Air Conditioner Calculator

Why Efficiency (COP) Varies by System Type

The Coefficient of Performance (COP) measures cooling output relative to electrical input. Installed split systems represent the gold standard, achieving COPs of 3.6 or more (A++ class) because their heat-rejection condenser sits entirely outdoors. Portable single-hose units are mobile but inefficient (COP of 2.6) due to negative pressure: venting hot exhaust air out of a window hose pulls hot air from the rest of the building back into the cooled room through gaps, undermining the cooling cycle.

Why Our Sizing May Differ From Other Calculators

This calculator uses 20–25 BTU/hr per square foot as its base rate, in line with UK residential conditions. Many widely-circulated sizing charts quote 40+ BTU/hr per square foot instead — often noticeably higher than the figure used here. That gap usually comes down to the climate and construction standards the chart was written for, not an error on either side:

• Climate and humidity load: Many popular charts are calibrated for US climates with much higher design temperatures and humidity than typical UK summers. Removing moisture from the air (latent heat) takes real cooling capacity that a mild UK climate simply doesn't demand as much of.
• Ceiling height: US residential ceilings commonly run 9–10ft, versus the UK's more typical 2.4m (around 8ft). More room volume per square foot of floor means more air to cool, which a flat rate calibrated to taller rooms will overstate for a standard UK space.

Rather than applying one flat rate to every room, this calculator adjusts for insulation quality, sun exposure, ceiling height, occupancy, and equipment load, so the result reflects your specific room rather than a generic bracket. It's also worth noting that a more powerful unit will cool a room faster, but sizing well above the calculated load can lead to short cycling: the unit satisfies the thermostat before it's run long enough to properly dehumidify the air, leaving the room cold but clammy and increasing wear on the compressor. Sizing to the calculated load gives the best balance of running cost, comfort, and equipment lifespan.

Why the Recommended Unit May Look Larger Than Your Calculated Load

For smaller rooms — a box room, a single home office, a small bedroom — you may notice the recommended split system sits well above the calculated cooling load. A room needing 3,000 BTU/hr might still show a 9,000 BTU/hr recommendation. This isn't the calculator inflating the number; it reflects two separate things happening at once:

• A small safety margin (~10–15%) is added to the raw load before matching to a product, in line with standard CIBSE/ASHRAE sizing practice. This covers hotter-than-average days and minor variance in room conditions, and is the same margin professional installers build in.
• UK wall-mounted split systems are rarely sold below 9,000 BTU (2.6 kW). Manufacturers simply don't produce smaller units in this format — anything under that figure gets rounded up to the smallest product that actually exists on the market, not the smallest number that would technically satisfy the load.

The market floor is doing most of the work in these cases, not the safety margin. A 3,000 BTU room isn't being sized up to 3,400 or 3,500 BTU with headroom — it's being matched to the smallest available product, full stop, because nothing between 3,000 and 9,000 BTU is manufactured as a UK split system.

This matters less in practice than it once did, because almost all modern UK split systems use inverter compressors rather than fixed-speed ones. An inverter unit modulates its output up and down to match demand, rather than switching fully on and off. A 9,000 BTU inverter system cooling a 3,000 BTU room will idle at a fraction of its rated output most of the time — it won't short-cycling the way an older fixed-speed unit would in the same situation, and the room won't feel "oversized" in normal use. It will run somewhat less efficiently than a unit matched more closely to the load, since compressors have a sweet-spot modulation range and spend more time near the bottom of it in this scenario, but this is a mild efficiency cost rather than a comfort problem.

If your room's calculated load is well below the smallest available split system, a portable unit — many of which are genuinely available from 5,000 BTU — may be a more proportionate match for the actual demand, at the cost of the lower efficiency (COP 2.6 vs. 3.6) covered above. Rooms flagged with a note reading "Smallest standard UK entry unit: 9,000 BTU / 2.6 kW" in the results table are hitting this market floor rather than a calculation issue.

How Sizing Adjusts for Solar and Internal Gains

Calculating the cooling capacity is more complex than heating because solar radiation and occupants are massive heat contributors. Standard rules of thumb (20 BTU/sq ft) only cover empty, standard-glazed rooms. Adding occupants (+600 BTU per person), moving the unit into a highly glazed conservatory (+25%), or choosing a south-facing room (+10% solar gain) will heavily increase the load. If you are calculating winter heating requirements instead of summer cooling, you can use our standard Radiator BTU Calculator (designed using CIBSE guidelines) to map room heat loss and radiator size.

How UK Electricity Surcharges are Calculated

To calculate operational costs, we divide cooling requirements (kW) by EER/COP to yield the active electrical draw, then multiply by average UK electricity tariffs. A modern 3.5 kW split unit drawing ~0.97 kW costs only ~24p per active hour. Ensuring units have high efficiency and setting moderate target temperatures are key to maintaining low cooling bills during heatwaves.

Converting BTU to kW and Air Conditioning Tons

For trade professionals and engineers requiring precise thermodynamic unit conversions rather than residential rules of thumb, the explicit formulas are governed as follows:

• BTU/hr to Kilowatts (kW): 1 Kilowatt of cooling capacity equates to exactly 3,412.142 British Thermal Units per hour. Formula: kW = BTU ÷ 3,412.142.
• BTU/hr to Air Conditioning Tons: 1 Ton of refrigeration (TR) represents the rate of heat transfer required to melt 1 short ton of ice in 24 hours, equating to exactly 12,000 BTU/hr. Formula: Tons = BTU ÷ 12,000.
• Kilowatts (kW) to Tons: One Ton of cooling capacity equates to 3.517 kW of heat extraction rate. Formula: Tons = kW ÷ 3.517.

These equations allow rapid conversions of heat exchange loads between imperial thermal metrics and metric-based mechanical engineering specifications.