AC soft starters explained

A soft starter is an electrical device that limits the surge of current an air conditioner’s compressor draws the moment it switches on. Compressors are motors, and motors need much more current to start turning than they do to keep running—this initial surge is often called “inrush current.” A soft starter ramps the motor up more gradually instead of hitting it with full voltage all at once, which reduces that starting spike. For homeowners, the practical reason this matters is usually generator sizing: a smaller backup generator may be able to run central air conditioning if the compressor’s starting surge is tamed, since the generator only has to handle a lower peak load. None of the sources below spell out exact percentage reductions or amperage numbers for soft starters, so treat specific manufacturer claims about “X% less starting current” as something to verify against your equipment’s own documentation rather than a fixed rule.

What a soft starter actually changes

A soft starter sits between the power supply and the compressor motor and controls how quickly full voltage is applied. Rather than the motor jumping straight to full speed, it eases into it over a short period. This lowers the peak current draw at startup compared to a standard direct-on-line start. It does not change how much power the air conditioner uses once it’s running normally—only the brief startup moment is affected. Because the actual amount of reduction depends on the specific soft starter and compressor, homeowners should rely on the equipment manufacturer’s published specifications rather than general assumptions when sizing a generator or breaker for a soft-started unit.

Why starting current matters if you run AC on a generator

If you’re planning to power central air conditioning from a portable or standby generator during an outage, the compressor’s starting surge is often the limiting factor—not its steady running load. A generator that can easily handle the AC once it’s running may still stall or trip if it can’t supply that brief startup spike. A soft starter is one way installers address this, but generator capacity, wiring, and how the generator connects to the house all matter too. For general background on preparing a home’s electrical needs before an outage, see the related generators section.

What to record

Do not wire a generator into the electrical panel without transfer equipment: an improper connection can backfeed utility lines and endanger utility workers (FEMA). A soft starter also changes the compressor circuit, so its voltage, wiring, and compatibility must match the exact equipment.

What to check

If a soft starter or a generator large enough to handle full inrush current lets you run air conditioning during a power outage, the standard generator safety rules still apply. Generators and their fuel should always be used outdoors, at least 20 feet from windows, doors, and attached garages, because carbon monoxide from a generator is colorless and odorless and can be fatal (Ready.gov; FEMA). Install battery-backed carbon monoxide detectors on every level of your home, keep the generator dry, and let it cool before refueling to avoid ignition from spilled fuel (Ready.gov). These precautions matter regardless of whether the AC circuit uses a soft starter.

Installation and wiring

Talk to a licensed electrician before adding a soft starter to an existing air conditioner, before connecting any generator to your home’s wiring, or if your AC unit is tripping breakers or straining at startup. An electrician can confirm what your specific compressor and generator can handle safely, which is not something to estimate on your own given how much it varies by equipment.

Sources

Power Outages | Ready.gov, accessed 8/12/2026
Use Generators Safely at Home | FEMA.gov, accessed 8/12/2026