Air Conditioner Size Calculator (BTU)

Unit size by room area, for the room type selected

Getting the size wrong costs you money in both directions. A unit that is too small runs constantly without ever cooling the room properly, uses more electricity and wears out sooner. A unit that is too large cools quickly then shuts off repeatedly, which means it never runs long enough to pull humidity out of the air — the room feels cold and clammy at the same time — and it costs more to buy and install for no benefit.

In Thailand’s climate the air conditioner is usually the largest single item on an electricity bill, so this is worth getting right.

The basic formula. Multiply the room area in square metres by a factor based on how the room is used. Thai installers generally work with 700 to 1,000 BTU per square metre.

Why the room type matters. Two rooms of identical size can need different units. A north-facing bedroom that never gets direct sun needs about 700 BTU per square metre. A living room with large windows taking the afternoon sun may need 900. A kitchen with a stove and a refrigerator giving off heat all day can need 1,000.

Other things that push the requirement up: ceilings higher than normal, several people in the room, large glass windows, a top-floor room directly under the roof, and poor insulation. Top-floor rooms in particular are worth allowing extra for in Thailand, where roof spaces get extremely hot.

Round up to the nearest standard size, never down. Units are sold in fixed sizes — 9,000, 12,000, 18,000 and 24,000 BTU among others. If your calculation gives 16,000, choose 18,000 rather than 15,000.

How the running cost is estimated. Power draw is approximated as the BTU rating divided by the SEER figure, which is the efficiency number on the energy label. A higher SEER means less electricity for the same cooling. Multiply the power by hours used and by your cost per unit.

Treat the cost figure as an upper bound. It assumes the unit runs at full power continuously. In practice an inverter unit throttles back once the room reaches the set temperature, so the real cost is typically 30 to 40% lower. A non-inverter unit cycles fully on and off instead, which uses more.

Cutting the running cost. Setting 26°C instead of 25°C cuts consumption by roughly 10%. Running a fan alongside circulates the cool air and lets you set a higher temperature comfortably. Clean the filters every couple of weeks. And setting the temperature very low when you first switch on does not cool the room any faster — the unit is already running at full power either way.