What size generator do I need for central AC?
Short answer: A 3-ton central AC needs about 3,500 running watts but 11,000 or more for the second its compressor starts. Without a soft-start kit, plan on a 14–18 kW standby or a 12,000 W portable running little else; with a soft-start kit, a 7,500 W (3-ton) to 9,500 W (4-ton) portable can usually start it.
Read the nameplate, not a chart
Your outdoor condenser has a data plate. Two numbers matter for a generator:
- RLA (rated load amps): the compressor's running current. Running watts ≈ RLA × 240 V, plus about 2–3 A for the outdoor fan and the indoor blower.
- LRA (locked-rotor amps): the current the compressor pulls for a fraction of a second as it starts. LRA × 240 V is the worst-case surge.
A typical 3-ton unit shows an RLA around 14–16 A and an LRA around 75–85 A. That is roughly 3,500 W running and, in the worst case, close to 19,000 VA at start. Generators can briefly deliver more than their rating, which is why in practice a 3-ton AC starts on generators rated well below 19 kW — but the margin is thin, and lights dim hard when it does.
Generator size with and without a soft starter
Computed with our generator calculator for central AC plus essentials (fridge, furnace blower, lights, internet, phone charging), motors started one at a time:
| Central AC | Running W | Peak W | Smallest common generator |
|---|---|---|---|
| 3 ton | 4,725 | 12,225 | 14 kW standby |
| 3 ton + soft start | 4,725 | 7,350 | 7,500 W portable |
| 4 ton | 5,925 | 15,225 | 18 kW standby |
| 4 ton + soft start | 5,925 | 9,180 | 9,500 W portable |
A soft-start kit (sold as a "hard start" replacement or by names like SoftStart or Micro-Air EasyStart) ramps the compressor up over about a second and cuts the inrush by roughly 60–70%. For around $300–400 in parts, it is the cheapest way to fit a central AC on a smaller generator, and it is the standard recommendation for RVs for the same reason.
What about the transfer switch?
A portable generator connected through a 30 A inlet (L14-30) delivers at most 7,200 W, whatever the generator's own rating. Running a 3-ton AC plus essentials needs a 50 A inlet and interlock or transfer switch. See 30 A vs 50 A inlets.
Safety. A standby generator large enough for central air is a permanent installation: gas line, concrete pad, automatic transfer switch and a permit. Get a load calculation from a licensed electrician before buying.