Buying an AC: tonnage, running cost and used units

Air conditioning is the largest single load in most Pakistani homes during summer, so the choice affects your bill far more than the purchase price suggests. Tonnage and inverter type are the two decisions that matter.

Getting the tonnage right

Undersize and the unit runs continuously without cooling the room; oversize and it short-cycles, which wastes power and leaves the air damp. Tonnage follows room volume, how much sun the room takes, ceiling height and insulation — a top-floor room under an uninsulated roof needs materially more than the same floor area on a shaded ground floor. Work from the room, not from what the neighbours bought.

What inverter actually saves

A conventional unit runs at full power and switches off when the room is cool. An inverter varies compressor speed and holds the temperature, which uses less power over a long run. The saving is real but it comes from long continuous running — if you use the unit for an hour at a time, the payback stretches out considerably. Match the technology to how you actually live.

Running it on backup power

Air conditioning is a heavy load and a demanding one to start. If you intend to run it on a generator or a solar system, confirm the start-up surge is within what your supply can handle. Inverter units are generally gentler on start-up than conventional ones, which matters more here than the efficiency rating.

Buying used

See it running and cooling before you pay, not just powering on. Check the outdoor unit for corrosion and bent fins, ask whether the gas has been refilled and how often — repeated refills usually mean a leak nobody has found. Ask whether the compressor has ever been replaced. Installation is a real additional cost, so factor it in when comparing a used unit against a new one with fitting included.

Common questions

What tonnage AC do I need?

It follows room volume, sun exposure, ceiling height and insulation. A top-floor room under an uninsulated roof needs more than the same area on a shaded ground floor.

Is an inverter AC worth the extra cost?

If you run it for long continuous periods, yes. The saving comes from holding a temperature rather than cycling, so short bursts stretch the payback.

Can I run an AC on solar or a generator?

Only if the supply handles the start-up surge, which is much larger than the running load. Inverter units are generally gentler to start.

What should I check on a used AC?

See it actually cooling, not just switching on. Check the outdoor unit for corrosion, and ask how often the gas has been refilled — repeated refills usually mean an unfound leak.

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