How Powerwall time-based control works
A Powerwall has only a handful of settings, but together they decide when it charges, when it discharges and whether it ever sells to the grid. Understanding them is the difference between a battery that saves a little and one that pays its way.
The two operation modes
Self-Powered
In Self-Powered mode, the Powerwall's only aim is to keep your house off the grid. Spare solar charges the battery. When the house needs more than the panels make, the battery covers the difference, until it reaches the backup reserve. Then the house draws from the grid.
This is simple and works well on a flat tariff, where every kWh costs the same whenever you buy it. It ignores price completely, though. On a winter night it will happily empty itself at 9 pm, and you then buy expensive evening energy the next day with an empty battery.
Time-Based Control
Newer versions of the Tesla app call this mode Savings mode; it is the same thing.
In Time-Based Control, the Powerwall is given a tariff: a set of time periods, each with a buy price and a sell price. It then tries to minimise what the day costs against that tariff. In practice:
- It saves battery energy for the expensive periods rather than spending it as soon as the sun goes down.
- With grid charging allowed, it can buy energy in cheap periods to use in expensive ones.
- With export set to include the battery, it can sell stored energy when the sell price makes it worthwhile.
Tesla doesn't publish exactly how the Powerwall decides, and it also takes account of its own solar forecast and your usage pattern. The principle is clear, though: the Powerwall optimises against the tariff you give it, not against what your supplier actually charges.
The settings that steer it
| Setting | What it does |
|---|---|
| Backup reserve | The share of the battery held back for a power cut. Everyday use never discharges below it. Raising it above the current charge makes the Powerwall top up to it, using the grid when solar isn't enough (owners report this can happen even with grid charging off). |
| Grid charging | Whether the battery may charge from the grid at all. Without it, the battery fills only from solar. |
| Export | Solar only sells spare solar but never battery energy. Solar + battery (shown as "Everything" in the Tesla app) lets the battery export too. No export, usually set by the installer where the network requires it, keeps everything on site. |
| Storm Watch | If you turn Storm Watch on, the Powerwall charges to full ahead of severe-weather warnings, overriding its normal mode until the alert ends. |
| The tariff | The periods and prices Time-Based Control optimises against. It only matters in Time-Based Control. |
Why the Powerwall's tariff doesn't have to be your real one
This is the least obvious and most useful idea. The tariff on the Powerwall is an instruction, not a bill. The Powerwall acts on whatever prices it is shown, so you can steer it by showing it prices:
- Force a charge: mark a window as very cheap to buy in, with grid charging on, and it will fill up in that window.
- Force an export: mark a window with a very high sell price, with battery export allowed, and it will sell flat out in that window.
- Hold energy back: mark the evening as expensive to buy, and it will save the battery for it rather than spending it in the afternoon.
Your real bill is still worked out on your real tariff. That is why SunHarvest keeps the two apart. Your real tariff drives the money figures, the rules and the AI, while the Powerwall's tariff is just a lever. See Prices and tariffs in the guide.
Where time-based control falls short
The Powerwall's tariff is built for tariffs that repeat: the same cheap window every night and the same peak every evening. Smart tariffs don't work like that.
- Half-hourly prices change every day. On Octopus Agile, tomorrow's prices are published each afternoon and can look very different from today's. A tariff typed into the Powerwall once is out of date the next day.
- Weather changes the right answer. Charging from the grid overnight is smart before a grey day and wasteful before a sunny one, when the panels would have filled the battery for free.
- Your priorities aren't prices. You may want a healthy reserve in storm season, or a full battery before a holiday. The tariff can't express that.
That gap is what automation is for. SunHarvest can push each day's real prices to the Powerwall just after midnight, so Time-Based Control follows the real day. Rules can change the reserve, grid charging and export on price, forecast and battery level. The AI planner can write a fresh set of those rules every few hours toward a goal you describe.
A sensible starting point
- On a flat tariff with solar, Self-Powered with a modest reserve (10–20 %) is usually right.
- On a tariff with a cheap overnight window, use Time-Based Control with grid charging on. Put the real window on the Powerwall, and add a rule that holds more in reserve before grey days.
- On a half-hourly tariff like Agile, use Time-Based Control with the real prices pushed daily, plus rules for the cheapest slots. Or let a planner decide.
- Whatever you do, watch the change log for a week. Seeing what changed, and who changed it, is the fastest way to learn how your system behaves.
General information, not financial advice. Tesla may change how the Powerwall and its app behave; the Powerwall's own documentation is the authority on your hardware.