What this calculator does
kW per tonne is the standard way to describe how efficiently a chiller or air conditioning plant turns electricity into cooling. It divides the electrical power the compressor draws by the cooling capacity it delivers, measured in tonnes of refrigeration. A lower kW/tonne figure means the plant is producing the same cooling for less electricity, so smaller numbers are better here, which trips people up the first time they see it.
One tonne of refrigeration is a fixed unit: it is the rate of heat removal needed to freeze one short ton of water into ice in 24 hours, which works out to exactly 3.517 kW of cooling. This calculator lets you enter cooling capacity in either tonnes or kW directly, since spec sheets use both, and converts between them so you can compare figures on the same basis.
The formula
Enter the electrical power the plant draws, in kW. Enter its cooling capacity, choosing whether you are entering it in tonnes of refrigeration or directly in kW of cooling. The calculator converts everything to a common basis using the fixed relationship 1 tonne of refrigeration = 3.517 kW, then divides power input by cooling capacity in tonnes to give kW/tonne. It also reports the coefficient of performance (COP), the same efficiency expressed as cooling output over power input rather than the other way round.
| Term | Meaning |
|---|---|
| kW/tonne | Electrical power input divided by cooling capacity in tonnes of refrigeration. Lower is more efficient. |
| Tonne of refrigeration | A fixed unit of cooling rate equal to 3.517 kW. |
| COP | Coefficient of performance: cooling output ÷ power input. Higher is more efficient, the inverse relationship to kW/tonne. |
The inputs explained
| Field | What to enter |
|---|---|
| Electrical power input (kW) | The electrical power drawn by the compressor and associated equipment, from the nameplate or a power meter reading. |
| Cooling capacity is in | Whether the cooling capacity you have is expressed in tonnes of refrigeration or in kW of cooling. |
| Cooling capacity | The cooling capacity figure itself, in whichever unit you selected above. |
When to use it
Comparing two chiller quotes
Two suppliers quote different power draws for plants of similar cooling capacity. Converting both to kW/tonne puts them on the same scale so you can see which one is genuinely more efficient rather than just smaller.
Checking a plant against its rated efficiency
Most chiller nameplates list a rated kW/tonne or COP figure. Measuring actual power draw and cooling output in the field and running them through this calculator shows whether the plant is still performing to spec or has drifted with age and fouling.
Sizing electrical supply for a new cooling load
Once you know the kW/tonne a plant is expected to run at, multiplying by the planned cooling capacity in tonnes gives the electrical load to design the supply around.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How does kW/tonne change with cooling capacity at a fixed power draw?
Holding electrical power input constant and varying cooling capacity shows how kW/tonne falls as the same power delivers more cooling.
| Cooling capacity (tonnes) | Efficiency (kW/tonne) |
|---|---|
| 50 | 2.000 kW/tonne |
| 75 | 1.333 kW/tonne |
| 100 | 1.000 kW/tonne |
| 125 | 0.800 kW/tonne |
| 150 | 0.667 kW/tonne |
| 200 | 0.500 kW/tonne |
Questions
What is a good kW per tonne figure for a chiller?
It depends heavily on chiller type, size and climate, and this calculator does not judge a figure as good or bad. Compare your result against the manufacturer's rated efficiency for that specific unit, or against other plants of the same type, rather than a single benchmark number.
Why is a lower kW/tonne better?
Because kW/tonne is power input divided by cooling output, a lower number means less electricity is needed to produce the same amount of cooling. It is the inverse of COP, where higher is better.
Where does the 3.517 kW figure come from?
One tonne of refrigeration is defined as the rate of heat removal needed to freeze one short ton (2,000 lb) of water into ice at 0°C over 24 hours. Converting that heat quantity into a continuous power rate gives exactly 3.517 kW, a fixed unit definition rather than an assumption.
Can I use this for a home air conditioner?
Yes, as long as you know the electrical power draw and cooling capacity. Most home units are rated in kW or BTU/h of cooling rather than tonnes, so select the kW option for cooling capacity and convert BTU/h to kW first if needed.
For related efficiency and energy figures, see the battery runtime calculator and the flow rate calculator.