kW to BTU/h - Heat Pump Capacity for a Manual J Load

Putting a Kilowatt Heat Pump Spec onto a US Load Calculation

Heat pump data sheets written for European or Asian markets state capacity in kilowatts. The Manual J worksheet, the AHRI listing and the permit application on an American installer's desk all speak BTU/h. Before a unit can be matched to a design heat loss, or defended to a plans examiner, its capacity table has to be restated line by line.

Conversion factor: 1 kW = 3 412.141635 BTU/h. A 7 kW air-to-water unit is therefore 23 885 BTU/h at its rated point — and if the same sheet shows 5.6 kW at −7 °C, that is 19 108 BTU/h, the figure that actually has to cover the design day.

The Conditions Hiding Behind a Capacity Figure

The rated point on the data sheet

European heating capacity is normally quoted at 7 °C outdoor air, with a stated water flow temperature. It is a laboratory reference point, not the weather your customer heats through.

Winter design temperature

Manual J sizes against a local design condition — the temperature the location beats for all but a small percentage of hours. That is where the capacity has to be checked.

Supplemental heat allowance

Below the balance point a strip heater, furnace or boiler makes up the shortfall. Its size falls straight out of the gap between the converted BTU/h and the calculated load.

Two rating standards meeting

A kW figure measured to EN 14511 and a BTU/h figure from an AHRI listing use different test conditions, so converting the number is only the first half of comparing them fairly.

From European Data Sheet to Permit Paperwork

Convert the capacity table rather than a single headline number. A heat pump that looks generous at 7 °C can be the wrong unit entirely once the design day is on the page.

Start with the rated capacity

Type the nominal kW figure into the left field — 5, 7, 8.5, 12 — and read the BTU/h equivalent immediately. Spaces are ignored and a comma is accepted as the decimal separator, which suits data sheets written for European markets.

Convert the cold-weather line as well

Go back to the capacity table and take the value at the lowest outdoor temperature published, then convert that too. This is the number that belongs beside your design heating load, not the rated one.

Set both figures against the calculated load

With the heat loss in BTU/h from the room-by-room worksheet, the comparison becomes direct: where the two curves cross is the balance point, and whatever is left below it is the backup heat you have to specify.

Turn an AHRI listing back into kilowatts

Checking a North American model against a European specification instead? The swap button (↔) runs BTU/h → kW, so a 36 000 BTU/h listing reads as 10.55 kW on the same scale as the imported sheet.

A capacity figure means nothing without its conditions: the same air-to-water unit publishes very different kilowatts at 35 °C flow for underfloor pipework and at 55 °C flow for existing radiators. Convert from the row that matches the system you are actually connecting to.

Heat Pump Capacity at Rated and Low Ambient

Representative heating capacities for common heat pump classes, at the rated point and at a cold outdoor condition, each converted to the BTU/h a load calculation is written in.

Unit class	Rated capacity (kW)	Rated (BTU/h)	At −7 °C (kW)	At −7 °C (BTU/h)
Single-zone mini-split, standard range	2.5 kW	8 530 BTU/h	1.8 kW	6 142 BTU/h
Single-zone mini-split, cold-climate model	3.5 kW	11 942 BTU/h	3.5 kW	11 942 BTU/h
Multi-split outdoor unit	7 kW	23 885 BTU/h	5.6 kW	19 108 BTU/h
Air-to-water monobloc, small dwelling	6 kW	20 473 BTU/h	4.8 kW	16 378 BTU/h
Air-to-water monobloc, mid range	8 kW	27 297 BTU/h	6.5 kW	22 179 BTU/h
Ducted air-to-air, whole dwelling	10 kW	34 121 BTU/h	7.5 kW	25 591 BTU/h
Air-to-water monobloc, large dwelling	12 kW	40 946 BTU/h	9.5 kW	32 415 BTU/h
Ground source, brine to water	9 kW	30 709 BTU/h	9 kW	30 709 BTU/h

The two right-hand columns are the point of the exercise. A standard mini-split loses roughly a quarter of its output by −7 °C, a cold-climate model holds its rating well past that, and a ground-source machine barely notices because its source is metres underground at a near-constant temperature. Picking a unit on the rated column alone is how a house ends up heated by its backup heater.

What Speeds Up a Capacity Table Conversion

Both fields move while you read the capacity table

Each new kilowatt value converts as it is typed, so a whole column of temperature-by-temperature figures goes across in one pass.

Reverse the direction for an AHRI listing

The swap button switches to BTU/h → kW, which is the way round you need when a North American model has to be judged against an imported specification.

Whatever unit the specification happens to use

Both dropdowns are searchable and carry every power unit in the app, so a sheet quoting watts or another rate still converts without changing page.

Figures that drop straight into the load sheet

Copy hands over the number by itself, with no unit or spacing to strip out before it lands in a worksheet cell or a permit form.

Heat Pump Capacity Questions

Why does a heat pump lose capacity as the weather gets colder?

It is moving heat out of the outdoor air, and colder air is thinner in every sense that matters: the refrigerant absorbs less per pass, the compressor has a bigger pressure difference to work across, and time is lost to defrost cycles. Output therefore falls exactly when the building needs the most, which is the awkward geometry every load calculation has to deal with. A unit rated 7 kW (23 885 BTU/h) at 7 °C might hold only 5.6 kW (19 108 BTU/h) at −7 °C.

What is the balance point, and when does backup heat take over?

Plot the building's heat loss against outdoor temperature — a line rising as it gets colder — and the heat pump's capacity on the same axes, a line falling. Where they cross is the balance point: above it the heat pump covers the house alone, below it something else has to fill the gap. Convert both curves to BTU/h so they share a scale. If the crossing sits at −5 °C and the design condition is −12 °C, the shortfall at −12 °C is the backup capacity you must install.

Can I size from square footage instead of running Manual J?

Not if you want the equipment to behave. A per-square-foot rule cannot know the insulation level, the window area and orientation, the air-tightness or the duct losses, and those swing the answer by a factor of two between a 1970s house and a recent retrofit. Rules of thumb also lean high, which suits a furnace but punishes an inverter heat pump: oversized, it runs in short bursts, never settles into the efficient low-speed operation it was bought for, and swings the room temperature. A room-by-room calculation is also what a plans examiner asks to see.

Is HSPF the same measurement as COP?

Related, but not interchangeable. COP is a dimensionless ratio of heat out to electricity in at one stated condition — a unit giving 7 kW of heat for 2 kW of input has a COP of 3.5. HSPF is a seasonal figure covering a whole heating season and, in its usual North American form, mixes units: BTU of heat per watt-hour of electricity. Dividing an HSPF by 3.412 gives a rough seasonal COP, so HSPF 10 is around 2.9. European sheets publish SCOP instead, which is dimensionless like COP.

Why do cold-climate data sheets publish capacity at two outdoor temperatures?

Because one number cannot describe an inverter machine. The rated point shows what the unit does in mild weather; the low-ambient point shows how much of that survives on the coldest days, and it is the whole basis of the cold-climate claim. Manufacturers of those units advertise the second figure precisely because it stays close to the first — the pairing is the selling point. When only a rated figure is published, assume the cold-weather output is unknown rather than equal, and ask for the full capacity table before committing the design.

Kilowatts to BTU per Hour