If you’re planning to run your air conditioner on a generator—whether for home backup during power outages, RV camping, or construction sites—choosing the right generator size is critical. Too small, and your AC won’t start or will overload your generator; too large, and you waste money and fuel. This guide breaks down everything you need to know to calculate, understand, and select the perfect generator size for your air conditioner.
AC Generator Size Calculator
1. Why Generator Size Matters for Air Conditioners
Air conditioners are among the most power-hungry household appliances because they have motors that require a surge of electricity at start-up. This starting wattage or surge wattage is typically 2 to 3 times higher than the continuous running wattage needed to keep the AC cooling once on.
A generator must supply this surge power without shutting down, tripping breakers, or damaging the unit. At the same time, it needs to provide enough continuous power to run the AC and any other essential appliances during an outage or while on the go.
2. Understanding the Key Terms: Running Watts vs Starting Watts
- Running Watts (Rated Watts): The continuous power the AC uses while operating.
- Starting Watts (Surge Watts): The extra power needed momentarily when the AC compressor motor starts. This can be 2 to 3 times the running watts.
For example, a 3-ton residential AC unit might have:
- Running watts: around 4,000 watts (4 kW)
- Starting watts: 10,000 to 12,000 watts (10 to 12 kW)
Generators are rated primarily based on running watts, but you must consider starting watts to size them correctly.
3. How to Calculate Your Air Conditioner’s Wattage
If you don’t have exact wattage ratings from the manufacturer label or manual, you can estimate using BTUs or tonnage:
- Step 1: Find the AC tonnage or BTUs. 1 ton of AC = 12,000 BTUs.
- Step 2: Convert BTUs to tons: BTUs ÷ 12,000 = tons.
- Step 3: Estimate running wattage: 1 ton = about 3,500 watts.
- Step 4: Calculate starting wattage: running watts × 3 (approximate surge factor).
Example:
A 3.5-ton AC = 3.5 × 3,500 = 12,250 running watts
Starting watts = 12,250 × 3 = 36,750 watts surge (this is a high estimate; consult your specs as it can vary)
Note: Actual starting wattage may be lower than 3x running watts depending on motor type and soft-start technology.
4. Using Nameplate Data for Accurate Sizing
Find the specifics on your unit’s nameplate or technical documents, which usually list:
- Voltage (typically 240V for residential central AC)
- Running amps (continuous current)
- Locked Rotor Amps (LRA) – the surge amps during startup
From this, you can calculate approximate watts:
- Watts = Voltage × Amps × Power Factor (usually around 1 for motors)
Typically, generators need to handle the Locked Rotor Amps (starting surge) without tripping.
5. How to Add Other Appliances to Your Generator Load
If powering only the AC, size the generator for AC start-up wattage. If you want to power multiple devices (refrigerator, lights, microwave), add their running watts to your total:
- List all devices and their wattages.
- Sum running watts for continuous power.
- Sum starting watts for devices with surge.
- Add a safety margin (~20-25%).
6. Recommended Generator Sizes for Common AC Units
| AC Unit Size (BTU) | Running Watts (Approx.) | Starting Watts (Approx.) | Recommended Generator Size (kW) |
|---|---|---|---|
| 9,000 BTU (0.75 ton) | 900 – 1,000 W | 2,000 – 2,200 W | 2.0 – 2.5 kW |
| 11,000 BTU (1 ton) | 1,250 – 1,400 W | 2,300 – 2,800 W | 2.5 – 3.0 kW |
| 13,500 BTU (1.1 ton) | 1,500 – 1,800 W | 2,800 – 3,500 W | 3.0 – 4.0 kW |
| 24,000 BTU (2 ton) | 3,500 – 4,000 W | 6,000 – 8,000 W | 7.5 – 8.5 kW |
| 36,000 BTU (3 ton) | 5,500 – 6,000 W | 10,000 – 12,000 W | 12 – 15 kW |
Always size up for safety and to extend generator life.
7. Factors to Consider Beyond Wattage
- Soft Start Technology: Some modern ACs have soft start devices that reduce surge wattage, allowing use of smaller generators.
- Generator Type: Inverter generators provide cleaner power, better for sensitive electronics in ACs.
- Voltage and Frequency Stability: Voltage sag and frequency fluctuations during start-up affect generator performance.
- Altitude and Temperature: High altitude and heat reduce generator capacity; consider derating.
- Run Time and Fuel Type: Consider fuel efficiency and fuel availability — gasoline, propane, diesel, or dual fuel.
8. Step-by-Step Calculation Example
Imagine a 3-ton central air conditioner:
- Step 1: Find running watts: 3×3,500=10,5003×3,500=10,500 watts
- Step 2: Estimate surge watts: 10,500×3=31,50010,500×3=31,500 watts (or 31.5 kW)
- Step 3: Add other appliance load (lights, refrigerator): 1,500 watts running + 2,000 watts surge
- Step 4: Total running watts: 10,500+1,500=12,00010,500+1,500=12,000 watts (12 kW)
- Step 5: Total surge watts: 31,500+2,000=33,50031,500+2,000=33,500 watts (33.5 kW)
- Step 6: Add safety margin: 33,500×1.25=41,87533,500×1.25=41,875 watts (41.9 kW)
- Step 7: Select generator: Choose a generator rated at least 42 kW to cover surge safely
This shows large residential ACs require substantial generators, often whole-house standby units.
9. Tips for Using Generators with Air Conditioners
- Start the AC first: Start your AC before switching on other heavy loads.
- Avoid overloading: Turn off other appliances if the generator struggles.
- Regular maintenance: Keep generator fuel fresh and perform routine checks.
- Consider automatic transfer switches: For standby generators to ease cutover during outages.
- Use soft-start accessories: To reduce surge power demand (like Progressive’s EasyStart).
10. Common Generator Sizing Mistakes to Avoid
- Underestimating starting surge wattage leads to generator failure.
- Forgetting to include other loads powered simultaneously.
- Choosing generators with insufficient fuel capacity for extended outages.
- Ignoring voltage drop and power quality, which can damage sensitive electronics.
- Not considering altitude or temperature effects on generator output.
11. Summary: How to Pick the Right Generator Size
- Determine your air conditioner’s running and starting watts from the nameplate or BTU conversion.
- List other appliances and calculate their wattage.
- Add running watts and starting watts separately.
- Add about 20-25% safety margin.
- Choose a generator rated for combined surge watts or more.
- Consider soft-start technology or inverter generators to reduce size and fuel consumption.
- Consider your running time, fuel availability, and maintenance preferences.
12. Additional Resources and Tools
- Online generator size calculators (like Cummins Home Generator Calculator)
- Manufacturer’s generator sizing guides
- Services like professional load analysis and consultation
- Blogs and forums with user experiences
This comprehensive approach helps you confidently select a generator that ensures your air conditioner starts reliably, runs efficiently, and keeps you cool no matter what.