Horsepower to Tons of Refrigeration for Compressors

Reading Cooling Capacity Off a Compressor Nameplate

You are stood in front of a condensing unit with a stamped plate that says 5 hp, and the customer wants to know how many tons of cooling they bought. The plate gives you shaft power. Tonnage is heat flow. They are two different quantities measured in the same units, and the trade has quietly blurred them for decades because, on a comfort-cooling machine, the two numbers happen to land close together.

Conversion factor: 1 hp = 0.212036134 TR — 745.7 W of shaft power set against the 3 516.85 W that defines a ton. A 5 hp motor is therefore 1.0602 TR of power. That same compressor still cools a four-ton house, because it pumps heat rather than producing it: multiply the shaft figure by the machine's COP to get delivered capacity.

Two Different Numbers on One Machine

Shaft work on the compressor plate

The hp figure stamped on a semi-hermetic body or an open-drive shaft is mechanical output at the crankshaft — what the motor has to push into the refrigerant, nothing more.

Tonnage on the equipment label

The ton in a condensing-unit model number is a heat-removal rate at stated rating conditions. It counts heat leaving the evaporator, not effort going into the compressor.

COP is the multiplier in between

Coefficient of performance is heat moved divided by work in. At a COP of 4.716 a machine shifts exactly one ton per horsepower of shaft power — which is where the old shorthand comes from.

Evaporating temperature moves the answer

The same 7.5 hp body gives roughly twice the capacity at a +20 °F suction as it does at −20 °F, because the gas arriving at the suction port is far denser.

Working From the Compressor Plate to a Tonnage Figure

Order matters on a service call: convert first, then apply the performance factor. Doing it the other way round is how a 3 hp scroll ends up quoted as a fifteen-ton machine.

Enter the horsepower stamped on the body

Type 3, 5, 7.5, 40 — whatever the plate reads. Tons appear alongside while you type, and a comma works as a decimal point if that is how the plate writes 7,5 hp.

Multiply by the performance factor

Take the tonnage you get back and multiply by the COP for the duty in front of you — somewhere near 4.5 to 5 for comfort cooling, closer to 1.5 for a low-temperature box. That product is the cooling the machine really delivers.

Reverse it when the label already reads tons

If all you have is a four-ton model number and you want the power behind it, press the swap arrow (↔) to run TR → hp. One ton is 4.7162 hp of heat flow; divide by COP to land on the shaft power the motor must supply.

Lift the number onto the ticket

The copy button puts the bare figure on the clipboard with no unit and no spacing, so it drops cleanly into a job sheet or a quotation line. Ctrl + C inside either field does the same thing.

A horsepower figure carries no conditions: published capacity always has a suction and a condensing temperature attached to it. Comparing two compressors by plate hp alone tells you nothing about which one will hold the box down.

Compressor Horsepower and the Tonnage It Actually Delivers

The middle column is the pure conversion — motor horsepower expressed in tons. The right-hand column is what the machine puts on the evaporator once you allow for the duty it was built for. The gap between the two is the entire point of a refrigeration cycle.

Compressor and duty	Motor	Shaft power as TR	Typical COP	Cooling delivered
Scroll, R-410A, comfort cooling (45 °F suction)	3 hp	0.6361 TR	4.7	≈ 3.0 TR
Scroll, R-410A, packaged unit (45 °F suction)	5 hp	1.0602 TR	4.6	≈ 4.9 TR
Semi-hermetic recip, R-448A, medium temp (+20 °F)	7.5 hp	1.5903 TR	3.0	≈ 4.8 TR
Semi-hermetic recip, R-448A, low temp (−20 °F)	7.5 hp	1.5903 TR	1.5	≈ 2.4 TR
Scroll, R-134a water chiller (44 °F leaving water)	15 hp	3.1805 TR	5.2	≈ 16.5 TR
Open-drive recip, R-404A, transport (+10 °F)	20 hp	4.2407 TR	2.6	≈ 11.0 TR
Parallel rack, R-448A, supermarket low temp (−25 °F)	40 hp	8.4814 TR	1.4	≈ 11.9 TR
Screw, R-134a, water-cooled chiller (44 °F leaving water)	100 hp	21.2036 TR	6.1	≈ 129 TR

Read the two 7.5 hp rows together and the trap becomes obvious: identical plate, identical body, and capacity halves purely because the box is colder. At the other end, the water-cooled screw returns more than six tons of cooling for every ton of shaft power, which is why chiller selections come from a performance table and never from a motor rating.

What This Page Does on a Service Call

Two live boxes while you work down a condensing-unit list

Both fields are editable and track each other as you type, so a yard full of units can be worked through without clearing anything between entries.

Flip the arrow when the plate already reads tons

The swap button turns the page into TR → hp, the direction you want when a model number gives capacity and you are hunting for the compressor behind it.

Any power unit for a plate that quotes BTU/h

Searchable dropdowns on both sides carry kW, BTU/h, kcal/h, PS and the rest, so an imported nameplate in unfamiliar units still lands somewhere useful.

A bare figure for the service ticket

Results run to eight decimals where they need to, and copying hands over the plain number with no unit attached — ready for a job sheet or a spreadsheet cell.

Compressor Nameplate Questions

Why do technicians say one horsepower equals one ton of cooling?

Because it is roughly true for air conditioning, and only for air conditioning. A ton of heat flow is 4.7162 hp, so a machine has to reach a COP of 4.716 for one horsepower of shaft power to shift one ton — and that is an EER of about 16, right in the band a modern comfort-cooling unit occupies. Carry the same shorthand into a freezer application and it fails by a factor of three.

Does a 5 hp compressor really only move 1.06 tons of heat?

No — 1.0602 TR is what 5 hp is worth as a rate of energy, the answer you would get if the motor were simply burning that power as heat. A compressor is a pump, not a heater: it spends 5 hp of work collecting several times that much heat from a cold space and delivering the total to the condenser. Delivered capacity is 1.0602 TR multiplied by COP, which for a comfort scroll comes out near 4.9 tons.

How much does evaporating temperature change what a compressor delivers?

Enormously. Capacity is swept volume multiplied by suction gas density, and density collapses as suction pressure drops. Between a +20 °F medium-temperature duty and a −20 °F low-temperature one, the same body typically loses around half its capacity while the motor still draws comparable power — so tons per horsepower falls off a cliff. Always read capacity from the manufacturer's table at your actual suction and condensing temperatures.

The condensing unit is sold as 4 ton but the compressor is 3 hp — which figure is wrong?

Neither. The 3 hp belongs to the compressor alone and describes work in; the 4 ton belongs to the whole condensing unit — compressor, condenser coil and fan together — and describes heat out at published rating conditions. A condensing unit will always carry a larger tonnage number than its compressor's horsepower, and if it does not, something is derated or badly matched.

Do semi-hermetic and scroll compressors give the same tons per horsepower?

Not at the same duty. A scroll compresses continuously with no suction valves and very little re-expansion loss, so at air-conditioning conditions it generally beats a reciprocating machine of similar displacement. Semi-hermetic recips earn their keep further down the range: they tolerate higher compression ratios and can be opened up in the field, which is why they still populate low-temperature racks where a scroll would struggle. Compare published capacity at your conditions, never plate horsepower.

Horsepower to Tons of Refrigeration