AC LRA Calculator
Calculate Starting Watts for your Air Conditioner using Locked Rotor Amps (LRA), Running Amps (RLA), or Tonnage. See how a Soft Start kit can lower your generator requirements.
STEP 1 Enter AC Specifications
Found on the condenser unit's data plate.
Note: LRA is estimated as 6x RLA. Check data plate for exact value.
Estimates LRA based on standard efficiency units.
Required Starting Power
Peak Surge (milliseconds)
Typical AC Specifications Reference
Approximate values for standard efficiency central air conditioners.
| Tonnage | Running Watts (Approx) | Standard LRA | Soft Start LRA |
|---|---|---|---|
| 1.5 Ton | 1,800 W | 45 A | 16 A |
| 2.0 Ton | 2,400 W | 55 A | 19 A |
| 2.5 Ton | 3,000 W | 65 A | 23 A |
| 3.0 Ton | 3,600 W | 78 A | 27 A |
| 3.5 Ton | 4,200 W | 90 A | 32 A |
| 4.0 Ton | 4,800 W | 105 A | 37 A |
| 5.0 Ton | 6,000 W | 135 A | 47 A |
How to Use the AC LRA Calculator
Step 1: Choose Your Input Method
The calculator offers three ways to determine your power needs, depending on what information you have available:
- LRA (Locked Rotor Amps): Select this if you can find the data plate on your AC unit. Look for "LRA" or "Locked Rotor Amps". This is the most accurate method.
- RLA (Running Load Amps): Select this if you only know the running amps (often labeled RLA). The calculator will estimate the surge (LRA) as approximately 6 times the running amps.
- Tons (AC Size): Select this if you know the size of your unit in Tons (e.g., 3 Ton). The calculator uses standard industry averages to estimate the power requirements.
Step 2: Enter the Values
Input the number corresponding to your chosen method. Then, ensure the Voltage is correct (usually 240V for central air, 120V for small window units).
Step 3: Check "Soft Start" (Optional)
If you have a Soft Start kit installed (or are planning to buy one), check the box. You will see the "Required Starting Power" drop significantly, showing you how much generator capacity you can save.
The Formula
Starting Watts = LRA × Voltage
Example: 78 LRA × 240 Volts = 18,720 Watts
What is LRA?
LRA (Locked Rotor Amps) is the massive current surge an AC motor draws the instant it starts up, before the rotor begins spinning. This surge lasts only milliseconds but determines whether your generator will handle the load or trip its breaker.