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What should I set my backup reserve to?

Short answer

The reserve is the charge kept back for power cuts; the battery stops normal daily discharge when it reaches it. Many owners use 10-20%, raising it before storms. Tesla's Storm Watch does this automatically in the UK. If a Powerwall falls below 10% in an outage it goes to standby, and below 5% it holds energy to restart from solar.

Last checked 24 September 2026 · 8 sources

What should I know before deciding?

Reserve trades savings for resilience: every kWh reserved is not used to avoid peak import. Tesla UK support: under 10% Powerwall enters standby and stops powering the home; if an outage starts under 5% the home loses power immediately; with solar it tries to recharge hourly 8am-4pm. Storm Watch (Tesla, available in the UK) charges to 100% from the grid ahead of severe-weather alerts, then returns to your normal mode. EST: systems usually stop discharging at a set level, 'maybe 20%', and should never reach zero. A reserve does nothing if the battery has no backup wiring (EPS or gateway).

How much battery should I keep in reserve for power cuts, and why do things reboot when the power fails?

The reserve is the share of battery capacity the system will not use day to day so it is available in an outage. 20% of a 13.5 kWh battery is 2.7 kWh, a few hours of overnight load. Raise it before storms. Brief blackouts during switchover can reboot routers and computers; a small UPS avoids this.

Tesla: set a higher Backup Reserve if prone to cuts; Storm Watch pre-charges before severe weather; standby below 10%. EPS systems can take ~5 s (GivEnergy 2023 guide); gateways claim 20 ms (GivEnergy) or 'fraction of a second' (Tesla); owners report 0.5-3 s blackouts on some systems. Reserve reduces daily savings: each kWh held back is a kWh not used to avoid import.

My Powerwall had a 20% backup reserve, so why did the house lose power overnight during a long power cut?

Backup reserve only guarantees how much energy is kept at the start of a cut; during the cut Powerwall keeps discharging below it. Tesla's UK guidance says that below about 10% it goes into standby and only retries hourly, 8am to 4pm, when solar is available. So a long overnight cut can drain it.

Backup Reserve page: during an outage Powerwall may discharge below the set reserve; after the outage it recharges to the reserve before normal operation. Reserve below 20% shows a 'Low Backup Reserve' indicator; if an outage starts with stored energy below 5% the home loses power immediately and Powerwall waits for morning solar. UK power-cut page: below 10% remaining Powerwall enters standby; with solar it retries hourly 08:00-16:00; restart by on/off switch or 'Restart Powerwall' in the app only when sunny.

What to do next

Check the manufacturer manual and warranty before changing settings. Use the installer or manufacturer support route for persistent faults.

Sources

Last checked: 24 September 2026