You need a minimum 17–20 kW (17,000–20,000 watt) generator to run a 5-ton air conditioner. A 5-ton AC draws approximately 5,000–6,000 running watts but requires 10,000–15,000 watts of surge power at startup when the compressor kicks in. If you install a hard-start kit on your AC, you can reduce the surge requirement and use a 14–16 kW generator instead.
Here's the complete sizing breakdown, including what happens when you're running other household loads alongside your AC.
Interactive AC Generator Calculator
Calculate the exact generator size needed for your central AC plus household essentials:
Generator Sizing for AC Units: Quick Reference
| AC Size | Running Watts | Surge (Starting) Watts | Minimum Generator | Recommended Generator |
|---|---|---|---|---|
| 1.5 ton | 1,800–2,200 | 4,500–6,000 | 7 kW | 8–10 kW |
| 2.0 ton | 2,400–3,000 | 6,000–8,000 | 10 kW | 10–12 kW |
| 2.5 ton | 3,000–3,600 | 7,000–9,500 | 12 kW | 12–14 kW |
| 3.0 ton | 3,500–4,200 | 8,000–11,000 | 14 kW | 14–17 kW |
| 3.5 ton | 4,000–4,800 | 9,000–12,500 | 16 kW | 16–18 kW |
| 4.0 ton | 4,500–5,400 | 10,000–14,000 | 17 kW | 17–20 kW |
| 5.0 ton | 5,000–6,000 | 10,000–15,000 | 17–20 kW | 20–24 kW |
Do not size your generator for running watts only. The compressor startup surge lasts 200–500 milliseconds but draws 2–3× the running wattage. If your generator can't handle the surge, it will trip its overload protection or stall — potentially damaging both the generator and your AC compressor.
Understanding Your 5-Ton AC's Power Requirements
Running vs. Starting (Surge) Watts
A 5-ton AC unit has two distinct power states:
| Power State | Duration | Watts | Amps (240V) |
|---|---|---|---|
| Running (steady state) | Continuous when cooling | 5,000–6,000 W | 21–25 A |
| Starting (surge/LRA) | 0.2–0.5 seconds | 10,000–15,000 W | 42–63 A |
| Blower fan only | Continuous when fan is on | 400–800 W | 2–3 A |
LRA (Locked Rotor Amps) is the maximum current draw during compressor startup. You can find your specific unit's LRA on the data plate. Multiply LRA × 240V to get surge watts.
Finding Your AC's Exact Power Draw
Check your AC data plate for these specifications:
| Specification | What It Means | Typical 5-Ton Value |
|---|---|---|
| RLA (Rated Load Amps) | Normal running current | 21–25 A |
| LRA (Locked Rotor Amps) | Startup current | 42–63 A |
| MCA (Minimum Circuit Ampacity) | Minimum wire/breaker size | 27–31 A |
| MOP (Maximum Overcurrent Protection) | Maximum breaker size | 45–60 A |
| FLA (Full Load Amps) | Full-speed motor amps | Similar to RLA |
To calculate your exact needs:
- Running watts = RLA × 240V (e.g., 24A × 240V = 5,760W)
- Surge watts = LRA × 240V (e.g., 55A × 240V = 13,200W)
Generator Sizing With Other Household Loads
You're probably not running just the AC during a power outage. Here's how to calculate total load:
Typical Household Load Calculator
| Appliance | Running Watts | Surge Watts | Essential? |
|---|---|---|---|
| 5-ton central AC | 5,000–6,000 | 10,000–15,000 | High priority |
| Refrigerator | 150–400 | 800–1,200 | Essential |
| Freezer | 100–300 | 500–900 | Essential |
| Well pump (1/2 HP) | 750 | 1,500–2,000 | If on well water |
| Sump pump (1/3 HP) | 500 | 1,000–1,500 | If basement floods |
| Lights (LED, whole house) | 200–500 | None | Essential |
| Internet/router/modem | 50–100 | None | Important |
| TV | 100–300 | None | Nice to have |
| Microwave | 1,000–1,500 | None | Nice to have |
| Electric range (one burner) | 1,500–2,500 | None | Avoid if possible |
| Electric water heater | 4,000–5,500 | None | Use sparingly |
| Garage door opener | 600 | 1,400 | Low priority |
Sizing Scenarios for a 5-Ton AC
| Scenario | Running Watts | Max Surge | Generator Size |
|---|---|---|---|
| AC only | 5,500 | 13,000 | 14–17 kW |
| AC + essentials (fridge, lights, well pump) | 6,800 | 15,000 | 17–20 kW |
| AC + essentials + kitchen | 8,500 | 15,000 | 20–22 kW |
| AC + everything except range & water heater | 9,500 | 16,000 | 22–24 kW |
| Whole-house backup | 12,000+ | 18,000+ | 24+ kW |
The sweet spot for a 5-ton AC with essentials is a 20–22 kW generator. This gives you enough headroom for the AC compressor surge plus refrigerator, lights, well pump, and a few convenience loads. A 20 kW Generac or Kohler whole-house standby generator costs $5,000–$7,000 installed and handles this easily.
