What size air conditioner do I need? BTU sizing guide for Irish homes
The short answer
The textbook cooling-only rule is around 340 BTU per m² (roughly 100 watts per m²). Irish installers typically size higher than that. Usually 500–600 BTU per m². Because most installs cover heating too and our housing stock varies a lot in insulation quality. A standard double bedroom of 12–16m² typically gets 9,000 BTU. A standard living room of 18–25m² typically gets 12,000 BTU. Right-sizing matters more than people think.
BTU is a small word that decides whether your air conditioner actually works. Buy too small and the unit runs flat-out forever and never gets the room cool. Buy too big and it costs you more, cycles on and off too fast, and does a rubbish job of dehumidifying. Most people land in the right zone by accident, but it pays to do the maths once before you sign a quote.
What BTU actually means
BTU stands for British Thermal Unit, a measure of energy. In the AC world, the number you'll see on every product page is BTU per hour. How much heat the unit can remove from a room in 60 minutes. Higher BTU means more cooling power.
The numbers you'll see on units sold in Ireland are almost always in standard sizes: 7,000, 9,000, 12,000, 14,000, 18,000, and 24,000 BTU. They map roughly to kilowatts of cooling power (divide BTU by 3,412 to get kW), but you'll rarely see kW on the box. Pick a BTU rating that matches your room and forget the kW conversion.
The basic sizing rule
The textbook HVAC rule for cooling alone is about 340 BTU per m², or roughly 100 watts per m². That's what you'd find in a manufacturer's pure cooling-load calculator for a well-insulated room with double glazing.
In practice, Irish installers usually size higher than that for two reasons: most modern AC units in Irish homes also get used for heating in shoulder season (heating needs more output than cooling for the same temperature swing), and Irish housing stock is a mix of well-insulated modern builds and draughtier older properties with single glazing. The realistic Irish working ranges:
- ~340 BTU per m² for well-insulated rooms, cooling only, no other adjustments
- 400–500 BTU per m² for a modern home where the unit will also do shoulder-season heating
- 500–600 BTU per m² for typical Irish housing stock used for both cooling and heating
- 600–800 BTU per m² for older homes with poor insulation, single glazing, or south-facing rooms
- Roughly 1.5–2x your normal-room baseline for conservatories, sun rooms, or rooms with heavy glazing
The rest of this guide uses around 500-600 BTU per m² as the working number because that's what most Irish installers fit for typical residential use.
The adjustments that matter in Ireland
The base m² rule gives you a starting point. Real Irish homes need adjustments because no two rooms are the same.
Insulation
A modern A-rated home built post-2010 cools efficiently. An older home with cavity wall insulation added later sits in the middle. A pre-1970s house with no wall insulation may need 30–50% more BTU than the base rule suggests because heat leaks back in as fast as you remove it.
Glazing
Single glazing acts like a heat tap that's always running. Double glazing is the Irish norm. Triple glazing is excellent. If a room has large single-glazed windows, add 20–30% to the BTU calculation. If it's triple-glazed, you can sometimes go 10% lower.
Window orientation
South-facing rooms with lots of glass get hammered by direct sun. Add 15–25% for a south-facing living room with large windows. North-facing rooms can often go slightly lower.
Ceiling height
Standard Irish ceilings are around 2.4m. The base rule assumes that. A room with 3m+ ceilings (Georgian or converted attic) has more volume to cool. Add roughly 10% per extra metre of height.
Occupancy and equipment
Rooms with a lot of people, computers, or kitchen appliances generate heat that AC has to remove on top of the ambient load. A home office with 2-3 monitors running all day needs more BTU than a guest bedroom of the same size.
Sizing table for Irish rooms
| Room | Typical size | BTU rating |
|---|---|---|
| Small bedroom / home office | Up to 10m² | 7,000 BTU |
| Standard double bedroom | 12–16m² | 9,000 BTU |
| Large master bedroom | 18–22m² | 9,000–12,000 BTU |
| Small living room | 15–20m² | 9,000–12,000 BTU |
| Standard living room | 20–28m² | 12,000 BTU |
| Large living room / open-plan | 28–40m² | 14,000–18,000 BTU |
| Conservatory (heavily glazed) | 12–18m² | 12,000–14,000 BTU |
| Open-plan over 40m² | 40m²+ | Multi-split recommended |
Why oversizing is a real mistake
Most people assume bigger is safer. With AC, it isn't. Three things go wrong when you over-spec:
- Short cycling. An oversized unit hits the target temperature too fast and shuts off. Then the room warms up, the unit kicks back in, cools fast, and shuts off again. This start-stop pattern is inefficient and wears the compressor out.
- Poor dehumidification. AC dehumidifies as it cools. Moisture condenses on the cold coil and drains away. That takes time. A short cycle doesn't run long enough to pull much moisture out, so the room ends up cold and clammy instead of cold and dry.
