Air-Conditioner Sizing & Running Cost Calculator
Air-conditioner sizing and running-cost calculator — enter your room size, sun exposure and how many hours a day you run it to get the right BTU and horsepower, then compare inverter vs non-inverter monthly running cost and CO₂, with ASEAN electricity tariffs and the payback on the inverter premium. Runs in your browser.
Air-Conditioner Sizing & Running Cost Calculator
How to Use the Air-Conditioner Calculator
Enter the room
Set the floor area, choose how exposed it is to the sun, and how many people are usually in it. The gauge shows the cooling load in BTU/h.
Read the recommended size
The tool rounds the load up to the nearest standard unit and shows the horsepower class to look for.
Set hours and tariff
Enter daily running hours and your region or exact tariff to get monthly running cost and CO₂ for inverter and non-inverter.
Check the payback
Enter the price premium of the inverter model to see how quickly the energy savings pay it back.
Sizing and Running an Air-Conditioner
Choosing an air-conditioner is really two questions: how big should it be, and what will it cost to run? Both matter more in a tropical climate than almost anywhere else, because the unit works hard for long hours all year round. This calculator tackles both. For sizing, it estimates the cooling load of your room — the amount of heat the air-conditioner has to remove every hour to keep you comfortable — and expresses it in BTU per hour, the standard unit of cooling capacity. The load grows with floor area, with sun exposure, and with the number of people in the room, since bodies, lighting and electronics all add heat. The tool applies a tropical baseline of roughly 600 BTU/h per square metre, adjusts it for how much sun the room takes, adds a margin for extra occupants, and then rounds up to the nearest standard unit size, shown in both BTU and the horsepower classes that air-conditioners are sold by across Southeast Asia.
Getting the size right is not just pedantry. An oversized unit reaches the set temperature so quickly that it switches off before it has wrung enough moisture out of the air, leaving a room that feels cold and damp, and the constant on-off cycling wastes electricity and shortens the compressor’s life. An undersized unit has the opposite problem: it runs continuously, never quite catches up on the hottest afternoons, and still costs a fortune because it never gets to rest. Matching capacity to load — neither too big nor too small — is what delivers steady comfort at the lowest running cost.
The second half of the tool turns capacity into money. It converts the unit’s cooling output into the electrical power it actually draws using a coefficient of performance, a measure of how much cooling you get per unit of electricity. Inverter units, which vary their compressor speed to hold a steady temperature, achieve a higher coefficient than older non-inverter units that simply switch on and off, so they draw less power for the same comfort. The calculator works out the monthly kilowatt-hours, cost and carbon emissions for both types side by side using your local electricity tariff and grid carbon factor, and then lets you enter the price premium of the inverter model so you can see how many years of energy savings it takes to pay that premium back. For a household that runs the air-conditioner for many hours a day, that payback is often surprisingly short — and every year after it is pure saving. Everything is computed in your browser from transparent factors you can inspect, with no data leaving your device.
The cheapest air-conditioner to run is the one that is sized correctly and then left at a sensible temperature — not the coldest or the biggest.
10 Facts About Air-Conditioning
Cooling capacity is measured in BTU per hour.
In SE Asia, 1 hp ≈ 9,000 BTU/h.
An oversized unit short-cycles and dehumidifies poorly.
An undersized unit runs flat-out and never cools.
Inverters vary compressor speed instead of on/off cycling.
An inverter can use 30–50% less energy in steady use.
West-facing rooms need more cooling from afternoon sun.
Each extra person adds roughly 600 BTU/h of load.
Every 1 °C lower on the thermostat raises energy use.
This calculator runs in your browser — nothing is uploaded.
Frequently Asked Questions
- As a tropical rule of thumb this tool uses about 600 BTU/h per square metre for a normally-exposed room, adjusts for sun exposure, and adds roughly 600 BTU/h for each occupant beyond two. It then rounds up to the nearest standard unit size. Enter your room area and exposure and it shows the recommended BTU and horsepower.
- In Southeast Asia air-conditioners are commonly sold by horsepower rather than BTU. As a guide, 1 hp is about 9,000 BTU/h, 1.5 hp about 12,500 BTU/h and 2 hp about 18,000 BTU/h. The calculator maps your cooling load to the nearest standard hp class.
- No. An oversized air-conditioner cools the air quickly but switches off before it removes enough humidity, leaving the room cold and clammy, and the frequent on/off cycling wastes energy and wears the compressor. Right-sizing — matching capacity to the load — gives the best comfort and efficiency.
- A non-inverter compressor runs at full speed then switches off when the set temperature is reached, cycling on and off. An inverter varies the compressor speed to hold the temperature steadily, which avoids the energy spikes of restarting and is markedly more efficient during long runs — exactly the usage pattern in a tropical climate.
- It converts the unit’s cooling capacity to the electrical power it draws using a coefficient of performance (COP) — about 3.5 for an inverter and 2.8 for a non-inverter — multiplies by your daily hours and 30 days to get kilowatt-hours, then applies your electricity tariff and grid carbon factor. You can override the tariff with the rate from your bill.
- Usually, if you run the unit for many hours. Enter the price difference between the inverter and non-inverter models and the calculator divides it by the annual energy saving to show the payback period in years. Heavy users in hot climates typically recoup the premium within a few years and save thereafter.
- Yes. A room with large west-facing windows takes a heavy heat gain from the afternoon sun, so it needs more cooling capacity than a shaded room of the same size. The exposure setting applies a multiplier to the base load to account for this.
- Around 24–25 °C is comfortable and efficient for most people in the tropics. Every degree lower increases energy use noticeably, so combining a moderate setpoint with a fan to move air is usually cheaper and just as comfortable as a very low thermostat.
- They are good estimates for planning and comparison, not a guaranteed bill. Real performance depends on the specific model’s rating, insulation, outdoor temperature, how well the room is sealed, and maintenance. Use the result to size sensibly and compare options, then check the model’s official rated input power.
- Completely free, with no account or usage limit. It runs entirely in your browser, collects no data, and works offline once the page has loaded.
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