What a Powerwall does in a power cut
The moment the grid fails, a Powerwall has about a hundredth of a second to notice, cut itself off from the street and become the house's own little grid. Most owners never see the switch happen; the lights flicker, or they don't, and the fridge carries on. This article explains what actually happens in a cut, what keeps running, why the solar carries on, how long the battery lasts, and how to set the reserve for the cuts you really get.
The switch itself
A Powerwall 2 is paired with a Backup Gateway, the grey box near your meter; a Powerwall 3 has the equivalent built in. The gateway watches the grid supply continuously. When the voltage or frequency goes outside limits, it opens a contactor between the house and the grid and the Powerwall's inverter starts generating its own 50 Hz supply for the house. The changeover takes a few hundredths of a second: short enough that computers and routers ride through it, long enough that some older clocks and a few microwave displays reset.
From then on the house is an island. Nothing you do can push power out to the street, which is what the safety rules require, so the engineers working on the line are not surprised by a live cable. The Tesla app shows "Off-Grid" and the gateway keeps checking the supply; when the grid has been stable for a few minutes it resynchronises and closes the contactor again, usually without a flicker.
What keeps running
That depends on how your installer wired it. In most UK homes on a single-phase supply the whole house is backed up: everything after the gateway is on the island. Some installations back up only a chosen set of circuits, typically lighting, sockets and the boiler, with the cooker and the shower left on the grid side. Your installer's paperwork, or a look at which breakers sit after the gateway, tells you which you have.
Whole-house backup has a limit, which is the Powerwall's power rather than its energy. A Powerwall 2 supplies 5 kW continuously, with a short burst to 7 kW; a Powerwall 3 supplies 11.5 kW. An oven, a kettle and an electric shower together can pass 5 kW with ease. On the grid that excess simply comes from the street. On the island there is no street, so the Powerwall trips to protect itself and the house goes dark until you turn something off and the gateway tries again. The practical rule in a cut on a Powerwall 2 is one big appliance at a time, and no electric shower.
Three-phase homes are a separate case. A single Powerwall backs up one phase, so a three-phase house gets backup on whatever is wired to that phase, and some installations have no backup at all for that reason.
Why the solar carries on
A solar inverter needs a grid to lock onto; without one it shuts down. In a cut, the Powerwall's inverter is that grid, so the panels keep producing and the house uses the solar first, with the battery covering the rest and charging from any surplus. On a sunny day a cut can be almost free.
There is one subtlety with a Powerwall 2 and a separate solar inverter. If the battery is full and the house is using less than the panels are making, there is nowhere for the surplus to go. The Powerwall handles this by nudging the island's frequency upward, which the solar inverter reads as a signal to back off, a standard behaviour called frequency shifting. You may notice the solar figure in the app dropping on a bright afternoon during a cut; that is the system working, not failing. A Powerwall 3 has its own solar inverter and manages this internally.
How long it lasts
A full Powerwall holds 13.5 kWh. A house ticking over on fridge, freezer, router, lights and a boiler's pump uses perhaps 300 to 500 W, which is a day or more. Cooking a meal electrically takes 1 to 2 kWh; a tumble dryer about 2 kWh a load; an electric shower 1 kWh for ten minutes, if the power allows it at all. Heating is the big one: a heat pump at 2 to 3 kW will empty the battery overnight, which is why a heat-pump home should treat a winter cut as a reason to keep the house a little cooler rather than run as normal.
How full the battery is when the cut starts matters as much as any of that, and that is what the reserve is for.
What the reserve actually does
The backup reserve is the share of the battery the Powerwall will not use in normal operation. With a 20 % reserve, the battery never discharges below 20 % to run the house or to export, so there is always at least 2.7 kWh waiting for a cut. In a cut, the reserve stops being a floor: the whole battery is available, down to empty.
Every percentage point of reserve is energy you paid for and keep paying to hold, as the backup reserve article works through. The sensible way to set it is from the cuts you really get, not the ones you imagine. Most UK homes see a handful of short cuts a year, measured in minutes to a couple of hours. A 10 to 20 % reserve covers those comfortably. If you are on a rural line that goes down for a day in every storm, a higher reserve in storm season is worth its cost, and Storm Watch, covered in its own article, fills the battery ahead of a forecast storm regardless of the reserve.
What the reserve cannot do is be set from inside a cut. If the grid goes down with the battery at 15 % on a 20 % reserve, you have 15 %. The reserve is a decision made in advance.
Things to know before the first cut
- Find out what is backed up. Ask your installer or read the labels at the consumer unit. A whole-house setup and a partial one feel very different at 8 pm in December.
- Know the power limit. One big load at a time on a Powerwall 2. Tell the household.
- Gas boilers need electricity. The boiler's controls and pump run on mains, so a backed-up house keeps its gas heating; a house with the boiler on a non-backed circuit does not.
- The internet may not survive. Your router will, but the street cabinet or the mast may not. The Tesla app needs the internet to show the Powerwall's status; the gateway keeps working without it.
- The EV charger probably won't. Most chargers are on the grid side, and a 7 kW charger would trip a Powerwall 2 anyway.
- Check the app afterwards. The Tesla app's history shows the cut as a grid outage, with the energy the battery supplied. SunHarvest's grid status signal reads off-grid while the house is islanded, so a rule can react to a cut and another to the grid's return, for example by filling the battery in the next cheap window ahead of its usual turn.
When the grid comes back
The gateway waits for the supply to be steady, then reconnects. The Powerwall goes back to whatever mode it was in, and if the cut drained the battery it recharges on the normal rules: from solar in Self-Powered mode, or in the next cheap window under Time-Based Control. It does not rush to refill at full price unless Storm Watch is active or you tell it to. If a long cut has left you nervous, a rule in SunHarvest can raise the reserve for a few days and let you put it back when the weather settles.
In SunHarvest
The grid status signal reads on-grid or off-grid, so a rule can act when a cut starts and another when it ends: raise the reserve for the rest of the week after a long outage, or fill the battery in the next cheap window ahead of its usual turn so the next evening starts full. Home shows the same state live, and the day's history shows the cut as hours with no import and no export. Settings › Site shows the hardware, capacity and peak power Tesla reports for your site, and the Recent changes list records anything that was switched around the cut, by you or by Tesla.
General information. Backup wiring, gateways and changes to your supply must be done by a qualified electrician to your network operator's requirements; check your own installation before relying on it in a cut.