What Size Generator Runs an Air Conditioner?

An air conditioner can need several times its running power for a fraction of a second when the compressor starts. That startup demand, not the cooling tonnage alone, determines whether a generator will run it reliably.

Use the exact AC nameplate and manufacturer data, add every load that will operate at the same time and check both generator running watts and surge capability. A generic 2,000W, 4,000W or 6,000W answer is not safe across window, RV and central systems.

Find the Air Conditioner’s Running and Starting Load

Look for rated input watts, rated load amps and locked-rotor amps in the manual or equipment label. If only volts and amps are available, multiplying them gives volt-amperes, but motor power factor means that result may not equal watts.

For inverter-driven or variable-speed AC, use the manufacturer’s maximum input and generator guidance. These units may start more gently than fixed-speed compressors, but controls and power electronics can still have specific voltage and waveform limits.

For fixed-speed compressors, locked-rotor current is not automatically the exact generator surge requirement. Ask the equipment maker or a qualified contractor for starting data with the proposed generator.

Calculate the Required Generator Capacity

1. Add the running watts of the AC and all simultaneous loads.
2. Identify the largest additional startup demand.
3. Add that startup demand to the running total.
4. Confirm the generator can supply the surge for the required duration.
5. Apply altitude, temperature and fuel derating from the generator manual.

Example: if an AC uses 1,500 running watts and needs 3,500 total watts while starting, and other loads total 600 watts, the generator must supply at least 2,100 watts continuously and about 4,100 watts during startup. Add reasonable margin rather than operating at the limit.

For central systems, include the indoor blower or air handler and any pumps. Do not include electric heat strips unless they are intended to run, but make sure controls cannot energize them unexpectedly in generator mode.

Window, RV and Central AC Considerations

Window and portable AC: Check rated input and compressor startup. Small units may run from a modest inverter generator, but extension-cord length and voltage drop matter.

RV rooftop AC: A 15,000 Btu unit may use around 1,800 running watts in a planning worksheet, with additional startup demand. Total RV load, altitude and optional soft-start equipment change the result.

Central AC or heat pump: These systems often need a much larger generator or listed load-management equipment. The generator must handle the outdoor unit, indoor blower and other essential house loads through a transfer switch.

A soft starter can reduce compressor inrush on compatible equipment. It does not increase generator continuous output, correct undersized wiring or make every compressor-generator combination compatible.

Power Quality, Connection and Safety

• Confirm acceptable voltage, frequency and waveform for the AC controls.
• Use conductors and plugs rated for the current and installation.
• Never backfeed house wiring through a receptacle.
• Use a permitted transfer switch or listed interlock installed by a qualified electrician.
• Stagger other motor loads so they do not start with the compressor.
• Test the system under realistic load before an emergency.

Run portable generators outdoors only, at least 20 feet from the home, with exhaust directed away from buildings, windows and vents. Never operate one in a garage, shed, basement, porch or carport.

Bottom line: Add total running load to the largest startup requirement, then apply generator derating and margin. Exact equipment data beats any generator-size chart based only on AC tonnage or Btu.

References: Generac wattage worksheet and CPSC carbon-monoxide guidance.