What BTU aircon do I need?

NEA's rule of thumb for aircon BTU and room size

The National Environment Agency (NEA) gives a simple formula for choosing cooling capacity. Take the total floor area the aircon will cool, in square metres, and divide it by 5. The answer is the capacity you need in kW. To convert it to BTU per hour (BTU/hr), multiply the kW figure by 3,412. The formula is set out in NEA's tips on buying energy-efficient appliances.

NEA's Resource Efficiency Guide for New Home Owners works through an example: a 35 square metre area divided by 5 gives 7kW. Multiplied by 3,412, that is about 23,900 BTU/hr.

Using NEA's rule of thumb, a 10 square metre bedroom divided by 5 gives 2kW. Multiply 2 by 3,412 and you get 6,824 BTU/hr. Treat the result as a starting point for an ordinary room, then adjust for the factors below.

Factors that need more capacity

The rule of thumb looks only at floor area. NEA's appliance buying tips say a larger aircon may be needed for rooms with double ceiling height, a large window area such as full-height glass, windows facing the sun (east or west), or extra heat sources such as a bar fridge.

The formula does not say how much extra to allow, so ask each supplier how they have allowed for these features in the capacity they recommend.

Bedroom or living room sizing

Work out each room on its own. Measure the floor area of the bedroom, study or living room the unit will cool, and apply the formula to that area only. If the living and dining areas form one open space cooled by the same unit, use the combined floor area, since NEA's rule is based on the total area being cooled.

If you are weighing single split units against a multi-split system, see types of aircon. For a multi-split system, NEA's tips page says the total capacity is usually the sum of the rooms' capacities. A smaller total may be enough if all the rooms are not cooled at the same time.

Ask the supplier for the capacity of each indoor unit and of the whole system, so you can check each figure against the formula.

Why a bigger aircon is not better

NEA advises choosing the right cooling capacity, not the largest. On its tips page, NEA says oversized units cost more to run and may not dehumidify well, because they cool the room too quickly. Undersized units do not cool or dehumidify the room adequately. NEA also suggests comparing life cycle cost, not just the purchase price: life cycle cost is the purchase price plus the annual energy cost multiplied by the lifespan in years.

Reading capacity on the Energy Label

NEA's Energy Label covers air-conditioners, refrigerators, clothes dryers, televisions and water heaters. For an aircon, it shows the cooling capacity in kW, so you can compare it with the figure from the rule of thumb. It also rates energy efficiency in ticks, from 1 to 5. More ticks means a more energy-efficient model.

According to NEA's Energy Label page, the label also shows an annual energy cost and consumption, worked out under fixed assumptions. For aircons, these include 8 hours of daily use plus 16 hours on standby, so your own use may give different figures. For split and multi-split aircons, the model on the label is the outdoor unit's model number.

If you are also choosing a compressor type, see inverter vs non-inverter aircon.

When to stop and call a professional

NEA's rule of thumb gives you a starting figure, not a full assessment. For a detailed assessment, NEA says to consult your aircon supplier, as its guide for new home owners sets out.

Ask for a closer look if your home has:

• an odd-shaped room, such as an L-shaped living area
• an open-plan layout where the living, dining and kitchen areas share one space
• strong west sun or a large area of glass
• several rooms on one multi-split system

In these cases, ask the supplier whether they can do a heat load assessment, and how they arrived at the capacity they recommend for each room.

When you collect quotes for aircon installation, ask each company to state the capacity it proposes for each room, so you can compare quotes on the same basis.

The price depends on the job; see aircon installation cost in an HDB flat. For more on choosing a unit, see aircon in Singapore.

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