What size generator runs a 3-ton AC?
Short answer for the most common case: with a typical essentials list and a 1 HP well pump, a 3-ton air conditioner lands you in the 22 kW standby class; on city water with no well pump, the 18 kW class usually covers it. The reason is one number: the AC's startup surge, and this page shows the whole calculation so you can redo it with your own load list.
The AC's two numbers
From our appliance wattage dataset, a 3-ton central air conditioner is rated:
| Figure | Watts | What it means |
|---|---|---|
| Running watts | 3,500 | What the compressor and fans draw once running, noticeable, but not huge. |
| Starting watts | 9,800 | The momentary surge as the compressor starts. This is the number that sizes your generator. |
| Surge increment | 6,300 | Starting minus running, the extra capacity that must exist for a second or two. |
Tonnage is a cooling rating, not an electrical one: two 3-ton units can draw differently depending on age and efficiency, and the dataset notes to check your nameplate's locked-rotor amps (LRA) for the exact surge. Treat 9,800 W as a planning figure, then confirm against the plate on your condenser.
The worked example: essentials + 3-ton AC
Start from the sizing calculator's "Outage essentials" preset, hit that button, then add the 3-ton AC, and the calculator does exactly this math:
| Appliance | Running (W) | Starting (W) |
|---|---|---|
| Refrigerator | 700 | 2,200 |
| Freezer (chest or upright) | 400 | 1,200 |
| Furnace blower (gas furnace) | 800 | 1,800 |
| Well pump (1 HP) | 2,000 | 4,000 |
| Sump pump (1/3 HP) | 800 | 1,300 |
| Lighting and outlets (whole home) | 1,500 | 1,500 |
| Central air conditioner (3 ton) | 3,500 | 9,800 |
Running watts total 9,700 W. Following the calculator's method, add the single largest surge increment, the AC's 6,300 W, since motors rarely start in the same second, for a starting load of 16,000 W. Apply 20% headroom so the generator isn't running at its limit: about 19.2 kW, which rounds up to the 22 kW standby class. The well pump is doing quiet damage in this example: its 2,000 W running load is in the total the whole time. Remove it (city water) and the same house runs 7,700 W, peaks at 14,000 W, and needs about 16.8 kW with headroom: the 18 kW class. Our well pump page shows why pumps pull size classes upward on their own.
Check the size on the fuel you'll burn
Standby sizes are propane numbers. On natural gas, a "22 kW" Generac Guardian in our model database delivers 19.5 kW, barely over this example's 19.2 kW requirement, while a Kohler 26RCA delivers 24 kW on NG. If your calculation lands near a class boundary, match models on your fuel, not the brochure badge: the calculator's recommendations already do this, and you can see every model that covers 19.2 kW on natural gas highlighted in the database.
What about a soft starter?
Our dataset's note on the 3-ton entry says it plainly: a soft-start kit can cut the startup surge substantially. That can matter here because the AC's 6,300 W increment is the largest in the example: shrink it and the 16,000 W starting load shrinks with it, potentially by enough to change the size class. What we won't do is put a percentage on it: the actual reduction depends on your specific compressor, its nameplate LRA, and the soft-start unit, and the honest way to get the number is a clamp-meter measurement and the soft-start manufacturer's specification for your unit. If a smaller generator hinges on a soft starter working, have an HVAC technician or licensed electrician verify the starting amps after installation, before the generator purchase depends on it.
What pushes you up a class anyway
- A second compressor, a second 3-ton AC adds another 3,500 W running and another 9,800 W starting figure to plan around (a 3-ton heat pump is heavier still in our dataset: 4,700 W running, 10,500 W starting). Load management that prevents simultaneous starts becomes the cheaper answer.
- Electric backup heat. Auxiliary heat strips are 10,000 W of running load in our dataset. If they can engage during an outage, they dwarf the AC itself; most designs lock them out on generator power.
- Kitchen and laundry. An electric range (5,000 W) or dryer (5,400 W) running while the AC cycles can eat the headroom the calculation reserved. Managed circuits exist precisely for these.
This is sizing guidance built on typical figures, not electrical advice. Confirm the final size, transfer equipment, and load-management design with a licensed electrician against your equipment's nameplates.
Sources & notes
- AC and appliance figures: TallyGauge appliance wattage dataset (Central air conditioner (3 ton): 3,500 W running, 9,800 W starting; dataset note flags nameplate LRA as the exact figure).
- Method: the sizing calculator sums running watts, adds the largest single surge increment, applies ~20% headroom, and rounds to the next standard standby class (7.5, 10, 14, 18, 22, 24, 26 kW).
- Per-fuel model outputs: TallyGauge model database. Soft-start effect is described qualitatively from the dataset note and manufacturer practice; no reduction percentage is assumed anywhere in the math above.