Storm Watch explained: what it does, what it costs and whether to leave it on
Storm Watch is the Powerwall feature that fills the battery before bad weather arrives and holds it full until the warning passes. It is simple, it is usually right, and it occasionally costs you a few pounds at exactly the wrong moment. This article explains what triggers it, what it overrides, what it does to a tariff-driven battery, and whether to leave it on.
What it does
With Storm Watch on, the Powerwall watches for severe weather warnings for your area. When one is issued, the Powerwall charges to 100 % as soon as it can, from solar if there is any and from the grid if not, and stays there until the warning ends. Then it goes back to whatever it was doing. In the Tesla app you see a Storm Watch banner while it is active, and the energy graph shows the fill as grid import.
The point is backup. A storm is when the grid is most likely to fail, and a full battery at the start of a cut is the difference between a comfortable night and a dark one. What a Powerwall does in a power cut covers what happens next.
What triggers it
Tesla takes the warnings from the official weather service for your country. In the UK that is the Met Office's severe weather warnings; Storm Watch responds to the more serious ones for wind, rain, snow and ice, not to every yellow warning. Tesla does not publish the exact mapping and has changed it over time, so you will sometimes see a fill for a warning you would have ignored and no fill for weather that turned out worse. The warning area is also large: a county-wide warning fills your battery even if your village sees nothing.
You cannot tune the threshold, and you cannot trigger it yourself. What you can do is switch the feature off, in the Tesla app under the Powerwall's settings, and run your own version with rules.
What it overrides
Storm Watch sits above everything else the Powerwall does:
| Setting | During Storm Watch |
|---|---|
| Operation mode | Ignored: the battery charges to full whatever the mode |
| Backup reserve | Effectively 100 %: the battery will not discharge to run the house |
| Grid charging | Allowed, at whatever the tariff price is now |
| Time-Based Control's schedule | Suspended until the warning ends |
| Solar | Used to charge first; export only once the battery is full |
Two of those rows have a cost. The battery charges from the grid the moment the warning arrives, which may be the middle of the evening peak on a time-of-use tariff, and it then runs the house from the grid instead of from the battery until the warning ends, which can be a day or two. On a flat tariff that is a few pounds at most. On Cosy, Agile or Flux it can be a fill at the dearest price of the day followed by two days of peak-rate evenings, perhaps £10 across a long warning. That is the price of certainty, and whether it is worth paying depends on how often your grid actually fails.
Storm Watch and a tariff-driven battery
A battery run on cheap windows has its own rhythm: fill in the window, run the house between, cover the peak. Storm Watch breaks that rhythm by design. The clash shows up in three ways.
First, the fill. If the warning arrives at 17:00, the Powerwall charges at the peak rate, when a rule would have waited for the 22:00 window. Second, the hold. For the warning's duration the house runs on the grid and the battery sits full, so the evening peaks you would normally have covered from the battery are bought at full price. Third, the hand-back. When the warning ends the battery is full and your rules carry on as if nothing happened, which is fine, except that a fill rule firing into an already-full battery does nothing and a morning skip rule that relies on the forecast may misjudge the day.
None of this is a fault. It is Storm Watch doing what it says. The question is whether its judgement of risk matches yours.
Leave it on, or run your own?
Leave it on if your grid fails in storms. A rural line, a history of cuts in high wind, a house with a medical need for power: the few pounds a warning costs are nothing against a night without electricity, and Storm Watch will act when you are asleep or away.
Consider switching it off if your grid is reliable and your tariff is time-of-use. Urban supplies rarely fail, and the warnings that trigger Storm Watch are far more common than the cuts they predict. You can keep the protection without the cost by writing it yourself:
- In SunHarvest, Storm Watch's own state is a signal, so a rule can react when Tesla raises it. A rule that waits for a cheap window before letting the fill happen is not possible, because Tesla charges immediately, but a rule can switch Storm Watch off again and raise the reserve instead, which keeps whatever is in the battery without a peak-rate fill.
- Better, use the weather signals directly. A rule on "max wind in the next 12 hours above 20 m/s" can raise the reserve to 100 % for the night and fill in the next cheap window rather than at once. The forecast is the same one the warning came from, a few hours earlier and under your own threshold.
- Put the storm rules in their own group, so that a calm week can run without them and a forecast you don't believe can be overruled with one switch.
The AI planner in SunHarvest sees the wind forecast and Storm Watch's state, so it can fill in the cheapest window before the weather arrives rather than the instant the warning is issued, which is the compromise most people want.
Things to know
- It needs the internet. Storm Watch is decided in Tesla's cloud and sent to the gateway. A Powerwall that has lost its connection does not get the warning.
- It shows in your figures. A Storm Watch fill is grid import at the price of the moment, and SunHarvest values it at that price, so a stormy week looks dearer than a calm one. The Recent changes list records it as Tesla's doing, not yours.
- It ends when the warning ends, which can be hours after the weather has passed. If the sky is clear and the battery is still being held, that is why.
- You can switch it off mid-warning. The hold stops at once and the battery goes back to its mode. Switching it back on later does not refill until the next warning.
In SunHarvest
Storm Watch appears twice among the rule signals: the setting, enabled or disabled, and the event, active while Tesla is holding the battery. The set Storm Watch action switches it either way, so a rule can turn it off when an event fires and raise the reserve instead, or turn it on for the winter and off for the summer. The forecast signals, max wind over the next hours among them, let a rule fill in the cheap window ahead of the weather on your own threshold. A storm fill shows in the money figures at the price of the moment, and the Recent changes list attributes it to Tesla, not to you.
Storm Watch is Tesla's feature; its triggers and behaviour are described as they were understood in autumn 2026, and Tesla changes them without notice.