Why Do My Lights Dim When the AC Turns On?

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Updated April 27, 2023

One common concern among central AC owners is that they notice the lights dim when the air conditioner comes on. There are many reasons ‌this could be the case, and it can be a problem with even the best air conditioners and the wiring of your electricity. Below, we’ll go into the common reasons ‌this happens and what you can do about it. And for those with portable units, we have another article covering typical portable AC unit repairs.


  • If your lights are only dimming slightly when the AC turns on, this likely isn’t a cause for concern.
  • If lights flicker, dim severely, or a crackling sound is heard, you have a problem with your central AC.
  • Overloading a circuit breaker can cause major electrical issues like shocks or fires.

Why Your Lights May Dim When the Air Conditioner Comes On

The quick dimming of lights aren’t always symptoms of a call for concern over your electrical circuits, especially if you’ve recently tested the air conditioner capacitors. Central air conditioners take a lot of energy to power on. If this has happened ever since the unit was installed, this most likely means that it’s just a cause of your central AC using that initial power voltage to turn on its motors. You can install a wire disconnect box for your air conditioner if that helps.

Insider Tip

If you live in an older house, check if your AC shares a circuit with any other major appliances.

However, suppose your lights are going very dim or continue to flicker once the AC is on. In that case, this could be a sign of a more significant problem that could cost you money. And if energy efficiency is a concern, we have another great article that covers leaving your air conditioner on when not at home.

The most common issue is a problem with the AC’s capacitor. This component‌ provides the AC unit with the initial burst of energy to power on and cool your home. You can also see this if you use the top-rated WiFi smart air conditioners, as they use a bit of power.

Damaged/Loose Wire

If the lights flicker rapidly whenever the AC kicks in, this most likely means a loose or damaged connection with one of the wires. If this continues, ‌turn your AC unit off and call professional electricians to check out what’s going on and find a solution.

Damaged/Old Capacitor

As stated before, the capacitor is the part of the machine that gives it an extra boost of power when starting. So, if your lights are severely dim whenever the AC kicks in, you likely have a faulty capacitor. You can test the capacitor yourself with a multimeter or call a professional service to check it.

Overloaded Circuit

The electrical code in the United States declares that an air conditioning unit should be on a separate circuit. However, suppose you share the central AC with other appliances or light bulbs. In that case, an overloaded circuit with its limited power supply combined with the burst of power needed by a modern air conditioner unit causes the lights to dim or create a crackling noise when the AC starts. If you find this to be the case, unplug any other appliances for the time being to prevent any emergencies like house fires and call an electrician to inspect your situation.


Not having a dedicated circuit for your AC increases the chances of fires and electric shock.


How much does it cost to replace an AC capacitor?

On average, replacing an AC capacitor will cost around $170.

Can I make AC repairs myself?

Many AC repairs, like changing a capacitor, are easy and can be done as DIY projects for those confident in their electrical skills.

Is it always bad when the light dims when I turn on appliances?

Dimming lights aren’t always a significant concern. They sometimes can mean that you are running too many appliances at once. However, it was good to get the problem checked out if the lights dim significantly.

STAT: Experts say that, if your lights only lose about 5% of their brightness, this isn’t a sign that someone is wrong with your AC unit. (source)

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