What Size Generator for Central AC (3-Ton)? (Startup vs. Running Watts)

A 3-ton central air conditioner compressor draws roughly 3,500W while running — totally manageable. The problem is Locked Rotor Amps (LRA): the massive current spike that occurs in the first fraction of a second when the compressor motor tries to overcome its own inertia from a dead stop.

During LRA, a standard 3-ton unit demands 10,000–14,000W for 1–3 seconds. Most portable generators simply can't supply this, causing the breaker to trip or — worse — the generator to stall under load.

There are two solutions. First, size your generator for the LRA: you need a generator rated at 15,000W starting power or higher to handle a 3-ton unit without a soft-start kit. Second, install a capacitor-based Soft Starter (e.g., Micro-Air EasyStart or Honeywell SoftStart). These devices electronically ramp the motor up slowly, reducing the LRA surge by 50–70%, which means a 7,500–10,000W generator can often handle the same AC.

Always check your AC unit's nameplate for the MCA (Minimum Circuit Amperage) and MOCP (Maximum Overcurrent Protection) values, and calculate starting watts as: LRA × 240V.