kW/ton), where 3.517 is the conversion factor between ton-refrigeration and kilowatts.
Part-load efficiency degrades because chillers lose efficiency as they unload below 50–60% of rated capacity. The part-load multiplier used in this tool is (0.2/PLR + 0.8), which approximates the ASHRAE 90.1 Appendix G IPLV (Integrated Part Load Value) degradation curve. At PLR = 0.5, a plant that runs 0.70 kW/ton at full load might consume 0.84 kW/ton.
ASHRAE 90.1 Table 6.8.1 sets minimum efficiency requirements for chillers in kW/ton and IPLV. Water-cooled centrifugal chillers must meet approximately 0.56 kW/ton full load and 0.54 kW/ton IPLV for large units. ASHRAE 90.1 Appendix G defines the IPLV weighting factors (A=1% at 100%, B=42% at 75%, C=45% at 50%, D=12% at 25% load) used to compute integrated seasonal efficiency. The California Energy Code Title 24 and LEED v4 both cite ASHRAE 90.1 as the efficiency reference baseline.
All-variable-speed chiller plants (variable-speed chillers, cooling tower fans, condenser water pumps, and chilled water pumps) routinely achieve 0.4–0.6 kW/ton at design conditions and 0.3–0.4 kW/ton at part load. Fixed-speed plants typically operate at 0.8–1.2 kW/ton. Chilled water supply temperature reset from 44°F to 48–52°F at part load improves chiller COP by 2–4% per degree raised; ASHRAE 90.1 Section 6.5.2 mandates chilled water temperature reset on most new systems. Condenser water temperature reset (lowering condenser water temperature as ambient conditions allow) provides similar or greater savings by reducing chiller lift.
Enter the electrical demand of each major plant component in kW: the chiller(s), cooling tower fan(s), condenser water pump(s), and chilled water pump(s). Enter the cooling load in tons and the part-load ratio (PLR) at the operating point of interest. The tool reports system kW/ton, chiller-only kW/ton, COP, and estimated kW/ton at the part-load ratio. Compare results against the benchmark bands: under 0.6 kW/ton is excellent, 0.6–1.0 kW/ton is good to fair, and over 1.0 kW/ton is poor.
An all-variable-speed water-cooled chiller plant should achieve 0.4–0.6 kW/ton at design conditions. Fixed-speed plants typically run 0.8–1.1 kW/ton. Air-cooled chillers are inherently less efficient, typically 1.0–1.4 kW/ton, because they reject heat to warm outdoor air rather than cooled tower water.
IPLV (Integrated Part Load Value) is a weighted average efficiency at 100%, 75%, 50%, and 25% load using ASHRAE 90.1 Appendix G weights. Because most chillers run at part load most of the time, IPLV is a better indicator of annual energy consumption than full-load kW/ton. A high full-load efficiency with poor part-load performance can result in a worse IPLV.
Raising the chilled water supply temperature setpoint by 1°F improves chiller COP by approximately 1.5–2%. Resetting from 44°F to 48°F (a 4°F raise) can save 6–8% of chiller energy. ASHRAE 90.1 Section 6.5.2.1 requires chilled water temperature reset on most new systems larger than 300,000 BTU/hr.
Yes — whole-plant kW/ton includes all components that consume energy to deliver cooling: the chiller compressor, cooling tower fans, condenser water pumps, and chilled water pumps. Excluding pumps understates the true plant efficiency and misses optimization opportunities from variable-speed drives on pump motors.
COP (coefficient of performance) is a dimensionless ratio of cooling output to electrical input using consistent units (kW/kW). EER (Energy Efficiency Ratio) uses BTU/hr divided by watts, numerically equal to COP × 3.412. IPLV and kW/ton are more commonly used in the commercial chiller industry than COP or EER.
Try our Smart Buildings Studio
More calculators, simulators, and guides for this discipline.