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Emergency power supply not working

Warning Do not attempt any electrical or gas work yourself. Always use a qualified, MCS-certified, or Gas Safe registered professional for repairs.

What you’re seeing (symptoms)

The grid has gone down (a power cut), but your house is completely dark. Even though your battery was 50% full, none of your lights or sockets are working. The inverter may be showing a “Grid Lost” message but isn’t providing “Backup” power.

What’s normal vs not normal

Normal:

  • Brief Flickering: A 1-2 second delay while the inverter switches from “Grid-Tied” to “Standalone” mode.
  • Limited Power: Only certain circuits (like lights and the internet router) working, while “heavy” appliances like the oven remain off.

Not normal:

  • The battery being full but providing no power at all during a grid failure.
  • The backup power working for a few seconds and then tripping the whole house.
  • The inverter app showing “EPS Active” but no power reaching your sockets.

Likely causes (ranked)

  1. No EPS circuits installed: Most UK battery installations do NOT include whole-house backup by default. Only specific “essential” circuits must be wired into the EPS output.
  2. Changeover switch in wrong position: If you have a manual changeover switch (I-0-II), it hasn’t been flipped to the “Backup” position.
  3. Overload at startup: You tried to run too many appliances at once. Home batteries can usually only provide 3kW-5kW of backup power; a kettle and a toaster together will trip it.
  4. Missing earth rod: For backup to work safely, your house needs a dedicated earth rod. If this wasn’t installed, the inverter may refuse to enter EPS mode for safety.
  5. SoC Reserve too low: The battery was already at its “Minimum SoC” when the power cut happened, leaving no energy for backup.

Quick checks you can do safely

  • Check the app: Does your battery app show an “EPS” or “Backup” section? Is it enabled?
  • Check the reserve: Look for a setting called “Backup Reserve” or “EPS Reserve.” We recommend keeping at least 20% of your battery held back specifically for power cuts.
  • Check your consumer unit: Look for a separate, smaller fuse box labelled “Emergency” or “Backup.” Are the breakers in the “Up” position?
  • Reduce the load: Turn off all heavy appliances (electric car, oven, dishwasher). Reset the inverter and see if the lights come back on.

When to call an installer

  • If your system was sold with “Full Backup” but provides no power during a cut.
  • If the backup power trips your RCD every time it tries to turn on.
  • If you want to add an “Automatic Changeover Switch” so your lights don’t even flicker during a cut.

Questions to ask your installer

  • “Which specific circuits in my house are wired for EPS backup?”
  • “Is there an earth electrode (rod) installed specifically for the backup system?”
  • “What is the maximum ‘Continuous Discharge’ rate of the inverter in EPS mode?”

Related NetZeroNow tools & guides

FAQs

Does every battery have backup? No. EPS (Emergency Power Supply) is often an optional extra. It requires additional wiring, a separate consumer unit for essential circuits, and often a dedicated earth rod. Can I run my whole house? Unlikely with a standard single-battery system. Most can only provide 3.6kW. That’s enough for lights, your router, and a TV, but not for heating, cooking, or charging an EV. How long will it last? A typical 5kWh battery will run basic lights and a fridge for about 10-15 hours. If you use the microwave or kettle, that time drops significantly. Will it charge from solar during a cut? Only if your system is “Black-Start” capable. Some inverters can use solar power to keep the battery charged while the grid is down; others cannot. Is it automatic? It depends on your installation. Some switch over in milliseconds; others require you to go to the garage and flip a manual switch.\n

Written by NetZeroNow | Last updated on 2026-03-02