Cooling load is driven by solar gain through glazing, occupancy, equipment, construction and ventilation – not by floor area. Oversizing produces short-cycling, poor dehumidification and a cool clammy room, which is worse than slightly undersizing.
Why the online calculators mislead
Every online BTU calculator asks for room dimensions and produces a number. It is a starting point and it is frequently wrong by a wide margin, because it ignores everything that actually determines the load.
Consider two identical 4 by 4 metre bedrooms in the same house. One faces north with a small window. The other faces west with a large window, is directly under an uninsulated roof, and has two people and a television in it. Their cooling loads are not remotely similar, and a calculator based on floor area gives both the same answer.
What actually drives the load
| Factor | Effect | Notes |
|---|---|---|
| Glazing area and orientation | Frequently the largest single element | West-facing glass takes an enormous afternoon gain. North-facing takes a fraction |
| Solar control and external shading | Can halve the glazing gain | Blinds inside are far less effective than shading outside |
| What is above the room | Substantial | A room under an uninsulated roof is a much harder proposition than a middle floor |
| Occupancy | Each person is a continuous heat source | A meeting room with twelve people is a serious load |
| Equipment | Every watt consumed becomes heat | Decisive in a home office or comms room |
| Construction and insulation | Determines how fast heat enters | A well-insulated room holds cool air |
| Ventilation and infiltration | Every cubic metre of warm outside air must be cooled | Matters more in draughty buildings and in commercial ventilation rates |
| Lighting | Less with LED, not zero | Significant in large retail |
| Adjacent spaces | A hot conservatory next door is a load | Frequently overlooked |
Floor area appears nowhere in that list except indirectly. It is a proxy at best.
Why oversizing is the worse mistake
People instinctively round up, reasoning that more capacity is safer. With air conditioning it is not.
- Poor dehumidification. Much of the comfort air conditioning delivers comes from removing moisture, and that requires the coil to be cold and the unit to run for a sustained period. An oversized unit reaches setpoint quickly and stops, having removed little moisture. The result is a room that is cool and clammy – measurably at target temperature and uncomfortable.
- Short-cycling. On, off, on, off. Every start is a current surge and a thermal shock, and the compressor spends its life starting rather than running.
- Worse efficiency. An inverter compressor is most efficient running slowly at part load. An oversized system cannot slow down enough to match a small load, so it cycles – losing the entire advantage of inverter control.
- Temperature swing. The room overshoots, then drifts, then overshoots again.
- Shorter compressor life, because starts are what wear compressors.
- Higher capital cost for worse performance.
A slightly undersized unit runs for longer at full output, dehumidifies properly, and takes a little longer to pull the room down on the hottest days. An oversized one is uncomfortable every day of the season. Given the choice between the two available sizes either side of the calculated load, the smaller is frequently the better answer.
Doing it properly
- Measure the roomDimensions, and critically the glazing area and its orientation.
- Establish the constructionWalls, roof above, floor below, insulation, glazing type.
- Count the internal gainsPeople, at a typical rate each. Equipment, at its actual power draw rather than its badge rating. Lighting.
- Consider the ventilation rateParticularly in commercial settings where it is set by occupancy.
- Calculate the load at design conditionsThe peak outdoor temperature the system is designed to cope with, and at the time of day the solar gain peaks for that orientation.
- Select a unit whose capacity matchesAt the actual operating conditions, not at the nominal rating conditions.
- Sanity-check against the room’s useIntermittent occupancy, or continuous. A room used for two hours an evening has a different requirement from a home office.
It is not a long job and it produces a defensible answer rather than a guess.
Rated capacity and real capacity
A unit’s headline capacity is quoted at standard rating conditions. Real performance varies with:
- Outdoor temperature. Cooling capacity falls as it gets hotter outside – exactly when you need it most. A unit rated at standard conditions delivers less on a genuinely hot day.
- Indoor conditions, including humidity.
- Pipe run length, which reduces capacity over distance.
- Height difference between indoor and outdoor units.
- Multi-split diversity, where the connected indoor capacity exceeds the outdoor unit’s output.
Good design accounts for these. A selection made from a headline figure without correcting for the actual conditions is optimistic.
Units, briefly
Capacity is quoted in kilowatts or in BTU per hour, and both describe the same thing. Roughly, 1 kW is about 3,400 BTU/hr – so a 2.5 kW unit is commonly sold as around 9,000 BTU, and a 3.5 kW as around 12,000.
Two cautions. First, the BTU numbers used in marketing are frequently rounded to familiar figures rather than being exact. Second, a capacity figure means nothing without knowing the conditions it was measured at.
And do not confuse capacity with power consumption. A 3.5 kW cooling unit does not consume 3.5 kW of electricity – it consumes considerably less, because it is moving heat rather than creating it. That ratio is the efficiency.
Getting a quote? Ask what the calculated load is and what it was based on. An answer that mentions glazing and orientation is a different quality of answer.
Questions we get asked about this
How do I work out what size air conditioning I need?
From the room’s heat gain – glazing area and orientation, what is above it, occupancy, equipment and construction. Floor area alone is a poor guide and the online calculators that use it frequently get it wrong.
Is it better to go bigger to be safe?
No. Oversizing causes short-cycling and poor dehumidification, leaving a room that is cool and clammy and a compressor that wears faster. Slightly undersized is usually the better error.
What is the difference between BTU and kW?
Two units for the same thing. Roughly 1 kW is about 3,400 BTU per hour. Marketing figures are usually rounded to familiar numbers.
Does a 3.5 kW unit use 3.5 kW of electricity?
No. That is its cooling capacity, not its consumption. It uses considerably less electricity because it moves heat rather than creating it.
Why does my unit struggle on the hottest days?
Cooling capacity falls as the outdoor temperature rises, so a unit delivers less on a hot day than its rating suggests. If it was sized from the headline figure without correcting for real conditions, it will be marginal exactly when you want it.
Can I fit a smaller unit and just run it longer?
Within reason, yes – and that is closer to how a well-sized inverter system should behave anyway. Too far undersized and it will not reach setpoint on the hottest days, which is the limit.
Need this doing?
Call and describe the problem — you will speak to an engineer, not a call centre. Written quotes, no estimates over the phone.