Choosing an AC: tonnage, star rating and the mistake most people make

The common instinct is to buy more tonnage than you need, on the theory that a bigger unit will cope on the worst day. It will — and it will short-cycle on every other day, reaching the set temperature before it has pulled any moisture out of the air. You get a room that is cold and clammy, and a compressor that starts and stops far more often than it should.

A serviceable rule for Indian conditions: about 0.08 tons per square metre of floor area for a normal bedroom, adjusted up for a west-facing wall, a top floor, a large window or more than two occupants. A 12x12 ft bedroom is usually a 1 ton machine, not the 1.5 ton one that gets sold.

On star ratings, the arithmetic is simple and worth doing before you decide. The difference between a 3-star and a 5-star inverter unit is typically recovered in three to four years of daily summer use, and sooner if the room runs at night. If the room is used occasionally — a guest bedroom — the cheaper unit is the rational buy.

Get the drain slope right at installation. Almost every complaint about water dripping down a wall traces back to a drain pipe run flat or, worse, uphill, because the outdoor unit was placed for convenience rather than for fall.

The common instinct is to buy more tonnage than you need, on the theory that a bigger unit will cope on the worst day.