What Size Generator Do You Need to Run a 5-ton Air Conditioner?
Quick answer
A 5-ton central air conditioner draws about 5,200 running watts and roughly 10,400 starting watts - the starting surge is what sizes the generator. Added to a typical home's essentials (about 3,225 W of running load), the peak lands around 13,625 W, so with 20% headroom you need about 16.4 kW: the 18 kW standby class, or 20 kW if a well pump, electric water heater, or range joins the outage load list. These AC figures are typical values for modern residential condensers, labeled as such - your unit's nameplate (RLA/LRA) is the exact source.
The math
| Load | Running watts | Starting watts |
|---|---|---|
| Refrigerator | 175 | 900 |
| Furnace blower (gas furnace) | 700 | 1,400 |
| Sump pump, 1/3 hp | 800 | 1,600 |
| Lighting (LED, whole house) | 250 | 250 |
| Router, TV, laptop, phones | 200 | 200 |
| Microwave | 1,100 | 1,100 |
| Central AC, 5-ton | 5,200 | 10,400 |
Running total with the AC on: 8,425 W. Peak adds the largest surge difference - the AC's own 5,200 W - for 13,625 W, and 20% headroom gives the 16.4 kW requirement above.
A soft-start kit (around $300-400 installed) cuts the AC's starting surge substantially and can drop a borderline house a full class - worth asking an installer about before upsizing the generator.
Standby picks in the 18 kW class
| Model | Output | Approx. unit price | Warranty |
|---|---|---|---|
| Generac Guardian 18kW | 18 kW | Around $5,100 | 5-year |
| Kohler 20kW (20RCA) | 20 kW | Around $6,000 | 5-year |
| Cummins QuietConnect 20kW (RS20A) | 20 kW | Around $5,800 | 5-year |
A note on climate
The math above assumes a mixed climate. Two adjustments matter at the extremes, and both are rules of thumb, not measurements. In a cold climate, heating dominates: the furnace blower runs near-continuously, and if the house has a heat pump with electric auxiliary heat strips (typically 5-15 kW), those strips are the single biggest sizing load in the house when they engage - count them at full draw or use load management to shed them. In a hot climate, the air conditioner is the dominant load and runs near-continuously through a heat-wave outage, so installers commonly add roughly 10% extra headroom on top of the standard margin. Our generator size calculator applies both adjustments openly - you can see exactly what changed.
FAQ
Can a portable generator run a 5-ton AC?
Only the largest portables are even in the conversation: the unit needs starting watts above the combined peak (13,625 W here), not just above the AC's running draw. In our dataset, only the biggest dual-fuel portable clears a peak like this; anything smaller will stall or trip on compressor start. A soft starter on the AC changes this math significantly.
Why does starting wattage matter more than running wattage?
A compressor motor draws two to three times its running current for a fraction of a second at startup. A generator that can't supply that surge sags in voltage, the compressor fails to start, and the transfer switch may drop the load. Running watts decide fuel burn; starting watts decide whether the AC runs at all.
Is a 5-ton AC the only big load I should count?
It is usually the largest single one, but a well pump (about 2,000 running / 4,000 starting watts for 1 hp) and an electric water heater (4,500 W) are the two that most often push a house up a class after the AC is counted.