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

Modified on Mon, 10 Nov at 1:20 AM

Running high-demand appliances such as induction cooktops, air-conditioners, and compressor-based fridges off-grid requires careful inverter sizing. Even if your inverter’s continuous wattage looks high enough, surge (startup) current can easily overload or trip it.

This guide explains Locked-Rotor Amperage (LRA), soft-start technology, and how to choose the right inverter for reliable operation of cooktops, A/Cs, and other heavy loads in caravans, RVs, and tiny homes.

? Understanding Continuous vs Surge Power

Most quality pure sine wave inverters (like those on Star Power Advance Solar Technology) list two ratings:

Rating	Description	Example (3000 / 6000 W Inverter)
Continuous Power	The maximum sustained load the inverter can supply indefinitely.	3000 W
Surge (Peak) Power	Short bursts (1–3 seconds) to start motors, compressors, or induction coils.	6000 W

⚠️ If surge demand exceeds the inverter’s peak rating, it will shut down or fault, even if your average load is below the continuous wattage.

? Induction Cooktops: Instant Surge Loads

Induction cooktops use rapid pulsed power to heat cookware via magnetic induction. When they cycle on high settings or both burners engage, they momentarily draw 2–3× their rated current.

Cooktop Rating	Typical Draw	Recommended Inverter
1800 W single burner	Surge ≈ 2500–3000 W	≥ 3000 W (6000 W surge)
2000 W dual burner	Surge ≈ 3500–4000 W	≥ 4000 W (8000 W surge)
Portable induction hob (camping type)	Surge ≈ 1500 W	≥ 2000 W inverter

Tips:

Always run induction cooktops on medium power levels (60–80 %) to reduce surge.
Avoid sharing the same inverter circuit with fridges or air-con when cooking.
Confirm your battery bank can deliver the surge current (see below).

?️ Air-Conditioners & Locked-Rotor Amperage (LRA)

When a compressor starts, it behaves like a stalled motor — drawing the Locked-Rotor Amperage, which can be 4–8 × normal running current.

Appliance	Running Watts	Typical Surge (LRA)	Recommended Inverter
9000 BTU caravan air-con	900 W	2500–3000 W	3000 W / 6000 W
12000 BTU split-system	1200 W	4000–5000 W	5000 W / 10000 W
240 V compressor fridge	150 W	500 W	1000 W / 2000 W

? What Soft-Start Devices Do

A soft-start module (also called EasyStart or SoftStartRV) reduces compressor surge by ramping up current slowly over ~1 second.
Installing one can cut LRA by up to 65 %, allowing smaller inverters to handle air-conditioners that would otherwise trip them.

Example:

A 1200 W air-con with 4500 W surge → after soft-start → ~1800 W surge.
✅ Now runs comfortably on a 3000 W inverter (6000 W peak).

Benefits:

Fewer inverter overload faults.
Reduced strain on LiFePO₄ battery BMS.
Quieter startups and longer compressor life.

? Battery Bank & Cable Considerations

Surge current doesn’t just affect the inverter — it pulls high amps from your batteries.
At 12 V, even a 3000 W load equals 250 A continuous and >500 A surge.

System Voltage	Peak Surge (6000 W)	Recommended Cable
12 V	500 A	70 mm² (00 AWG)
24 V	250 A	35 mm² (2 AWG)
48 V	125 A	25 mm² (4 AWG)

Use short, heavy-gauge cables, solid crimped lugs, and quality fuses/breakers.
Undersized wiring or loose lugs cause voltage drop, leading to premature inverter low-voltage shutdown.

⚙️ Example Sizing Scenarios

Appliance Setup	Battery System	Recommended Inverter
1800 W induction cooktop + lights	12 V 200 Ah LiFePO₄	3000 W / 6000 W
Caravan A/C (9000 BTU) + fridge	12 V 300 Ah LiFePO₄	3000 W / 6000 W + soft-start
Split-system A/C + microwave	24 V 400 Ah LiFePO₄	5000 W / 10000 W
Small induction hob only	12 V 150 Ah LiFePO₄	2000 W / 4000 W

? Quick Troubleshooting if Inverter Trips

Symptom	Likely Cause	Fix
Inverter beeps or shuts down at startup	Surge exceeds peak rating	Upgrade inverter or install soft-start
Inverter resets under heavy load	Voltage drop from undersized cables	Shorten or upsize wiring
Cooktop flickers ON/OFF	Load modulation confusing inverter	Use single burner mode or add load stabilizer
A/C starts then stops	BMS limiting discharge current	Increase battery C-rate or add parallel pack