Answer first, context after
Why do my lights dim for a second when the AC kicks on?
A brief, faint flicker at startup is usually normal: an AC compressor draws a large gulp of current in its first moments, enough to sag the house's voltage for a blink. What is not normal is dimming that has grown more dramatic, lasts more than a moment, or comes with a struggling, stuttering start, because those describe a compressor working too hard to wake up.
This is one of the most commonly Googled AC behaviors, usually at 11 p.m. by someone who just noticed it and wants to know whether to worry. Short version: probably fine, with a specific list of exceptions worth knowing.
Why it happens at all: an AC compressor motor draws several times its running current in the instant it starts, a brief surge called inrush. That gulp momentarily pulls down the voltage available to everything else in the house, and incandescent bulbs and some LED drivers show it as a flicker. One quick, subtle dim at each startup, unchanged for years, is ordinary house physics, the same reason lights once dimmed when old refrigerators kicked on.
The changes that turn it into a symptom: dimming that has visibly worsened over a season, lights that dim hard rather than flicker, a startup you can hear straining, several seconds of hum before the compressor catches, or dimming paired with a breaker that trips. The usual translation: a weakening run capacitor is giving the compressor less starting help, so the motor leans harder and longer on the house’s wiring to get moving. That is a $189 to $240 flat repair when caught, and a compressor-killer when ridden all summer.
The fix a struggling start sometimes earns, and the upsell it sometimes is: a hard start kit gives an aging compressor a stronger opening jolt, shortening the strain. It is a legitimate part with a legitimate use and a famous history as a tune-up upsell, so it deserves a measurement before it deserves your money: the diagnosis is the capacitor test and a startup amp reading, not a glance and a story.
The version that belongs to an electrician: if lights dim when several different appliances start, or flicker with no appliance at all, the story may be loose connections or a service issue on the house side rather than the AC, and that is an electrician’s call, worth making promptly since loose connections and heat are a bad pair. If the dimming belongs only to the AC, it belongs to us: $89 to diagnose, waived with the repair, and a startup problem caught at the capacitor stage is one of the cheapest saves in air conditioning.
Still stuck? That's what the truck is for.
The diagnostic is $89, waived when you book the repair, with a written flat price before any work starts.