What backup reserve should you set on your Powerwall?
The single slider that decides how much of your battery is saved for outages vs. used for savings. Here's how to set it for your situation.
Your Powerwall's backup reserve is the percentage it always keeps in the tank for a power outage. Set it to 100% and the battery is a pure backup device — but it never helps your bill. Set it to 0% and you maximize daily savings — but you'd have nothing left if the grid went down at 9 PM. The right answer is almost always in between, and it depends on you.
What the reserve actually does
If your reserve is 30%, the Powerwall will use the top 70% of its capacity for daily cycling (storing solar, powering your evening), but it stops discharging at 30% so that slice is held for backup. During an outage, the full battery powers your home until it's drained.
Rules of thumb
- Frequent or risky outages (wildfire PSPS zones, storm-prone areas): keep a higher reserve — 50–100%. Peace of mind wins.
- Rare outages, focus on savings: a lower reserve — 10–20% — lets the battery do its real job of shifting energy off peak rates.
- Somewhere in between: 20–30% covers a typical evening outage while still freeing most of the battery for savings.
The nuance most people miss: it should change
A fixed reserve is a compromise. The smarter approach is to raise the reserve when an outage is likely and lower it the rest of the time. Heading into a storm or a planned PSPS event? Push it to 100% the day before. Calm week ahead? Drop it so the battery earns its keep. Some owners also lift the reserve seasonally (fire season) and relax it otherwise.
Don't forget your rate plan
The "savings" half of this decision only pays off if the battery discharges during your expensive hours. On a time-of-use plan that means holding charge until the 4–9 PM peak, not dumping it at 2 PM. If your battery is set to self-power naively, it can discharge at the wrong time and leave you buying grid power at peak. Matching discharge to your real peak windows is exactly what an app like SolDial automates.
Quick starting point
If you're unsure: try 20% if outages are rare, or 50% if they're common — then adjust after watching a few weeks. And if you'd rather not think about it, let an automation raise the reserve ahead of bad weather and relax it otherwise.