Cost to run air conditioning: per hour, per day, per summer

Air conditioning is the biggest summer line on the bill, and its range is enormous because the machines range from a 900 W bedroom portable to a 3,500 W central system. The honest anchor figures: a window or portable unit around 1,000-1,500 W costs roughly 18-28 cents or 20-35p per hour at average US/UK rates; central air near 3,500 W costs roughly 60-70 cents or up to a pound an hour while actually running (it cycles off at temperature, so real hourly averages land lower). Eight hot hours a night, every night of a hot month: anywhere from 45 to 170 dollars - which is why the settings matter more than the machine. Price your own unit through the electricity cost calculator.

The levers, biggest first

Thermostat discipline: every degree cooler adds real cost; a setpoint around 24-25 C (75-77 F) with a fan for the wind-chill effect is the efficiency sweet spot - fans on summer mode make a higher setpoint feel fine, and fans cool people, not rooms, so they go off when you leave. Block the sun before it enters: blinds closed on the sunny side, awnings or window film on the worst windows - preventing heat beats removing it. Maintenance: a choked filter makes the machine run long and hard; check it monthly in season. Timing: pre-cool before the peak-price hours if you are on a time-of-use plan (the tariff guide), and let the insulation hold the cool.

The sizing sanity check

An oversized unit cools fast, shuts off damp, and leaves a clammy room; an undersized one runs forever without getting there. The right size for a bedroom is far smaller than the sales floor suggests - and for many Tier-1 homes, the honest cheapest summer is one right-sized unit in the room you actually sleep in, not the whole-house default.