Guide · Updated September 24, 2026
Wall heat pump · BTU calculator
Calculating the Right Wall Heat Pump Size (2026 Guide)
By AirGreen Climatisation & ChauffagePublished April 11, 2023Updated 9 min read
Looking for a wall-mounted heat pump (mini-split) but not sure what size to choose? Working it out can seem daunting, but it’s an important step. A poorly sized unit costs more to run, heats less well and wears out faster. Here’s what you need to know to calculate the right size for your home or office, with tools that do it in seconds.
-23 °C
Montreal design temperature (National Building Code 2020)
12,000 or 15,000?
BTU/h for 750 sq ft: two Hydro-Québec pages, two answers
70%
Minimum output an ENERGY STAR “cold climate” model keeps at -15 °C
80 to 125%
The target range in Natural Resources Canada’s sizing guide
In short
In Québec, count on 16,000 to 20,000 BTU/h per 1,000 sq ft as a starting point (Hydro-Québec’s two rules; our estimators start at 18,000). Then adjust for insulation, ceiling height, windows and sun, and check what the unit delivers in your city’s cold: -23 °C in Montreal, -24 °C in Laval and Longueuil. A wall unit heats its own zone first: a closed room needs a dedicated head or keeps its baseboard heater.
9,000
BTU/h: about 450 to 560 sq ft
12,000
BTU/h: about 600 to 750 sq ft
18,000
BTU/h: about 900 to 1,125 sq ft
24,000
BTU/h: about 1,200 to 1,500 sq ft
Benchmarks at 16–20 BTU/h per sq ft, 8 ft ceiling. For your exact case, use the calculator below.
01 · Calculator
Your starting size in 60 seconds
An estimate, not an engineering calculation. It tells you where to start, what a real model delivers in the cold, and what to ask the estimator.
600 sq ft
Ceiling height
Insulation
Windows and sun
Regular occupants
2
Kitchen in the zone
2
Kitchen in the zone
Design temperature: -23 °C (-26 °C at 1%) · city hall. National Building Code 2020.
Recommended starting size
12,000BTU/h
Estimated need: 10,800 BTU/h · coverage 111%, within the target range
Too smallTarget range 80–125%Too big
Also possible: 9,000 BTU/h (83%). The estimator decides on site.
600 sq ft × 18 BTU/h (average)10,800 BTU/h
8 ft ceilingNo adjustment
Windows and sun: normalNo adjustment
2 regular occupantsNo adjustment
Kitchen in the zoneNo
Estimated need10,800 BTU/h
In your city’s cold
The GREE EXTRÊME 12,000 delivers up to 12,000 BTU/h at -15 °C. Design temperature in Montreal (city hall): -23 °C. NEEP publishes no data colder than -20.6 °C for this model, so we don’t make it up. Keep backup heat.
See the GREE EXTRÊME 12,000Recommended starting size
12,000BTU/h
Estimated need: 10,800 BTU/h · coverage 111%, within the target range
Too smallTarget range 80–125%Too big
Also possible: 9,000 BTU/h (83%). The estimator decides on site.
600 sq ft × 18 BTU/h (average)10,800 BTU/h
8 ft ceilingNo adjustment
Windows and sun: normalNo adjustment
2 regular occupantsNo adjustment
Kitchen in the zoneNo
Estimated need10,800 BTU/h
In your city’s cold
The GREE EXTRÊME 12,000 delivers up to 12,000 BTU/h at -15 °C. Design temperature in Montreal (city hall): -23 °C. NEEP publishes no data colder than -20.6 °C for this model, so we don’t make it up. Keep backup heat.
See the GREE EXTRÊME 12,00002 · The factors
Five basic factors, and two that get overlooked.
Several factors come into play. Here are the five that decide the size, what they are worth in numbers, and the two that most charts ignore.
Factor 1
Room size
The most important factor: the bigger the area to heat or cool, the more BTUs you need.
In numbers: 16 to 20 BTU/h per sq ft according to Hydro-Québec; 18 for our estimators.
