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Diagnostic guide · Tesla Powerwall

Tesla Powerwall backup not working during power cut

Your Powerwall is fully charged but when the grid drops, there is no power — or power cuts out after a few minutes. This is usually a backup reserve issue, a Gateway contactor fault, a load overload, or the critical circuits are not on backed-up wiring. This guide covers Powerwall 2 and Powerwall 3.
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Power cut — but no backup?

Tell us your Powerwall model, backup reserve setting, which circuits lost power, and whether the battery was fully charged. We review your Gateway configuration and backup status remotely.

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After 7 months of not being able to access/control my system following a change of wi-fi provider, followed by the news that GivEnergy had ceased trading i was left high and dry. I was unable to get any help whatsoever from my installer, tried various electrical tradespeople etc but no-one knew how to help me. Changed the Dongle but then couldnt get this change recognised in the inverter. Found Solar Tech Support while researching yet again online. Ronald has been amazing. Responding very quickly to my emails and very knowledgeable on the system. Managed to give us the information we needed to get the Dongle up and running and created me another account which now works as it should. His help has been invaluable. Thank you so much. Reviews I saw from other people said Solar Tech Support are very good, but i would say they are Outstanding!! Thanks again for all your help- Mick & Sandra

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Written by · Independent Solar Engineer Last updated

Before you start: check the Tesla app to confirm the Powerwall SOC is above the backup reserve level (Settings > Power > Backup Reserve). If the battery is at 15% and the reserve is set to 20%, the Powerwall will not provide backup until it charges above the reserve. Also note which lights and sockets stayed on during the outage — these tell you which circuits are backed-up.
Diagnosis

Step-by-step backup fault diagnosis

Work through these steps in order. Step 1 checks the simplest cause — backup reserve too low. Step 2 verifies your circuits are backed-up. Steps 3–6 cover Gateway contactor, Storm Watch, load capacity, and power cycling.

1
Check the Backup Reserve setting in the Tesla app

Open the Tesla app → Settings → Power → Backup Reserve. This is the percentage of battery that is kept in reserve for power cuts — the Powerwall will not discharge below this level during normal operation. Minimum recommended is 20%. If the reserve is set too high (e.g. 60%), or the battery is at 65% SOC and the reserve is 60%, the Powerwall will only have 5% usable capacity during a power cut. When the reserve is below 20%, the battery may not store enough energy to sustain critical loads during long outages. Check your current reserve and your home's daily consumption — ensure the reserve leaves enough headroom for backup power.

2
Verify your loads are on the backed-up circuit

The Gateway has a contactor that switches your home between grid and battery during an outage. Only circuits wired BEHIND the Gateway breaker receive backup power. Circuits that bypass the Gateway — such as a garage, external building, or a separate immersion heater circuit — will lose power during outages. During a power cut (or a test), walk around and note which lights and sockets work — these are backed-up. Critical appliances like your boiler, heating, fridge, and lights should be on backed-up circuits. If they are not, they will have no power during an outage. Contact your installer to review your wiring diagram and confirm which circuits are protected.

3
Check the Gateway contactor

The Gateway's contactor is an electromagnetic switch that isolates your home from the grid and connects the Powerwall when power drops. If it gets stuck or fails, it cannot switch — you will have no power even though the battery is charged. During a power cut, listen for a clicking sound near the Gateway as the power drops. If you hear a click, the contactor is engaging. If there is no click or a delayed click, the contactor may be faulty. Also check distance between Gateway and Powerwall — if they are more than 30 metres apart or have poor network connectivity, communication latency can prevent reliable switching. The Gateway must detect the grid loss and disconnect from the network within the 0.5 s limit set by the G98/G99 connection standards.

4
Check if Storm Watch is enabled

Storm Watch is available on UK Powerwalls and integrates with the Met Office forecast. When enabled, the Powerwall auto-charges to 100% when severe weather is forecast for your area, bypassing the backup reserve and maximising capacity before storms. Enable it in the Tesla app → Settings → Storm Watch. If disabled and a storm is approaching, the battery may only charge to the backup reserve level (e.g. 20%), not to 100%. When a power cut occurs, the battery will run out of energy faster than expected. Enable Storm Watch before severe weather season to ensure the system pre-charges to full capacity.

5
Check home consumption versus Powerwall capacity

Powerwall 2 continuous output: 5kW. Powerwall 3 continuous output: 11.5kW. If your backed-up loads exceed this rating, the Powerwall will overload and shut down, cutting all power. Common high-draw appliances: electric showers (7–10kW), immersion heaters (3kW), kettles (2–3kW), electric cookers (10kW+). During a power cut, if the Powerwall cuts out when you turn on the shower, that confirms an overload. The solution is to reduce simultaneous load during backup, wire high-draw appliances on non-backed-up circuits, or install additional Powerwall units for higher capacity.

