Why do the lights dim when my AC turns on?

It’s a question that our HVAC technicians and electricians get from a lot of homeowners: “Why do my lights dim when my AC turns on?” If you’ve ever noticed that the lights in your house noticeably get dimmer for a second when your air conditioner starts up, you’re not alone. This is relatively common and, in most cases, normal.

However, as we’ll explore in this article, there are cases when you need to take dimming lights when the AC turns on more seriously. If the dimming is especially intense or prolonged, you need to call the electricians here at Wagner so we can diagnose and fix the issue.

Why are my lights dimming when the AC comes on?

Air conditioners draw the most electrical power when they first start up. To ensure the system has enough power to get moving, your air conditioner’s condenser has a specialized capacitor that draws and stores electrical power. This ensures that the AC unit doesn’t draw too much power from the circuit.

Here’s a short answer to the main question of this article: it’s normal for your lights to dim briefly when your AC first comes on. Even with a properly functioning capacitor, the power draw is so great that you’ll likely see your lights just barely dim for about a second. Unless you’re looking, you may not even notice this happening.

What this “dimming” really represents is the air conditioner needing extra voltage to get moving. As it pulls from the rest of your home in the split-second of turning on, less voltage is sent to your lights and appliances for about a second. This has no negative effect on your electronics, but it’s why you’ll see your lights dimming when the AC starts.

How do I know if I have a more serious problem?

Earlier, we mentioned that your air conditioner’s capacitor is responsible for regulating the power draw needed to get your AC running. Typically, problems in this space start with the capacitor. As capacitors physically age, they can hold less and less stored energy. This requires a larger power draw (in voltage) from the circuit, which means you’ll see a longer, more pronounced dimming of lights. If your lights dim when the AC turns on and lose significant lighting for more than 4-5 seconds, that’s a sure sign that something is wrong with your condenser unit.

The most serious symptom of this is your air conditioner tripping the circuit breaker. This indicates that the power draw was significant enough to overload the circuit. If you have to reset the circuit at the breaker, you need to call our Wagner’s electricians to see what the problem is.

Excessive power draw isn’t always caused by a bad capacitor. In less common cases, the root problem could lie with the wiring or circuit panel itself. Today’s air conditioners require more power on startup than older systems. One phenomenon our electricians have seen is that older homes have installed a new air conditioner, but never upgraded the wiring connecting the condenser to the circuit. It’s just too much for that circuit to carry, and it leads to problems.

Your service panel could also be the problem. There are more electrical appliances in homes than ever before, each of them hungry for electrical current. This potentially includes in-garage charging stations for electric vehicles—not something that home builders or contractors had to think about even a decade ago.

As a result, many homes here in Albuquerque have outdated panels that just can’t keep up with the electrical needs of the home. In such cases, you’ll need to have our electricians install an upgraded panel.

Related: Make your life easier by adding smart switches and outlets to your home

Do I need to take action if I have dimming lights?

Dimming lights when the AC turns on is more than just a nuisance. It’s a sign that either something is wrong with your air conditioner’s electrical connection, it isn’t getting enough power, or—potentially—both. Insufficient voltage for your air conditioner during startup can cause long-term damage to the system, potentially shortening its lifespan. If the power draw is significant, it could also damage electronics in your home that experience a voltage drop every time your AC turns on.

If this is happening to you and your home, you need to take action. Call Wagner and have our electricians come out to your home to diagnose the problem and then fix it. Never try to handle this kind of electrical work yourself. It’s not only dangerous, but it’s likely to cause more significant problems for your air conditioner and your home.

Watch: How to fix flickering lights in your house

In this video, the team from This Old House walks you through the variety of reasons why your home’s lights might be flickering.

Will installing an energy-efficient air conditioner solve this problem?

While there are many good reasons to upgrade to an energy-efficient air conditioner, this isn’t really one of them. While high-efficiency air conditioners do use less electricity while operating, they use just about the same—if not more—when first starting up.

This is one of the reasons why variable-speed HVAC systems are able to operate so efficiently. Instead of turning off-and-on repeatedly, they simply switch to a “lower gear” so that they can keep running, just with less strain and energy use.

It’s also why homeowners lose so much money when their air conditioner is “short-cycling.” Short-cycling refers to a phenomenon where the air conditioner is constantly turning itself on, running for just a short while, and then switching off. That constant starting-and-stopping takes a lot of energy.

Short-cycling can happen for a number of reasons, from the air conditioner being too large for the square footage of the home to a mechanical issue. If you’re noticing that your AC is short-cycling, you need to talk to our HVAC professionals.

How much does it cost to replace a capacitor?

If our electricians do find that the problem is with your air conditioner’s capacitor, it’ll probably cost between $170 and $500 to have it replaced, depending on what kind of air conditioner you have and the make and model of the capacitor itself. Our electricians will provide you with an upfront estimate once we’ve diagnosed the issue with your home’s dimming lights.

Wagner is here to help

Electrical problems can be extremely frustrating. Fortunately, the experts at Wagner Electric are here to help. We have been serving Albuquerque homeowners for almost 100 years, and our team is equipped to handle any electrical issue you may be facing. If you are experiencing any electrical problems, please give us a call today. We would be happy to help you get your home back up and running.