Calculator: area →Factor 2
Climate
The colder it gets, the more output you need to keep the house comfortable. In Québec, heating is what decides.
In numbers: Design temperature: -23 °C in Montreal, -26 °C in Saint-Jérôme (NBC 2020).
Calculator: your city →Factor 3
Insulation
A well-insulated house holds its heat: it can do with a smaller unit.
In numbers: Well insulated 16, average 18, poorly insulated 20 BTU/h per sq ft in our calculator.
Calculator: insulation →Factor 4
Windows
More windows, or large sunny windows, call for more output, for heating as well as cooling.
In numbers: Very sunny room +10%, very shaded −10% (ENERGY STAR).
Calculator: windows and sun →Factor 5
Ceiling height
A higher ceiling means more air to heat and to cool.
In numbers: 9 ft ceiling +13%, 10 ft +25%, 12 ft +50% (air volume).
Calculator: ceiling →Overlooked
Occupants and kitchen
People and cooking give off heat. For cooling, it counts.
In numbers: +600 BTU/h per occupant beyond 2, +4,000 BTU/h for a kitchen (ENERGY STAR).
Calculator: occupants →Overlooked
Closed doors
A wall unit heats its own zone first. Behind a closed door, heat circulates poorly.
In numbers: A dedicated head (multi-zone), or the baseboard heater stays in place.
Room-by-room planner →03 · The thermometer
How many of your BTUs are left at -25 °C?
Slide to the left. The bars show the maximum output of four models we install, according to the AHRI-certified data published by NEEP.
Outdoor temperature
-15 °C
The ENERGY STAR “cold climate” test point
-30 °C-20 °C-10 °C0 °C+8 °C
Shortcuts
GREE EXTRÊME, maximum output at this temperature
GREE EXTRÊME 9,000Certified
13,000
144%
GREE EXTRÊME 12,000Certified
12,000
100%
GREE EXTRÊME 18,000Certified
20,000
111%
GREE EXTRÊME 24,000Certified
24,000
100%
At -15 °C, all four models still have data. Look at the percentage: it’s what remains of the nominal size.
Bar: maximum output. Line: nominal size. “Estimated” = between two certified NEEP points, on a straight line. Below the coldest certified point, we show nothing: we never extend a curve.
Outdoor temperature
-15 °C
The ENERGY STAR “cold climate” test point
-30 °C-20 °C-10 °C0 °C+8 °C
Shortcuts
Maximum BTU/h at the NEEP-certified points. The vertical line follows your temperature.
GREE EXTRÊME, maximum output at this temperature
GREE EXTRÊME 9,000Certified
13,000
144%
GREE EXTRÊME 12,000Certified
12,000
100%
GREE EXTRÊME 18,000Certified
20,000
111%
GREE EXTRÊME 24,000Certified
24,000
100%
At -15 °C, all four models still have data. Look at the percentage: it’s what remains of the nominal size.
Bar: maximum output. Line: nominal size. “Estimated” = between two certified NEEP points, on a straight line. Below the coldest certified point, we show nothing: we never extend a curve.
The certified curves at a glance
Maximum BTU/h at the NEEP-certified points. The vertical line follows your temperature.
9,00012,00018,00024,000
9,00012,00018,00024,000
04 · Your city
The cold that counts, city by city.
Size is judged by your city’s cold, not by a January average. Here are the official design temperatures for Greater Montreal.
City
January 2.5%
January 1%
CityMontreal
city hall
-23 °C
-26 °C
Montréal-Nord
-24 °C
-26 °C
Saint-Laurent
-23 °C
-26 °C
Dorval
-24 °C
-26 °C
Beaconsfield
-24 °C
-26 °C
Laval
-24 °C
-26 °C
Pincourt
-24 °C
-26 °C
January 2.5%
January 1%
Montreal
city hall
-23 °C
-26 °C
Montréal-Nord
-24 °C
-26 °C
Saint-Laurent
-23 °C
-26 °C
Dorval
-24 °C
-26 °C
Beaconsfield
-24 °C
-26 °C
Laval
-24 °C
-26 °C
Pincourt
-24 °C
-26 °C
How to read this table: in Laval, it is colder than -24 °C during only 2.5% of January hours. That is the design temperature in the National Building Code 2020 (Appendix C, Table C-2). Longueuil and Boucherville are not listed: we show the nearest station.