Hard-Start Kits: Reduce Your Generator Requirement
A hard-start kit (also called a compressor saver) reduces your AC's startup surge by 30–50%. This can let you use a smaller, less expensive generator.
| Factor | Without Hard-Start Kit | With Hard-Start Kit |
|---|---|---|
| Startup surge (5-ton) | 10,000–15,000 W | 6,000–9,000 W |
| Minimum generator | 17–20 kW | 14–16 kW |
| Cost of kit | — | $50–$150 (parts) |
| Installation | — | $100–$200 (labor) |
| Potential generator savings | — | $500–$2,000 |
How it works: A hard-start kit adds a start capacitor and potential relay that gives the compressor an extra "boost" during startup, reducing the peak amperage draw. It takes 15–30 minutes for a technician to install.
Recommended hard-start kits:
- 5-2-1 Compressor Saver: Most popular, widely available ($50–$80)
- Supco SPP6E: Universal fit, easy install ($40–$60)
- Rectorseal Kick Start: Compact design ($55–$75)
A hard-start kit is beneficial even without a generator. It reduces compressor wear, extends compressor life, and reduces the momentary "brown-out" you might notice when your AC kicks on (dimming lights). It's one of the cheapest HVAC upgrades with real benefits.
Types of Generators for AC
Portable vs. Standby
| Factor | Portable Generator | Standby (Whole-House) Generator |
|---|---|---|
| Power range | 3–15 kW | 10–48+ kW |
| Can run 5-ton AC? | Marginal (largest portables only) | Yes |
| Fuel | Gasoline, propane, or dual-fuel | Natural gas or propane |
| Automatic start | No (manual) | Yes (starts within 10–30 seconds) |
| Transfer switch | Manual (requires switching) | Automatic (built-in) |
| Cost | $1,000–$3,500 | $4,000–$15,000 installed |
| Noise | 68–80 dB | 60–70 dB |
| Runtime | 8–12 hours per tank | Unlimited (connected to gas line) |
| Best for | Occasional outages, portability | Frequent outages, whole-home backup |
Most portable generators cannot run a 5-ton AC. The largest portable generators top out at 12–15 kW, which may not be enough for the compressor surge. If you must use a portable, install a hard-start kit on your AC first, and don't run other large loads simultaneously during compressor startup.
Recommended Generators for 5-Ton AC
Standby Generators (Best Option)
| Model | Power | Fuel | Cost (Installed) | Notes |
|---|---|---|---|---|
| Generac Guardian 22 kW | 22 kW | Natural gas/LP | $5,500–$7,500 | Most popular, great value |
| Generac Guardian 24 kW | 24 kW | Natural gas/LP | $6,000–$8,000 | Extra headroom |
| Kohler 20RCAL | 20 kW | Natural gas/LP | $5,500–$7,500 | Premium quality, quieter |
| Briggs & Stratton 20 kW | 20 kW | Natural gas/LP | $5,000–$7,000 | Good value |
| Cummins RS20A | 20 kW | Natural gas/LP | $5,500–$8,000 | Industrial quality |
| Generac Protector 25 kW | 25 kW | Natural gas/LP | $7,000–$10,000 | Commercial grade |
Large Portable Generators (Budget Option)
| Model | Running/Surge Watts | Fuel | Cost | Notes |
|---|---|---|---|---|
| Generac GP15000E | 15,000/22,500 | Gasoline | $2,500–$3,200 | Largest portable, needs hard-start kit |
| DuroMax XP13000EH | 10,500/13,000 | Dual fuel | $1,300–$1,800 | Dual fuel is nice but tight on wattage |
| Champion 12,500-Watt | 12,500/15,625 | Dual fuel | $1,800–$2,400 | Needs hard-start kit for 5-ton |
Inverter Generators: Not for 5-Ton AC
Inverter generators (Honda EU series, Yamaha EF series) produce ultra-clean power but max out at 3,000–7,000 watts. They're great for electronics and small appliances but cannot power a 5-ton central AC system under any circumstances.
