The thermostat question: why the smartest air-conditioning setting is neither off nor arctic

The great summer argument — leave the air conditioning running while you're out, or switch it off entirely — turns out to have a third answer, and it is the one the experts favour. Associated Press reporting this week says the strongest strategy is neither extreme: set the thermostat several degrees higher for hours at a time, particularly during the workday, so the home uses less energy without letting heat and humidity build to levels that are expensive or unsafe to reverse.

What happened

The numbers behind the advice are concrete. AP cites US Department of Energy guidance that adjusting thermostats by 7 to 10 degrees Fahrenheit for eight hours a day can cut cooling costs by up to 10 per cent, with experts adding that each single-degree increase yields roughly 3 per cent in savings. The caveats matter as much as the figures: the real benefit depends on climate, construction, insulation and the health needs of the people inside. A well-insulated home in dry Arizona can safely warm through the day; in humid Florida, moisture control is part of the air conditioner's job, and switching off entirely risks discomfort and even mould.

Why it matters

Both extremes fail for different reasons. Chilling an empty house wastes money and energy; a full shutdown lets indoor temperatures climb until the unit must work punishingly hard later, increasing wear and erasing much of the saving. The setback approach treats cooling as a managed system rather than an on-off habit — and a programmable or smart thermostat automates the pattern, starting the cooldown before occupants return and avoiding the classic mistake of overcooling in a rush. Cheaper supporting measures compound the effect: blinds and reflective films block heat before the system has to fight it, fans improve comfort, and sealed leaks and clean filters let the unit work less aggressively.

There is a hard limit to the efficiency framing, though: heat safety comes first. A healthy adult in a shaded, insulated home has room to economise. An older person, a young child or someone with a medical condition may need a lower, steadier setting regardless of the bill. In dangerous heat, the first priority is preventing illness, not winning the savings league — rigid one-size-fits-all advice is precisely what the experts warn against.

The bigger picture

Multiply one household's thermostat by millions and the stakes change category. When homes demand maximum cooling simultaneously, grids strain; setbacks that trim unnecessary demand reduce the risk of heat-related power failures at exactly the moment cooling is most needed. The Guardian's reporting on Europe's air-conditioning debate shows how far the issue now extends beyond appliance tips: countries that historically relied on fans, shade and cool nights are reconsidering mechanical cooling, with all the questions about energy policy, inequality and care-home safety that follow. Air conditioning saves lives in extreme heat, yet access is uneven and the people most at risk are often least able to afford it.

What happens next

As heat waves grow more frequent, the thermostat is becoming a small control point in a much larger story about buildings, grids and climate resilience — and household advice will keep evolving with it. The current consensus is practical rather than ideological: do not assume switching off is efficient, do not assume running cold all day is necessary, set back gradually, respect humidity and health, and layer in the unglamorous fixes that make every degree cheaper. In a hotter world, the humble thermostat has quietly become a risk-management tool.