Why the range is so wide
Two systems cooling the same room can sit at opposite ends of 20p to 60p an hour. A modern inverter unit sips power once the room reaches temperature, adjusting its output instead of switching on and off; an older or cheaper machine runs harder for the same result. Efficiency ratings on the spec sheet translate directly into pence, which makes them worth reading before the brochure photography.
The room matters as much as the machine. A well-insulated bedroom holds its temperature; a glass-heavy loft room under a hot roof leaks cool air as fast as the unit makes it. The same blinds, curtains and draught-sealing that keep heat in through winter keep it out through summer, and every degree the room holds by itself is a degree you do not pay to make twice.
The habits that keep it cheap
Set a sensible target and leave it alone: every degree colder costs more, and a room a few degrees below the outside air already feels transformed. Close the door of the room you are cooling, shade the windows before the afternoon sun arrives rather than after, and let the unit's own thermostat cycle the compressor instead of switching the system on and off yourself.
Maintenance is the quiet lever. Clogged filters make the fan work harder for less air, so the occasional clean keeps the pence-per-hour where the spec sheet promised. None of this is exotic: an efficient unit, run steadily, in a room that holds its temperature, sits at the cheap end of the range for most of the summer.
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Compare air conditioning quotesThe heating side of the ledger
A reversible split system earns its keep twice, because the same machine heats in winter, and it heats efficiently: it moves warmth in from outside rather than generating it, delivering a multiple of what a plug-in electric heater gives you for the same electricity. For a single room, a loft office or an extension, that is often the cheapest warmth in the house.
Running cost is also a reason to buy well rather than cheaply: the efficiency gap between units outlasts the price gap within a couple of summers. Compare quotes from trusted air conditioning installers with the efficiency figures on each, and the running-cost question answers itself before anything goes on the wall.
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