05 · Planner
Place your rooms, and we’ll tell you how many heads.
Check the rooms to heat, adjust their area and tell us which ones stay closed. Two scenarios appear: a single wall unit, or a multi-zone system.
Your home
Insulation
Design temperature: -24 °C.
Your rooms
280 sq ft
140 sq ft
140 sq ft
170 sq ft
120 sq ft
100 sq ft
400 sq ft
Check the rooms to heat. An “open” room opens onto the living room with no closed door.
Your plan
Open area · wall unit 15,000 BTU/h
Living room
280 sq ft
Dining room
140 sq ft
Kitchen
140 sq ft · kitchen
Main bedroom
170 sq ft · 3,060 BTU/h
Baseboard or dedicated head
Bedroom 2
120 sq ft · 2,160 BTU/h
Baseboard or dedicated head
Open areaClosed doorOther floor
Your estimated total
19,300 BTU/h
Within Hydro-Québec’s range for a condo (12,000 to 24,000 BTU/h).
Scenario A · One wall unit, plus your baseboards
1 wall unit of 15,000 BTU/h for the open area
Estimated need: 14,080 BTU/h (living room, dining room, kitchen)
Main bedroom, Bedroom 2: your baseboards stay
The closed door keeps the wall unit’s heat on the other side.
Scenario B · A multi-zone system
Head 1: open area
Estimated need: 14,080 BTU/h
Head 2: main bedroom
Estimated need: 3,060 BTU/h
Head 3: bedroom 2
Estimated need: 2,160 BTU/h
1 multi-zone compressor for 3 heads
Chosen on site based on the total: 19,300 BTU/h.
Your rooms
280 sq ft
140 sq ft
140 sq ft
170 sq ft
120 sq ft
100 sq ft
400 sq ft
Check the rooms to heat. An “open” room opens onto the living room with no closed door.
Your plan
Open area · wall unit 15,000 BTU/h
Living room
280 sq ft
Dining room
140 sq ft
Kitchen
140 sq ft · kitchen
Main bedroom
170 sq ft · 3,060 BTU/h
Baseboard or dedicated head
Bedroom 2
120 sq ft · 2,160 BTU/h
Baseboard or dedicated head
Open areaClosed doorOther floor
Your estimated total
19,300 BTU/h
Within Hydro-Québec’s range for a condo (12,000 to 24,000 BTU/h).
Scenario A · One wall unit, plus your baseboards
1 wall unit of 15,000 BTU/h for the open area
Estimated need: 14,080 BTU/h (living room, dining room, kitchen)
Main bedroom, Bedroom 2: your baseboards stay
The closed door keeps the wall unit’s heat on the other side.
Scenario B · A multi-zone system
Head 1: open area
Estimated need: 14,080 BTU/h
Head 2: main bedroom
Estimated need: 3,060 BTU/h
Head 3: bedroom 2
Estimated need: 2,160 BTU/h
1 multi-zone compressor for 3 heads
Chosen on site based on the total: 19,300 BTU/h.
Why doors matter so much
Change the bedroom door and watch where the heat goes.
Living room
The wall unit blows here. It’s the zone it heats first.
Bedroom
The wall unit’s heat stays on the other side. The baseboard takes over, or a dedicated head if you go multi-zone.
What Natural Resources Canada says: an added wall heat pump is sized for the target zone, and can cover part of the needs beyond its room. Behind a closed door, heat circulates poorly: plan a dedicated head, or keep the baseboard heater.
06 · 4 rules, 4 answers
There is no “magic number”.
There is no single method for choosing the size. Some people simply multiply the square footage by a magic number to get BTUs. Slide the area: four published rules, four different answers.
750 sq ft
100 sq ft2,500 sq ft
Gap between the lowest and the highest rule
6,000 BTU/h
That’s 50% more between the lowest and the highest rule. That’s why we measure on site.