Installation and Wiring
Transfer Switch Requirements
You must use a transfer switch to connect a generator to your central AC. Never back-feed your electrical panel without one — it's dangerous and illegal.
| Transfer Switch Type | Cost | How It Works |
|---|---|---|
| Manual transfer switch | $200–$500 + installation | You manually switch circuits to generator power |
| Automatic transfer switch (ATS) | $500–$1,000 + installation | Automatically detects outage and switches to generator |
| Whole-house ATS | $800–$2,000 + installation | Switches entire panel, includes load management |
| Smart load management | $1,000–$2,500 + installation | Prioritizes loads to avoid overloading generator |
Smart load management is ideal for running a 5-ton AC on a generator. Systems like Generac's Smart Management Modules prevent the AC compressor from starting when other large loads (like a well pump) are running, avoiding dangerous overloads.
Electrical Requirements
| Requirement | Specification |
|---|---|
| Generator output | 240V split-phase |
| AC circuit breaker | 40–60 A (check MOP on data plate) |
| Wire size to AC | 8 AWG (40A) or 6 AWG (50–60A) |
| Transfer switch rating | Must exceed total generator output |
| Generator to transfer switch wire | Per manufacturer specs (typically 4 AWG for 20 kW) |
Hire a licensed electrician. Generator installation involves working with 240V circuits, natural gas connections, and transfer switches. Improper installation can cause electrocution, fire, or damage to your AC system. Most jurisdictions require permits and inspections for generator installations.
Real-World Scenarios
Example 1 — Florida hurricane prep: The Turners have a 5-ton Carrier AC and wanted whole-house backup for hurricane season. They installed a Generac Guardian 22 kW standby generator with an automatic transfer switch. Total cost: $6,800 installed. It powers their AC, refrigerator, lights, well pump, and all outlets simultaneously with capacity to spare.
Example 2 — Texas grid instability: The Johnsons experienced Winter Storm Uri and wanted AC backup for future summer grid emergencies. They added a 5-2-1 hard-start kit to their 5-ton Goodman ($175 installed), then bought a Generac GP15000E portable generator ($2,800). With the hard-start kit, the 15 kW portable handles the reduced surge. Total investment: $2,975.
Example 3 — Arizona extreme heat: The Patels have a 5-ton Trane and medical conditions requiring consistent cooling. They installed a Kohler 24 kW standby generator with automatic transfer switch and load management. Cost: $8,500 installed. Automatic start ensures AC resumes within 15 seconds of an outage — critical when outdoor temps exceed 110°F.
Example 4 — Budget approach in Georgia: The Simmons family couldn't afford a standby generator. They installed a manual transfer switch ($350 installed) and use a borrowed 12 kW portable generator during outages. With a hard-start kit on their 5-ton AC, the portable generator handles AC + essentials, but they have to manually manage loads (no microwave while AC is running). Total invested: $525 for the transfer switch and hard-start kit.
Generator Fuel Consumption
Running a generator for your 5-ton AC uses significant fuel:
| Generator Type | Fuel | Consumption at 75% Load | Cost per 24 Hours |
|---|---|---|---|
| 15 kW portable | Gasoline | 2.5–3.5 gal/hr | $90–$130 |
| 20 kW standby | Natural gas | 240–300 cu ft/hr | $50–$80 |
| 20 kW standby | Propane | 3.5–4.5 gal/hr | $60–$95 |
| 22 kW standby | Natural gas | 260–320 cu ft/hr | $55–$85 |
| 22 kW standby | Propane | 4.0–5.0 gal/hr | $65–$100 |
Natural gas standby generators are the most economical for extended outages because they connect directly to your gas line — no refueling needed.
Key Takeaways:
- A 5-ton AC needs 17–20 kW minimum generator (5,500W running + up to 15,000W surge)
- A hard-start kit ($50–$150) reduces surge by 30–50%, potentially letting you use a 14–16 kW generator
- 20–22 kW standby generators ($5,000–$8,000 installed) are the best option for 5-ton AC + household loads
- Most portable generators can't handle a 5-ton AC without a hard-start kit
- Always use a transfer switch — never back-feed your panel
- Smart load management prevents overloads by sequencing large-load startups
- Natural gas standby generators cost the least to run during extended outages