Kilowatts to Kilocalories per Hour for Radiator Sizing

Sizing Emitters When the Radiator Catalogue Speaks kcal/h

A room's heat loss comes off the survey in kilowatts, but the radiator catalogue on the merchant's counter in Milan, Valencia, Istanbul or Athens may still list emitter output in kilocalories per hour — and so do plenty of price lists across Asia. Until both figures sit in the same unit, there is no way to tell whether the panel on the shelf actually covers the room.

Conversion factor: 1 kW = 859.8452279 kcal/h — in the van, multiply kilowatts by about 860. A type 22 panel rated 1.58 kW at Δt50 is the very same emitter as one listed at 1,359 kcal/h, and a room losing 6 kW needs 5,159 kcal/h of emitter surface.

Where the Kilocalorie Figure Still Turns Up

Emitter price lists in the old unit

Southern European and many Asian manufacturers keep printing kcal/h columns beside the kW ones, and distributor spare-part lists often reprint the kilocalorie data alone.

Room schedules written by two hands

Modern heat-loss software gives watts per room while the older technician's sheet stayed in kilocalories, so one job can carry both units down the same column.

Replacing one radiator in an old system

To match a cast-iron bank or a 1980s panel you need its rating in today's unit before you can pick a same-output replacement from a current range.

Underfloor loops quoted per square metre

Screed systems are specified as output per square metre. Converted to kilocalories, that figure lines up directly with the emitter tables in the same catalogue.

From Room Heat Loss to the Emitter on the Order

The sequence is the same whether you are filling in a full room-by-room schedule or pricing a single replacement panel.

Type the room's required output in kW

Enter 1.2, 2.5, 0.85 — whatever the heat-loss calculation gives for that room. The kcal/h figure appears as you type, and the field accepts a comma as decimal separator, which is how the number is written across most of the markets that still work in kilocalories.

Read across the catalogue at the same Δt

Compare your kcal/h number only with a column rated at the same temperature difference — nearly always Δt50 under EN 442. The copy button hands over the bare figure, ready to drop into a schedule cell or a quotation.

Correct for the flow temperature you will actually run

Catalogue output is a laboratory figure at high water temperature. Apply your Δt correction factor to the emitter rating first, then check the corrected output still covers the room load.

Turn an old survey sheet back into kW

Found an existing bank listed at 1,200 kcal/h? Press the swap button (↔) to run kcal/h → kW and read 1.396 kW — the figure a current catalogue and a heat pump quotation both want to see.

Δt belongs to the rating, not to the footnotes: an older kilocalorie table published to Δt60 shows roughly 27 % more output than the same radiator rated at Δt50. Check which temperature difference a table assumes before you set two emitters side by side.

Radiator and Underfloor Output at Δt50 in Both Units

Representative catalogue figures for the emitters that turn up on domestic jobs, all at Δt50 (75/65 °C flow and return into a 20 °C room), with the kilocalorie equivalent alongside.

Emitter	Size / basis	Output (kW)	Output (kcal/h)
Steel panel, type 11	600 × 1000 mm	0.91 kW	782 kcal/h
Steel panel, type 22	600 × 1000 mm	1.58 kW	1,359 kcal/h
Steel panel, type 22	600 × 1600 mm	2.53 kW	2,175 kcal/h
Steel panel, type 33	600 × 1000 mm	2.19 kW	1,883 kcal/h
Cast-iron column	4-column, 600 mm, per section	0.14 kW	120 kcal/h
Chrome towel rail	1200 × 500 mm ladder	0.40 kW	344 kcal/h
Underfloor in screed	150 mm pipe spacing, tiled, per m²	0.07 kW	60 kcal/h
Underfloor at the comfort limit	29 °C floor in an occupied room, per m²	0.10 kW	86 kcal/h

The logic behind the type numbers shows up down the panel rows: a second water panel with its fin set roughly doubles what the same wall length delivers, and a third adds about a further third again. The last two rows show the ceiling on a wet floor — even at the warmest comfortable surface temperature, a square metre gives less than a tenth of a single type 22 panel, which is why a floor system needs the whole room area working for it.

What the Converter Does on a Radiator Schedule

Walk a whole room list without clearing the box

Both fields stay live as you type, so bedroom, hall, bathroom and kitchen loads can go through one after another with the emitter figure read off for each.

Flip direction for a legacy survey sheet

The swap button turns the page into kcal/h → kW, the direction you need when the existing emitters are documented in the old unit and the new plant is not.

Reach watts and BTU/h from the same dropdowns

Searchable unit lists on both sides cover every power unit, so a per-square-metre figure in watts or an imported spec in BTU/h drops into the same comparison.

Clean emitter figures for the quotation

Results carry up to eight decimals with thousands spaced for reading, while the copy button gives a bare number with no unit stuck to it.

Emitter Sizing Questions from the Job

What does Δt50 on a radiator catalogue actually mean?

It is the gap between the mean water temperature inside the radiator and the room air. Δt50 means the test ran at 75 °C flow and 65 °C return into a 20 °C room: mean water is 70 °C, which sits 50 K above the air. Every kW and kcal/h number in the table is tied to that condition, so a rating quoted at any other Δt is not comparable until it has been corrected.

How far does emitter output fall when the flow temperature drops?

Steeply, because output follows the temperature difference raised to an exponent of roughly 1.3 rather than tracking it proportionally. Run the same radiator at Δt30 instead of Δt50 and it gives around 51 % of its catalogue figure; at Δt20 it gives about 30 %. The 1.58 kW type 22 panel above becomes a 0.48 kW emitter — 413 kcal/h — once the water is down at 45/35 °C.

Do the radiators need to grow if a heat pump replaces the boiler?

Usually yes, and by more than people expect. A room needing 1.5 kW on 75/65 °C water wants an emitter rated close to 4.9 kW at Δt50 — about 4,250 kcal/h in an older catalogue — to still deliver that 1.5 kW on 45/35 °C water. In practice that means stepping a type 11 up to a type 22 or 33, going longer along the wall, or moving the room onto a floor circuit.

What separates a type 11, type 22 and type 33 panel?

The two digits count water panels and convector fin sets: type 11 is one panel with one fin set, type 22 is two of each, type 33 is three. At an identical 600 × 1000 mm size that works out at roughly 782, 1,359 and 1,883 kcal/h at Δt50. The deeper types buy their extra output largely from convection through the fins, and convection weakens faster than the radiant share as flow temperature comes down — worth remembering before a type 33 is chosen for a low-temperature system.

Why do radiator catalogues in southern Europe and Asia still print kcal/h?

Heating engineering ran on the kilocalorie for decades before SI units took over the standards, and a whole generation of installers, merchants and heat-loss sheets learned emitter sizes in that unit. Manufacturers kept the column so long-standing customers could still recognise a model, spare-part lists inherited the old data, and the habit stayed put in Italy, Spain, Turkey, Greece and much of Asia.

Kilowatts to Kilocalories per Hour