Air conditioning units are sold by capacity, measured in kilowatts (kW) of cooling and heating. The right capacity for your room depends far more on how much heat gets into it than on how big it is. Two rooms with the same floor area can need very different units.
This guide explains what the numbers mean, what goes into a proper heat load assessment, and what happens if the size is wrong.
What the kW figures mean
A unit’s cooling capacity is how much heat it can remove from the room, and its heating capacity is how much it can deliver. Both are given in kW on the manufacturer’s data sheet, usually as a nominal figure with a minimum and maximum. Don’t confuse capacity with power input: a unit rated at a few kW of cooling draws much less electricity than that, because it moves heat rather than making it. Our plain English guide explains why.
Capacity isn’t fixed. Output in heating mode falls as it gets colder outside, and manufacturers publish tables showing how capacity changes with outdoor temperature. If you plan to heat with the unit, that matters as much as the cooling figure.
What really sets the size: heat gains
At survey we add up the heat that gets into a room on a hot day:
- Sun through glass. Usually the biggest factor. A large south or west facing window, roof light or glazed door can outweigh everything else. Blinds and solar film reduce it.
- Walls and roof. Solid walls, insulated cavity walls and timber frames behave differently. A room under a flat or poorly insulated roof, or a loft conversion, takes on a lot of heat.
- People. Each person adds heat, more when active. A bedroom with two people at night is different from a meeting room with ten.
- Equipment and lighting. Computers, screens, gaming PCs, servers, kitchen appliances and lighting all add heat.
- Air leakage and open doors. A room open to a hallway or kitchen shares its load with them.
- How you use the room. Cooling a bedroom overnight, a home office all day, or a dining room for a few hours each evening are different tasks.
For heating, we look at heat losses instead: glazing, walls, floor and roof, and how cold the room gets now.
Why floor-area rules mislead
Floor-area rules of thumb are everywhere online. They assume an average room with average windows, which most rooms aren’t. A small loft bedroom with a big roof light may need more cooling than a much larger north-facing lounge. A conservatory may need far more than its floor area suggests. Rules of thumb are fine for a first guess at budget, but not for choosing a unit.
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What goes wrong if the size is wrong
| Too small | Too big |
|---|---|
| Runs at full output on hot days and still can’t reach the set temperature | Reaches temperature fast, then cycles on and off |
| Louder, because it’s always working hard | Doesn’t run long enough to dry the air, so rooms can feel clammy |
| Higher running costs on peak days | Higher purchase price and often less efficient at low load |
| In heating mode, may not keep up in cold weather | Larger indoor and outdoor units than needed |
Modern inverter units can slow down a long way, which makes them more forgiving of a bit of oversizing than old on-off units. But they still have a minimum output, and a unit that’s far too big for a small room spends a lot of time at that minimum or switching off.
Multi-room systems
On a multi-split, each indoor unit is sized for its room, then the outdoor unit is chosen to suit the combination, within the limits the manufacturer sets for connected capacity. Not every room peaks at once: a bedroom and a home office rarely need full cooling at the same moment. We take that into account so the outdoor unit isn’t bigger than it needs to be.
What we check at survey
- Room dimensions and ceiling height.
- Window and roof light sizes, which way they face, shading and blinds.
- Wall, roof and floor construction, and what’s above and below.
- Typical occupancy and equipment.
- Whether you want heating as well as cooling, and which room is the priority.
- Where the indoor unit can go so its air reaches the whole room.
From that we recommend a capacity and a model, and the fixed-price quote shows it. If two sizes are close, we explain the trade-off.
Quick answers
What size air conditioner do I need for a bedroom?
It depends on the window size and direction, the roof above, and how many people sleep there. Many bedrooms take one of the smaller wall units, but a loft room or one with large south-facing glass may need more. We work it out at survey.
Is it better to oversize an air conditioning unit?
No. A unit that’s far too big cycles on and off, dries the air less well and costs more. Slightly generous is fine; well oversized isn’t.
Does the kW rating tell me how much electricity it uses?
No. Capacity is how much heat it moves. Electricity used is lower, and you’ll find it as the power input on the manufacturer’s data sheet.
Related services
- Air conditioning installation
- Wall-mounted air conditioning
- Multi-split air conditioning
- Domestic air conditioning
More advice
- What Happens at an Air Conditioning Site Survey
- How Air Conditioning Works: A Plain English Explanation
- Air Conditioning for Conservatories: Is It Worth It?
- Air Conditioning Energy Efficiency: SCOP, SEER and EPC Explained
- All advice
This guide is general information, checked against the sources linked on 30 October 2026. It isn’t legal, tax or financial advice. Rules, grants and product ranges change, so check the current position before you rely on it.