Hydro-Québec “adequate output” article
12,000 BTU/h
12,000 BTU/h per 750 sq ft
AirGreen our estimators’ starting rule
13,500 BTU/h
18,000 BTU/h per 1,000 sq ft, adjusted on site
Hydro-Québec “seven points” guide
15,000 BTU/h
15,000 BTU/h per 750 sq ft
ENERGY STAR room air conditioners
18,000 BTU/h
Chart by area bracket, cooling only
100 sq ft2,500 sq ft
Hydro-Québec “adequate output” article
12,000 BTU/h
12,000 BTU/h per 750 sq ft
AirGreen our estimators’ starting rule
13,500 BTU/h
18,000 BTU/h per 1,000 sq ft, adjusted on site
Hydro-Québec “seven points” guide
15,000 BTU/h
15,000 BTU/h per 750 sq ft
ENERGY STAR room air conditioners
18,000 BTU/h
Chart by area bracket, cooling only
Gap between the lowest and the highest rule
6,000 BTU/h
That’s 50% more between the lowest and the highest rule. That’s why we measure on site.
07 · Reference chart
The reference chart, redrawn and complete.
For reference only: the 14 brackets of the ENERGY STAR chart, with the four official adjustments and Hydro-Québec’s ranges for a whole home.
5,000
BTU/h
100 to 150 sq ft
6,000
BTU/h
150 to 250 sq ft
7,000
BTU/h
250 to 300 sq ft
8,000
BTU/h
300 to 350 sq ft
9,000
BTU/h
350 to 400 sq ft
10,000
BTU/h
400 to 450 sq ft
12,000
BTU/h
450 to 550 sq ft
14,000
BTU/h
550 to 700 sq ft
18,000
BTU/h
700 to 1,000 sq ft
21,000
BTU/h
1,000 to 1,200 sq ft
23,000
BTU/h
1,200 to 1,400 sq ft
24,000
BTU/h
1,400 to 1,500 sq ft
30,000
BTU/h
1,500 to 2,000 sq ft
34,000
BTU/h
2,000 to 2,500 sq ft
The 4 official adjustments (ENERGY STAR)
−10%
Very shaded room
+10%
Very sunny room
+600
BTU/h per occupant beyond 2
+4,000
BTU/h for a kitchen
For the whole home (Hydro-Québec)
Condo
12,000 to 24,000 BTU/h
Single-storey house
24,000 to 48,000 BTU/h
Two-storey house
up to 65,000 BTU/h
Cooling only. This chart comes from ENERGY STAR for room air conditioners. It says nothing about heating at -23 °C, which is what decides the size in Québec. The highlighted box follows the area in your calculation.
08 · Too big, too small
Why choosing the right size matters.
Wall heat pumps are popular for heating and cooling a zone: they are efficient and, when properly installed by certified refrigeration technicians, quiet. But their size is crucial to their performance and lifespan. Too small, it works harder and longer, which drives up the bill. Too big, it turns on and off more often, which reduces its efficiency and wears out its components.
Too big
CommonIt starts and stops constantly
Natural Resources Canada says it clearly: short cycles degrade performance and cause swings in comfort.
In summer, the air stays clammy
It cools the room before removing the humidity (ENERGY STAR). Hydro-Québec gives the same warning.
You pay for BTUs you don’t need
A bigger unit costs more to buy, without giving you more comfort.
The target range
80 to 125%
of the calculated load. That’s the range targeted by Natural Resources Canada’s sizing guide.
Too small
In extreme coldIt runs flat out for a long time
In extreme cold, it stays at maximum output and the room struggles to hold its temperature.
Your baseboards work harder
That’s the job of the backup heat Hydro-Québec recommends keeping, but it takes a bigger share of the bill.
Heat waves overwhelm it
During a heat wave, it can’t keep up in the most exposed rooms.
Already have one? Check what you’re experiencing.
Our reading
Check what you’re experiencing
We’ll tell you what it suggests: a unit that’s too big, too small, or a distribution problem.
