A chiller's annual electricity bill often exceeds its purchase price within a few years. That is why the most important question is the same both when choosing a unit and when evaluating an existing one: how much electricity does this chiller use for one unit of cooling? COP and EER answer this question, but when read wrongly they can also lead to a wrong decision.
What are COP and EER?
COP (Coefficient of Performance) is the ratio of the cooling capacity a chiller produces to the electrical power it draws, and it has no unit. For example, a chiller that produces 500 kW of cooling and draws 170 kW of electricity has a COP of about 2.94. That is, for every 1 kW of electricity it produces about 2.94 kW of cooling.
EER (Energy Efficiency Ratio) expresses the same ratio in cooling mode in Europe and in Eurovent documents; COP, on the other hand, is often used for heating mode. In US sources EER is given in a different unit, as Btu/h per watt; this value is converted to the kW/kW ratio by multiplying by about 3.412. When comparing, make sure both values are in the same unit.
What does kW/ton tell you?
kW/ton is the electrical power used for one ton of cooling (about 3.517 kW), and it reads like the inverse of COP: the lower the value, the more efficient the unit. It is used often in large water-cooled chillers and in international projects.
Converting is easy: the kW/ton value is found by dividing 3.517 by the COP. A unit with a COP of 3.0 uses about 1.17 kW/ton, and a unit with a COP of 6.0 uses about 0.59 kW/ton.
The ton can appear in two different ways in a catalog. Some manufacturers base it only on the compressor power, while others use the total power of the unit including fans and the control system. Before comparing values, check the footnotes to see which power is counted.
Why is the full-load value not enough?
Full-load efficiency shows the chiller's performance only on the hottest day and at the highest load. A comfort chiller, however, spends most of the year at 30-70 percent load, in cooler outdoor conditions. The value that really determines annual energy consumption is the efficiency at these part-load conditions.
That is why seasonal values were developed. In Europe SEER (under Ecodesign) and the previously used ESEER, and in the US IPLV and NPLV, reduce efficiency at different load and temperature points to a single value by weighting. In process cooling, the SEPR value, defined for a constant load all year round, is used.
| Value | What does it measure? | How is it read? |
|---|---|---|
| EER / COP | Cooling / electrical ratio at full load | Higher is more efficient |
| kW/ton | Electrical power per ton of cooling | Lower is more efficient |
| SEER / ESEER | Seasonal, part-load weighted efficiency (Europe) | Higher is more efficient |
| IPLV / NPLV | Part-load weighted efficiency (US) | Read according to its unit |
| SEPR | Seasonal efficiency for process cooling | Higher is more efficient |
When comparing two units, there can be a small difference in the full-load value and a large difference in the seasonal value. Units with inverter or staged capacity control usually have a clear advantage at part load.
Under what conditions are catalog values valid?
The efficiency values in a catalog are measured under standard test conditions. Under Eurovent conditions, 12/7 °C chilled water and 35 °C outdoor air are generally used for air-cooled chillers, and additionally 30/35 °C condenser water for water-cooled chillers. If the conditions in your project are different, the unit's real efficiency will be different too.
Two parameters affect efficiency the most. As the chilled water outlet temperature rises, the compressor does less work; in practice each 1 °C increase improves efficiency by a few percent. Efficiency also rises as the condensing temperature falls. That is why, in comfort applications, keeping the setpoint lower than necessary and leaving the condenser dirty turn even the most efficient unit into an average one.
How is real efficiency measured in the field?
How efficiently is your chiller running?
We calculate real efficiency on site from flow, temperature difference, and power measurements and give improvement recommendations in writing.
Real efficiency in the field is calculated by dividing the cooling the chiller produces by the measured electrical power. Cooling capacity is found from water flow and the evaporator inlet-outlet temperature difference: capacity (kW) is calculated for pure water with the formula flow (m³/h) × ΔT (°C) × 1.16. In glycol circuits the coefficient drops according to the glycol ratio.
For example, a 5 °C temperature difference at a flow of 86 m³/h means about 500 kW of cooling. If the electrical power measured at the same time is 190 kW, the field COP is about 2.6. If this value is clearly below the catalog value for the same conditions, a dirty heat exchanger, refrigerant shortage, or compressor wear should be investigated. We covered efficiency loss on the compressor side in our article on screw compressor overhaul.
A reliable measurement requires a flow meter or calibrated ultrasonic measurement, precise temperature sensors, and a power analyzer. Measurements should be repeated under stable operating conditions and, where possible, at different load points. Electrical measurements should be done by a qualified technician.
At the facility level, the chiller is not the only consumer. Chilled water and condenser pumps, cooling tower fans, and the control system also use energy. Total system efficiency is calculated as the ratio of the power drawn by the whole chiller room to the cooling produced. When an efficient chiller is used together with oversized, fixed-speed pumps, the facility's total efficiency can fall far below expectations. For water-cooled systems we explained the effect of the tower side in our cooling tower maintenance article.
What can be done to increase efficiency?
The efficiency of an existing chiller can often be increased without replacing the unit. The measures that make the most difference in the field are these:
- Regular cleaning of condenser coils and heat exchanger tubes
- Raising the chilled water setpoint according to the building's needs, and resetting according to outdoor air
- Correct refrigerant charging and superheat setting
- In water-cooled systems, treating the tower water and controlling the tower fans
- If there are multiple chillers, optimizing sequencing and load sharing
- Evaluating a switch to variable flow for fixed-speed pumps
When choosing a new unit, air-cooled and water-cooled options should be compared on total energy consumption; we covered this in our chiller selection guide.
How do we help with efficiency at YOİ?
During our maintenance visits we measure and record the chiller's flow, temperature, and power values and track the decline in efficiency over the years. In our calculations for improving an existing system or selecting a new unit, we present energy consumption and payback period together. For details you can look at our chiller services.
Frequently asked questions
What is a good COP value for a chiller?
There is no single right value. At full load, air-cooled chillers generally show 2.8-3.5, and water-cooled screw and centrifugal chillers show 5-6 and above. Comparisons should be made under the same test conditions and with seasonal values.
What is the difference between EER and SEER?
EER shows the instantaneous efficiency at full load, while SEER shows the seasonal efficiency found by weighting different load and outdoor temperature points. SEER is more meaningful for estimating annual energy consumption.
How do I convert COP to kW/ton?
Just divide 3.517 by the COP. For example, a chiller with a COP of 5 uses about 0.70 kW/ton.
Why does my chiller's efficiency drop over time?
A dirty condenser and evaporator, refrigerant leaks, compressor wear, and wrong setpoints reduce efficiency over time. Regular maintenance and measurement records let you notice this decline early.
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Detailed guide: Chiller maintenance: intervals, checklist, and maintenance form