What Does An AC Capacitor Do?

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Updated February 3, 2023

If your capacitor fails, you might wonder what a capacitor does in an air conditioner. Usually, several capacitors are included in the best air conditioner systems, so the answer may be slightly more complicated than you think. Your air conditioner most likely features multiple capacitors, each with its own specific function, including start capacitors and run capacitors. Essentially, these capacitors store energy expended during your air conditioning cycle.


  • A start capacitor stores excess energy used when your AC starts its cycle.
  • The run capacitors contain energy used throughout the AC cycle after the initial starting jolt.
  • Capacitors that fail can cause lowered strength and functioning of your air conditioning system.

What Does an Air Conditioner Capacitor Do?

As stated, capacitors store energy expended during your air conditioning cycle. However, the two types of capacitors use this energy in different ways. If you aren’t sure whether one of your capacitors has failed, it’s time to replace the capacitor on your air conditioner. Watch out for the symptoms listed below. You might consider an air conditioner circuit board replacement if you aren’t experiencing capacitor-related problems.

Faulty capacitors, even on the leading inverter A/Cs, can cause various issues, including lower strength and functioning of your unit. Sometimes, it may make it so that your AC has no power. Read on to learn how the capacitor affects your air conditioner and what can cause the capacitor to fail.

Insider Tip

Capacitors are used to start and run your AC, so you want to keep them in good condition.

Start Capacitors

The start capacitor provides the excess electricity necessary to start your AC unit, which your electrical grid cannot handle. This energy is gathered and stored as electrons while the capacitor is not in use.

Once the AC cycle begins, the stored energy combines with the power coming from the electrical grid to start your air conditioning system’s units and motors. In other words, the start capacitor provides a significant boost in energy.

STAT: When your AC starts, it uses about 300 and 500 percent more power than when it runs the rest of the time. (source)

Run Capacitor

Your air conditioning unit likely has several run capacitors housed inside, including those for the compressor motor, outdoor fan motor, and indoor blower motor. However, the most common compressor motor capacitor is a dual capacitor with three terminals.

Run capacitors store energy for use during the AC unit’s cycle, much like a start capacitor does. In this case, the unit uses the stored energy to power the system alongside your incoming electricity over long periods. This capacitor supports the system throughout its cycle. For example, while the air conditioner is off, both start and run capacitors collect energy that will be used the next time the unit turns on.


Capacitors look like large batteries, but they have two or three posts sticking out of the top. Each battery-like system consists of metal conductors with insulating materials in between. These large, cylindrical components are typically located in the AC’s housing unit.

In some systems, you may see a dual capacitor connecting the start and run capacitors to the compressor and fan motors.

Capacitor Failing

The capacitors attached to the compressors are most likely to fail, causing the A/C compressor to not work, so the unit loses strength and begins to function with reduced power. Sometimes, the AC may stop working altogether. If your AC seems to have a power supply issue, you may have a faulty capacitor and need service from a certified technician.


A faulty capacitor can cause your air conditioning to lose power, so ‌check your capacitor for signs of wear if you don’t think your air is running correctly.

Common causes for capacitor failure include age, low voltage rating, and heat exposure during the summer. Electrical overheating due to the AC running too long can damage your air conditioner’s elements. Finally, power surges affect efficiency and condition, even if they relate to a large appliance turning on.


How long do AC capacitors last?

An AC capacitor can last 10-20 years, depending on environmental and maintenance factors.

Start capacitors vs run capacitors: What’s the difference?

A start capacitor provides the extra energy needed to start your AC and its motors, while a run capacitor helps deliver the power to keep it running.

How can a bad capacitor affect your AC?

A faulty capacitor can cause lower strength and function. It may also cause the AC to release warm air.

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