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Sunsynk F18: AC Overcurrent Fault (Hardware)

Updated June 2026 · Reviewed for technical accuracy · ~3 min read

When your Sunsynk shows F18, it has detected more current on its AC output than the hardware can safely carry, and it shut that output down to protect itself. Nine times out of ten the trigger is on the load side: too much connected to the backup output, or a big appliance kicking in. It is usually a quick fix.

Where the problem usually is

F18 points at the AC output, not the panels. The likely culprits, in order:

  • An inrush surge. A pump, fridge, A/C or power tool draws a big gulp of current the instant it starts.
  • An overloaded backup output. The total connected load is over what your Sunsynk is rated to supply.
  • A faulty appliance pulling abnormal current or shorting.
  • An internal hardware fault, which is rare and only likely if F18 keeps returning with almost nothing plugged in.

How to clear it safely

Stay outside the box Everything here is done with the switches and the appliances. Do not open the inverter or touch DC wiring.
  1. Shed the load. Turn off the heavy items on the backup circuit first, anything with a motor, pump or heating element.
  2. Cycle the AC side. Switch the inverter's AC output breaker off, count to sixty, and switch it back on.
  3. Still showing F18? Do a full restart. AC breaker off, then the DC isolator off, wait five minutes, then DC back on followed by the AC breaker.
  4. Reconnect appliances one by one. Bring loads back gradually and watch which one tips it over. That is your problem device.
  5. Confirm you are within rating. Check that the combined backup load sits below your Sunsynk's rated output power on the spec label.

Quick decision flowchart

F18 appears
↓ shed load & power-cycle
✅ Clears and stays off with a sensible load → sorted. Keep within the rating.
❌ Comes back with little or no load → likely an internal hardware fault.
↓
If it keeps returning, stop resetting and call your installer.
When to bring in a professional If F18 returns straight away with almost nothing connected, or it trips again and again after a clean restart, the output stage may be damaged. Repeatedly power-cycling a hardware fault won't help. Contact your installer or Sunsynk support with the code and model number.

Related Sunsynk codes

F34, AC overcurrent F20, DC overcurrent F64, Heatsink high temp

FAQ

F18 won't clear even after a full restart.

Unplug every backup load and restart once more. If F18 still shows with nothing connected, it is no longer a load issue and points to the inverter's AC output stage. Stop resetting and call your installer.

It trips the moment my borehole pump or A/C starts.

That start-up surge is a classic F18 trigger. Staggering heavy start-ups, or confirming the appliance is within your Sunsynk's rating, usually solves it.

Will F18 have harmed my appliances?

Usually not. The inverter protects itself by cutting the output fast. But if one specific appliance trips F18 every time, have that appliance checked.

Sources

  • Sunsynk Hybrid Inverter User Manual (fault-code list, F18 = AC over current, hardware).
  • Sunsynk fault-code references and installer documentation.
⚠️ Safety disclaimer. Solar inverters carry lethal DC and AC voltage even when "off". This guide covers external, switch-level steps only. Never open the unit or work on wiring unless you're qualified. When in doubt, call a licensed solar installer or electrician.