How to Size a Generator for Dual Central Air Conditioners (2026)
Running dual central air conditioners requires either a 20kW+ generator or installing A/C soft-starters on both outdoor condensers. Soft-starters cut compressor locked-rotor amps by up to 70%, allowing a 10,000W generator to start both units sequentially.
01 The Challenge of Starting Two High-Tonnage Compressors
Two-story homes commonly feature separate central air conditioning systems for upstairs and downstairs living areas. Starting both compressors simultaneously produces staggering locked-rotor inrush spikes exceeding 20,000 watts.
Without soft-starters or time-delay relays, starting both compressors will instantly stall a portable generator or trip the 50-amp main breaker.
NEC 220 mandates calculating 125 percent of continuous load plus the single largest motor startup surge. Unassisted central air conditioners draw 3 to 5 times running current during startup.
- Two 3-ton compressors draw up to 22,000 starting watts without soft-starters.
- Dual soft-starters drop combined peak surge down to under 9,000 watts.
- Stagger thermostat setpoints to prevent simultaneous compressor starts.
02 Dual A/C Sizing Matrix
Installing a Micro-Air EasyStart on both condensers drops locked-rotor amps so that a 10,000-watt tri-fuel inverter handles both units comfortably.
Review our lab tested surge numbers for common two-unit residential configurations.
| A/C Setup | Stock Inrush Surge | Dual Soft-Starter Surge | Recommended Generator |
|---|---|---|---|
| Dual 2.5-Ton Units | 15,800 Watts | 6,200 Watts | Genmax GM10500XiT |
| 3-Ton + 2-Ton Units | 18,400 Watts | 7,100 Watts | Westinghouse iGen12000TFc |
| Dual 3.5-Ton Units | 22,600 Watts | 8,900 Watts | Generac Guardian 18kW |
| Dual 4-Ton Units | 27,200 Watts | 10,500 Watts | Generac Guardian 26kW |
NEC 220 mandates calculating 125 percent of continuous load plus the single largest motor startup surge. Unassisted central air conditioners draw 3 to 5 times running current during startup.
- Install soft-starters on both compressors to cut inrush current.
- Offset upstairs and downstairs thermostats by two degrees.
- Ensure the generator provides split-phase 240V power.
Frequently Asked Questions
Q: Can a 12,000-watt generator run two central air conditioners?
Yes, provided both air conditioners have soft-starters installed and the units do not kick on at the exact same fraction of a second.
Q: What happens if both compressors start at the same time?
If both start simultaneously, the total inrush may trigger the generator overload circuit. A simple digital time-delay relay on one thermostat prevents simultaneous starts.