What happens to your air con if it runs for hours when nobody is home

When the air conditioning is left running while no one is at home, the system keeps working to maintain a constant temperature… but there's nobody there to enjoy it.

This results in two very specific effects:

• Continuous unnecessary energy consumption, even if the unit is not working at full capacity.
• Increased wear and tear on the system, especially the compressor.

There’s also an important factor that is often overlooked: the condition of the unit itself. For example, dirty filters increase energy consumption and reduce efficiency, forcing the air conditioner to work harder to achieve the same result.

That’s why it’s highly recommended to clean the filters before summer begins to start the hot season with the system in optimal condition.

Cost breakdown: running it for 8 hours vs. turning it off and back on

This is where we find the real answer: by looking at the cost.

1.     Scenario A: leaving the air conditioning on

• Average power: 1,500 W (1.5 kW)
• Time: 8 hours
• Consumption: 12 kWh

Estimated cost: €1.80–€2.40 per day.

2.     Scenario B: turning it off and switching it back on

• The air con is off while you are out
• The air con is switched back on for about 1 hour when you get back
• Consumption: ~1.5 kWh

Estimated cost: €0.20–€0.30.

The difference between the two scenarios is clear: you can save up to €2 a day simply by switching the air con off when you’re not home.

Over a summer month, that difference can easily add up to over €50 on your electricity bill.

Home insulation changes everything: how to know if yours is up to par

Not all homes behave the same when you turn the air conditioning off. And this is key to deciding how to use it.

In well-insulated homes, the temperature rises slowly, meaning you can switch the system off for several hours without losing comfort.

In homes that are more exposed to heat, the process is faster and the house warms up more quickly.

You can use this simple rule of thumb:

• If your home stays cool for several hours → good insulation
• If it heats up quickly → insulation could be improved

While this doesn’t change the general recommendation, it does affect how long you’ll need to regain comfort when you return.

When it actually makes sense to leave it running: exceptional cases

Although turning the unit off is usually the best option, there are situations where leaving the air conditioning on might actually make sense:

• Extreme heat waves (≥ 40°C)
• Homes that take over 2 hours to cool down from scratch
• Homes with vulnerable people (babies, elderly people, or individuals with health conditions)

In these cases, the key is not to maintain maximum comfort all the time, but to prevent the house from overheating without driving up energy consumption.

How can you do this? It’s simple: raise the temperature to 27–28°C and turn on the eco mode or auto mode. This keeps the house at a safe and comfortable temperature, but with much lower energy consumption.

Eco mode, auto mode, and timers: the real alternative to leaving it on all day

Between leaving the air conditioning on all day and turning it off completely, there’s a much smarter third option: using the system efficiently with scheduling.

A simple habit can make a big difference:

• Turn the air con off about 15 minutes before leaving the house.
• Schedule it to switch on before you return.
• Set the temperature to a comfortable level (24–26°C).

The timer is your best ally here. If you’re not sure how to use it, you can follow a step-by-step guide on how to schedule your air con timer so the house is cool when you arrive.

Additionally, there are two key points worth remembering:

• Eco mode can reduce energy consumption by up to 30%.
• Every degree you lower the air con temperature increases consumption by around 8%.

If you want to take things a step further, you can also learn how to keep your air con at the right temperature without driving up your electricity bill.

What if I leave the air con on overnight? The bedroom scenario

At night, the situation changes… and so does energy consumption.

Outdoor temperatures typically drop, meaning your air conditioner no longer has to work as hard as it does during the hottest hours of the day. That’s why leaving it on while you sleep doesn’t have the same impact as leaving it running all day in an empty home.

In this case, it can make sense to run the unit for several hours straight, especially on particularly hot nights or if you struggle to sleep in the heat. However, there’s no need to keep it running at full power all night long.

The best option is to turn on sleep mode or night mode. This feature is designed to maintain comfort while you sleep by gradually adjusting the temperature and reducing the system’s workload once intense cooling is no longer necessary.

The result is simple: more comfort, lower energy consumption, and a more pleasant temperature throughout the night. Because sleeping cool in summer is great… but doing it without overspending is even better.

FAQ: common questions about leaving the air conditioning on

Does the air con use more electricity when turning on than when it’s already running?

This is one of the most common beliefs every summer, but in practice, that’s not really how it works.

In modern inverter units, starting up barely causes a significant spike in energy consumption. And although older models may use slightly more power when first switched on, that extra cost doesn't justify keeping the air con running unnecessarily for hours.

In other words: turning it off is still more efficient than leaving it on ‘just in case.’

What temperature should I leave the air con at if I’ll be away for 4 hours?

If you prefer not to switch it off completely, there’s a simple setting that can help you spend less without letting the house overheat too much.

We recommend setting it between 27 and 28°C in eco or auto mode. This allows the system to maintain a reasonable temperature while consuming significantly less energy than if you leave it at your usual comfort setting.

In fact, this small tweak can reduce consumption by between 30% and 40%.

How much does it cost to run the air con for 24 hours?

It depends on the unit and the selected temperature, but to give you a quick idea, a 1,500 W air conditioner running all day can consume around 36 kWh.

In practical terms, that translates to approximately €4.50–€6.00 a day just for air conditioning.

That’s why, when you need to use the system for many hours, adjusting the temperature and making use of features like timers can make a significant difference to your electricity bill.

Is eco mode better than turning it off?

The answer depends mainly on how long you’ll be away from home.

If you’re only leaving for a short time (say less than two hours), keeping the unit in eco mode may be enough to maintain comfort without using too much energy.

However, if you’re going to be away for longer, the most efficient move is usually to turn the air conditioning off and schedule it to switch back on before you return.

That way, you get a cool home when you need it without running the system unnecessarily all day long.