How to choose your air-con BTU for an ice-cold room that saves electricity and doesn't drain your wallet!

When you buy an air conditioner for your home, the most important thing not to overlook is choosing a BTU rating that matches the size of the room If the BTU is too low, the unit has to run hard all the time, the room never gets cool and your electricity bill goes up. If the BTU is too high, it cools quickly but the compressor cycles on and off too often, humidity builds up in the room, and you've spent more on the unit than you needed to.

An easy formula for air-conditioner BTU

The basic formula most people use is BTU = room area (width x length in metres) x heat factor The heat factor depends on how the room is used and which way it faces the sun:

• Ordinary bedroom (not in strong sun): use a factor of 700 - 800
• Living room / reception room: use a factor of 800 - 900
• Home office / room with many electrical appliances: use a factor of 800 - 900
• Top-floor room / west-facing room (afternoon sun): use a factor of 900 - 1,000+

Room size vs. recommended BTU

If you just want a quick estimate for a standard room size, compare against this summary:

• Room of 9 - 12 sq m: suits a 9,000 BTU unit
• Room of 13 - 18 sq m: suits a 12,000 BTU unit
• Room of 19 - 24 sq m: suits an 18,000 BTU unit
• Room of 25 - 32 sq m: suits a 24,000 BTU unit
• Room of 33 - 40 sq m: suits a 30,000 BTU unit

Extra factors that call for more BTU

Besides floor area, there are hidden heat sources in a room that you should account for by adding roughly 1,000 - 2,000 BTU:

• Ceiling height: if the ceiling is higher than 2.8 metres, there is more air volume than usual
• Large glass walls: glass lets heat from the sun outside in very easily
• Number of occupants and appliances: rooms with several people, or with computers, a microwave or a fridge, build up a lot of heat

Electrical and installation precautions

Once you have the right BTU size, safe installation is the most important thing not to overlook.

Important caution: Running the power cables, choosing a breaker sized for the BTU rating and installing the earth wire for the air conditioner must be done only by an electrician or a qualified specialist. Never connect the wiring yourself — this prevents short circuits, current leakage or fires.