- Money wasted. Bigger units cost more up front and more to run. You're paying for capacity you don't need and not getting better comfort for it.
Why undersizing is also bad
Going the other way isn't the answer either. An undersized unit:
- Runs at full power constantly and never reaches the target temperature on warm days
- Uses more electricity than a right-sized unit because it never gets to ease off into its efficient inverter sweet spot
- Wears out faster from constant high-load running
- Disappoints you on the hottest days, which is exactly when you wanted it most
Right-sized genuinely is better than either extreme. The unit cycles gently around the target temperature, runs in its efficient inverter range most of the time, dehumidifies properly, and lasts longer.
Multi-split sizing: it's not just adding up
If you're cooling multiple rooms with one outdoor compressor feeding several indoor units, the sizing has an extra layer.
Each indoor unit is sized for its room (using the rules above). The outdoor unit doesn't need to equal the sum of the indoor units. Why? Because you'll rarely run all the indoor units flat-out at the same time. Bedrooms cool overnight, the living room cools in the evening, the office cools during the day. Manufacturers like Daikin and Mitsubishi typically allow total indoor capacity to be 100% to 130% of the outdoor rated capacity. The exact ratio depends on how the installer designs around expected load diversity for your home.
Don't try to do this maths yourself. A multi-split design is exactly the sort of thing an experienced installer should be doing for you. They account for pipe lengths, refrigerant volumes, and load diversity factors that aren't worth a homeowner learning. But you can sanity-check what they propose by checking each indoor unit is roughly the right BTU for its room.
Common Irish scenarios
Cooling a single upstairs bedroom for sleep
Standard 14m² Irish double bedroom, decent insulation, north-facing, cooling + shoulder-season heating use. Base: 14 × 500 = 7,000 BTU. Round up to 9,000 BTU for a bit of headroom and because that's the standard available size. Single split, simple install.
Cooling a living room + adjoining kitchen-diner
Combined 35m² open-plan space, south-facing patio doors. Base: 35 × 550 = 19,250. South-facing adjustment +20% = ~23,100. Best handled as a multi-split with two 12,000 BTU indoor units, one per zone. A single 24,000 BTU unit could work too, but the multi-split gives better zonal control.
Cooling a home office for daily use
12m² south-facing room with two monitors. Base: 12 × 550 = 6,600. South-facing +15%, equipment load +500: ~8,200 BTU. Go with 9,000 BTU. The investment makes sense because you'll use it daily through summer.
Cooling a conservatory
15m² heavily glazed conservatory. Conservatories punish standard sizing rules because of the glass area and solar gain. Apply roughly 1.5–2x your normal room rate: 15 × 900 ≈ 13,500 BTU. Round up to 14,000 BTU. Conservatories also benefit hugely from external blinds before AC. The unit will struggle if direct sun is hammering the glass all afternoon.
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Get matchedCommon questions
How do I calculate BTU for a room in Ireland?
The textbook cooling-only baseline is around 340 BTU per m² (roughly 100W per m²). Irish installers typically size higher than that, usually 500 to 600 BTU per m², because most installs cover both cooling and shoulder-season heating and Irish housing varies a lot in insulation quality. Adjust up further for poor insulation, single glazing, or south-facing windows. A 20m² bedroom typically gets 9,000 to 12,000 BTU in practice.
Is it better to oversize or undersize?
Neither. Oversizing wastes money up front, runs in short cycles, and fails to dehumidify properly. Undersizing means the unit runs flat-out constantly and never reaches temperature. Right-sizing is genuinely better than either extreme.
What BTU do I need for a bedroom?
A standard Irish double bedroom of 12 to 16m² typically needs 9,000 BTU. A small single bedroom or home office up to 10m² may only need 7,000 BTU. A larger master bedroom with lots of glazing may need 12,000 BTU.
What BTU do I need for a living room or open-plan space?
A standard Irish living room of 18 to 25m² typically needs 12,000 BTU. Larger living rooms or open-plan kitchen-diner spaces of 25 to 40m² typically need 14,000 to 18,000 BTU. Open-plan over 40m² usually needs a multi-split.
What size AC do I need for a conservatory?
Conservatories need substantially more BTU per m² than internal rooms because of the glazing and direct sun. A typical rule is 1.5 to 2x the BTU per m² you'd use for a normal room of the same size. A 15m² conservatory might need 12,000 to 14,000 BTU rather than the 9,000 you'd use for a similar-sized bedroom. External blinds help a lot.
Does ceiling height matter?
Yes. Standard sizing assumes 2.4m ceilings. Higher ceilings (Georgian rooms, converted attics, double-height spaces) mean more air volume to cool. Add roughly 10 percent per extra metre of ceiling height.