6
Power cycle the Gateway and monitor

Turn off the Gateway circuit breaker, wait 30 seconds, turn it back on. Allow 5 minutes for the system to reconnect. Test backup by briefly switching off the main grid breaker — the Powerwall should power the home within 2 seconds. The Tesla app should show "Your Home Is Backed Up" and display power flow from the battery. If the Powerwall cycles on/off repeatedly during this test, there may be a firmware issue, a load that is too high, or a contactor fault. If it does not switch at all within 5 seconds, the Gateway may have a hardware fault. Contact STS for a remote diagnostic — we review your Tesla app backup status, firmware, and home consumption data.

Tesla Powerwall backup — how islanding works

When a power cut occurs, the Gateway detects the loss and opens its internal contactor, isolating your home from the grid within the 0.5 s limit set by the G98/G99 connection standards. The Powerwall then forms a microgrid — it powers your backed-up circuits independently, continuing to operate even if the Tesla app is offline or the internet connection drops. This islanding process is what keeps your home functioning during an outage. The Powerwall operates at its continuous rating (5kW for PW2, 11.5kW for PW3) and the backup reserve ensures enough energy is available when the grid fails. If the contactor does not switch, or if your backup reserve is too low, the backup will fail.

In the UK, Powerwall backup installations must comply with G98/G99 islanding requirements, the network operators' connection standards that govern how battery systems can safely isolate from the grid and operate independently. These standards require the contactor to switch within a strict timeframe, ensure the system can detect a grid fault, and prevent export to the grid during an outage. The Gateway and Powerwall are designed to meet these requirements out of the box, but wiring and configuration matter — if your installation bypasses the Gateway or uses non-standard backup circuits, islanding may fail. STS diagnoses backup faults remotely by reviewing your Tesla app data, backup status, and Gateway firmware to determine if the issue is a settings problem or a hardware fault.

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FAQ

Powerwall backup — common questions

The most common causes are: backup reserve is set too low and the battery did not have enough energy stored, critical loads are wired on non-backed-up circuits that bypass the Gateway, the Gateway contactor failed to switch, or the Powerwall is overloaded by appliances exceeding its continuous rating (5kW for PW2, 11.5kW for PW3). Also check Storm Watch — if disabled, the battery may not have pre-charged to 100%. Test by briefly switching off the main breaker — if the Powerwall does not power your home within 2 seconds, the issue is with the Gateway or contactor.
The backup reserve is the SOC percentage below which the battery will not discharge during normal operation — that energy is kept for power cuts. Default is typically 20%. A higher reserve (e.g. 50%) saves more energy for backup but reduces daily self-consumption and cost savings. A lower reserve (e.g. 10%) maximises daily savings but leaves less energy for backup. Minimum recommended is 20% for adequate backup capacity. Set it higher if you experience frequent long outages, or lower if you rarely have outages and prioritise daily savings.
Briefly turn off the main circuit breaker for 5 seconds. The Powerwall should detect the loss and switch over within 2 seconds, powering the backed-up circuits. Check the Tesla app — it should show "Your Home Is Backed Up" and display power flow from the battery. Lights and sockets on backed-up circuits should stay on. Non-backed-up circuits will lose power just as in a real outage. Turn the main breaker back on to reconnect. If the Powerwall does not switch, or cycles on and off, there is a Gateway contactor issue, firmware fault, or an overload problem.
Depends on wiring and load. If configured as whole-home backup, the Powerwall powers all circuits behind the Gateway as long as simultaneous consumption does not exceed the continuous rating: Powerwall 2 = 5kW, Powerwall 3 = 11.5kW. If partial backup was installed, only selected circuits are protected. High-draw appliances like electric showers (7–10kW), immersion heaters, or cookers will overload the Powerwall if run simultaneously with other loads. For whole-home backup with high-draw appliances, install multiple Powerwall units or upgrade to Powerwall 3 for its higher 11.5kW output.
Met Office integration that auto-charges the Powerwall to 100% SOC when severe weather is forecast for your area, bypassing the backup reserve to maximise capacity before storms. Available on UK Powerwalls. Enable in the Tesla app under Settings > Storm Watch. Overrides the normal backup reserve temporarily. When a severe weather alert is issued, the system pre-charges to full capacity, ensuring maximum backup energy is available if a grid outage occurs during the storm.
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Powerwall backup still not working?

If you have checked the backup reserve, verified your circuits are backed-up, tested the contactor, and the Powerwall still will not provide backup — the issue may be a Gateway hardware fault, a firmware bug, a communication lag, or an overload condition. We review your Tesla app backup status, Gateway firmware, and home consumption data remotely. Independent from Tesla and your installer.

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