Before drawing conclusions, have the maintenance and settings checked: a technician confirms the cause.
Our reading
Check what you’re experiencing
We’ll tell you what it suggests: a unit that’s too big, too small, or a distribution problem.
Before drawing conclusions, have the maintenance and settings checked: a technician confirms the cause.
09 · True or false
Six common beliefs. How many do you hold?
Answer, and we’ll tell you why, with the source. Two minutes, and you’ll be asking any salesperson the right questions.
01
Bigger is safer.
02
A heat pump’s heating capacity is rated at +8.3 °C.
03
There is a single official BTU-per-square-foot rule.
04
A “cold climate” heat pump keeps at least 70% of its output at -15 °C.
05
With a heat pump, you can remove the baseboards.
06
A wall unit in the living room also heats closed bedrooms.
Your score
0 / 6
6 questions to go.
10 · On site
What our estimator looks at, and no tool can see.
The on-site estimate is free. The office then sends you a written quote within 24 hours.
01
Open area or closed doors
A wall unit heats the zone it blows into first. We look at where heat can really circulate.
02
Ceilings and mezzanines
A cathedral ceiling or a mezzanine changes the air volume, and therefore the output.
03
Windows and orientation
A large south-facing window doesn’t have the same needs as a shaded room.
04
Insulation and drafts
Doors, windows, basement: we note what lets heat escape.
05
Existing heating
Baseboards, furnace, fireplace: we decide what stays as backup for extreme cold.
06
Outdoor unit
Location, clearance and your city’s bylaws. Each borough has its own rules.
07
Electrical panel
We check the space and the circuit available for the connection.
08
Noise and neighbours
We choose a spot that meets the municipal noise bylaw.
11 · Frequently asked questions
What we’re asked most often.
It depends on the rule. Hydro-Québec gives the equivalent of 16,000 BTU/h on one page and 20,000 on another. Our estimators start at 18,000. The ENERGY STAR chart gives 18,000 to 21,000 for cooling. Insulation, ceilings and windows do the rest.
It depends on the rule. Hydro-Québec gives the equivalent of 16,000 BTU/h on one page and 20,000 on another. Our estimators start at 18,000. The ENERGY STAR chart gives 18,000 to 21,000 for cooling. Insulation, ceilings and windows do the rest.
Final thoughts
Final thoughts
Calculating the right wall heat pump size is an important step for your comfort and for your unit’s efficiency. Consider the factors above to determine the size that suits you: you’ll save on energy costs and extend the life of your heat pump.
RBQ 5645-9605-01
Subcategories 15.8 Ventilation, 15.10 Refrigeration and 16 Electricity
Free estimate
An estimator measures the zone at your home, free of charge
Quote within 24 hours
The office sends you the exact price in writing
Showroom
1370 Joliot-Curie Street, Suite 712, Boucherville
Sources checked on September 24, 2026.
- Hydro-Québec, “Thermopompe : comment déterminer la puissance adéquate pour votre habitation ?” (French page, undated, accessed September 24, 2026).
- Hydro-Québec, “Sept points à considérer avant d’acheter une thermopompe” (French page, accessed September 24, 2026).
- Natural Resources Canada, Air-Source Heat Pump Sizing and Selection Guide, version 1.0 (December 21, 2020); toolkit updated December 23, 2024.
- National Research Council Canada, National Building Code of Canada 2020 (2nd printing, revisions of March 31, 2025), Division B, Appendix C, Table C-2.
- ENERGY STAR, Room Air Conditioners: sizing chart and adjustments (accessed September 24, 2026).
- ENERGY STAR, Heat Pump Specification, version 6.2: “cold climate” criteria.
- NEEP, Cold Climate Air Source Heat Pump List: AHRI-certified data for the GREE EXTRÊME 9,000 (204325176), 12,000 (9944776), 18,000 (9114294) and 24,000 (9123256), accessed September 24, 2026.
The sizes shown are starting estimates, not a CSA F280 calculation. Only an on-site visit can size your installation.