Central AC Generator Size Calculator (by Tonnage)
Data reviewed ·how we calculate
Central air conditioning is the heaviest, hardest-starting load in a typical American house, and the honest answer this page delivers is that whole-house cooling usually points past portable generators altogether. A 3-ton compressor runs at roughly 4,200W but can demand 11,600W or more at the instant it starts — the famous locked-rotor surge. Pick your tonnage below for real numbers, and read about soft-start kits, which are the one technology that genuinely changes this math.
Size a generator for this load
Computed on typical values; ranges shown below. 25% headroom applied.
How to size it step by step
Find your tonnage: the condenser model number usually encodes it (look for 24/36/48 = BTU in thousands ÷ 12 = tons), or check the nameplate’s RLA and LRA figures for exact amps.
Compute the surge requirement: running watts plus the compressor’s starting delta. For a 3-ton system that is roughly 4,200W running and 11,600W at the moment of start.
Apply 25% headroom: 11,600W × 1.25 = 14,500W — past the 12,000W portable ceiling, which is why the answer for most central AC is a 14–22kW standby unit installed with a transfer switch.
If you want portable-generator cooling, change the problem: install a soft-start kit (HVAC tech) to cut the surge ~60–70%, or cool one room with a window unit instead.
Pro tips
- Check your nameplate first — every figure on this page is a planning estimate, and the label on your specific unit beats any chart.
- After an outage, give the condenser 5+ minutes before restart (many thermostats enforce this) — restarting against unequalized refrigerant pressure produces the worst-case surge that trips generators.
- If you already own a 240V portable near the required size, a soft-start kit is far cheaper than the next generator class up — get an HVAC quote for the kit before shopping generators.
The data behind this calculator
| Figure | Value | Source |
|---|---|---|
| 2-ton (24,000 BTU) running / starting | ~3,000W / ~8,300W | Generator sizing charts; condenser nameplate LRA convention |
| 3-ton (36,000 BTU) running / starting | ~4,200W / ~11,600W | Generator sizing charts; condenser nameplate LRA convention |
| 4-ton (48,000 BTU) running / starting | ~5,500W / ~15,000W | Generator sizing charts; condenser nameplate LRA convention |
| Voltage | 240V — split-phase required | US residential condenser nameplates |
| Soft-start kit effect | starting surge reduced ~60–70% | Micro-Air EasyStart / Hyper Engineering SureStart manufacturer data |
Duty cycle: On a design-hot day central AC runs at high duty cycle for hours, and the compressor restarts many times a day — the generator must cover the full starting surge every single cycle.
Central air conditioner generator questions, answered
What size generator do I need to run central air conditioning?
For a typical 3-ton system: about 4,200W running, but roughly 11,600W the instant the compressor starts. With 25% headroom that is 14,500W, which lands in the 14–22kW home standby generator class — beyond any standard portable. A 2-ton system with a soft-start kit is the main case where a large 240V portable becomes realistic. This is the page where "add up the watts" honestly ends at "call an electrician about a standby unit".
What is a soft-start kit and does it really work?
It is a controller (Micro-Air EasyStart and Hyper Engineering SureStart are the known names) wired into the condenser that ramps the compressor up instead of slamming it across the line, cutting the starting surge by roughly 60–70% per manufacturer data. A 3-ton system’s ~11,600W start can drop near 4,000–5,000W — suddenly within a 9,000–12,000W portable’s ability. Installation belongs to an HVAC technician, and you should confirm compressor compatibility before buying.
Why does central AC need a 240V generator?
US residential condensers run on 240V split-phase power — both legs of your panel. A standard 120V-only generator physically cannot supply it, whatever its wattage. You need a generator with a 240V outlet (typically an L14-30R or 14-50R) AND a transfer switch or interlock installed by a licensed electrician to feed the AC circuit. There is no safe cord-and-plug route to a hardwired condenser.
Can I run just the furnace fan to circulate air instead?
Yes, and on brutal days it helps more than nothing: the air handler blower alone (roughly 700W running — see the furnace blower page) circulates air and can at least even out temperatures, run at night for free cooling, or push basement-cool air upstairs. Pair it with a window unit cooling one "refuge room" and you have the standard portable-generator heat-wave strategy at about a tenth of the standby generator’s cost.
Is a standby generator worth it just for air conditioning?
That depends on your climate and health situation, and this site won’t pretend otherwise. In the Gulf South, where multi-day post-hurricane outages coincide with dangerous heat, whole-home standby units earn their $10,000–$15,000 installed cost as health equipment. In milder climates, a $1,000 portable covering the fridge and a window AC delivers most of the resilience for a tenth of the money. Run the window AC numbers before signing a standby contract.
Related calculators
- Window AC Generator Size Calculator (by BTU)what size generator to run a window air conditioner
- Furnace Blower Generator Size Calculatorwhat size generator to run a furnace
- Whole-House Generator Sizing With Central ACwhat size generator for whole house with central ac
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