Running Induction Cooktops & Air-Con: Surge (LRA), Soft-Start & Inverter Sizing

Modern caravan and off-grid travellers love the comfort of home—especially induction cooktops and air-conditioners.
But these high-draw appliances have one big catch: surge power. Understanding surge current (LRA), inverter sizing, and soft-start options ensures your system runs safely without tripping or shutting down.

⚡ 1. Understanding Surge (LRA) Power

Every motor or compressor appliance demands a brief current spike on startup.
This spike is known as Locked Rotor Amperage (LRA)—often 3 × to 8 × the running current.

If your inverter can’t deliver this short-term surge, it may:

• Trip into overload protection

• Fail to start the appliance

• Produce audible buzzing or partial startups that stress both inverter and load

⚙️ 2. Choosing the Right Inverter Size

Step 1 – Find Running Power

Check the appliance label (or manual) for rated watts or amps.

Step 2 – Estimate Startup Surge

Multiply running power by a surge factor:

• Induction cooktops → × 2

• Compressor air-cons → × 3 to 5

• Power tools → × 2 to 3

Step 3 – Select an Inverter With Sufficient Surge Rating

Most pure sine wave inverters can supply 2 × their continuous rating for 1–3 seconds.
For example:

• A 3000 W / 6000 W pure sine inverter supports surges up to 6 kW briefly—enough for many cooktops and 1 HP air-cons.

• A 2000 W inverter may start a 1000 W cooktop but not a 1000 W air-con without soft-start assistance.

Step 4 – Check Battery & Cable Capacity

High surge loads draw massive DC current:

At 12 V, 3000 W = 250 A (and 500 A surge!)
Use 4 AWG or larger cabling, short runs (<1 m), and appropriate ANL or MIDI fuses.
Undersized cables cause voltage sag, inverter cutoff, or heat damage.

?️ 3. The Role of Soft-Start Devices

A soft-start module limits compressor or motor inrush current by ramping voltage over a few hundred milliseconds.

Benefits:

• Reduces LRA by 50–70 %

• Prevents inverter overloads

• Extends inverter and compressor lifespan

Examples:

• Air-con Soft Starter (e.g., Micro-Air EasyStart, SoftStartRV)

• Induction Cooktops with “Power Share” or “Eco Mode” functions

• Variable Frequency Drives (VFD) in larger AC units

With soft-start, a 3000 W inverter can often handle appliances previously needing 5000 W.

? 4. Battery Bank & DC Source Considerations

Even with a strong inverter, voltage sag on startup can trip the unit if the battery can’t supply the amps.

• Use LiFePO₄ batteries: high discharge (100 A + per 100 Ah cell)

• Parallel cells for higher current capacity

• DC bus bars to balance current evenly

• Thick cables (≥ 25 mm²) and solid crimp lugs

If running long surges (e.g., 10 s on induction), voltage must stay above 11.5 V DC to avoid low-voltage cutout.

? 5. Real-World Example

Setup:

• Gentrax 12 V 100 Ah LiFePO₄ × 2 (200 Ah total)

• 3000 W / 6000 W Pure Sine Inverter

• 1 kW Induction Cooktop

Results:
Startup ≈ 2200 W (0.5 s)
Steady ≈ 1000 W (≈ 85 A draw)

→ Runs smoothly; surge absorbed by battery + inverter reserve.
Without the dual battery bank, inverter tripped on startup.

? 6. Practical Tips

• Turn other heavy loads off (kettle, toaster, AC) before starting an induction cooktop.

• Pre-cool air-con on generator or mains before switching to inverter.

• Keep battery SOC > 60 % for better surge handling.

• In hot climates, mount inverter in a ventilated area to prevent thermal derating.

• Use pure sine wave only—modified sine inverters cause buzzing, poor induction heating, and potential appliance damage.

✅ 7. Summary Table