Backup reserve: what percentage to set, and what it costs you
The backup reserve is the one Powerwall setting almost everyone leaves where the installer put it. It is also the one with the clearest price tag: every percentage point held back is energy you paid for and cannot use on an ordinary day. This article explains what the reserve does, what it costs, and how to pick a number you can justify.
What the reserve is
The backup reserve is a percentage of the battery that everyday use never discharges below. The Powerwall will run your house, sell to the grid or do whatever its mode asks, but only down to that line. Below it, the energy is kept for one purpose: a power cut. If the grid fails, the Powerwall drops to the reserve and keeps going, and only then runs the house from what was held back.
Set it to 20 % and a 13.5 kWh Powerwall stops at about 2.7 kWh remaining. Set it to 50 % and it stops at about 6.75 kWh. Whatever the number, the battery behaves, day to day, as if it were that much smaller.
What it does in each mode
Self-Powered
In Self-Powered mode the battery discharges into the house whenever the panels cannot keep up, and the reserve is simply the floor. Once it is reached the house draws from the grid until the sun comes back. Nothing else changes.
Time-Based Control
In Time-Based Control the reserve is still a floor, and the Powerwall's price-driven decisions happen above it. There is one more behaviour that matters. If you raise the reserve above the current charge, the Powerwall treats the gap as something to fill. It will top up to the new reserve, using solar if there is any and, on most sites, the grid if there isn't; owners report this can happen even with grid charging switched off. That is why the reserve is such a useful lever for automation: raising it to 100 % at the start of a cheap window and lowering it at the end is the simplest way to fill the battery on schedule. How Powerwall time-based control works covers the rest of the settings.
What a reserve costs you
Held-back energy has a cost that repeats every day. Think of a home that uses 10 kWh a day and fills the battery from the cheap overnight window in winter. With no reserve, the battery covers the whole 10 kWh with room to spare. With a 30 % reserve it carries 9.45 kWh of usable energy and the last of the evening is bought at the day rate.
| Reserve | Held back | Usable each day | What the held-back energy would run |
|---|---|---|---|
| 0 % | 0 kWh | 13.5 kWh | Nothing, beyond the system's own floor |
| 10 % | 1.35 kWh | 12.15 kWh | Fridge, freezer, router and lights for an evening |
| 20 % | 2.7 kWh | 10.8 kWh | The same for most of a day, or a few hours of normal use |
| 50 % | 6.75 kWh | 6.75 kWh | A normal day for a light-use home, with no heavy loads |
| 100 % | 13.5 kWh | 0 kWh | Everything the battery can hold, for as long as it lasts |
The daily cost of a reserve is the saving those kWh would have produced on a day when you needed them. On a summer day with a full battery and a small evening load, a 20 % reserve costs nothing, because the top 80 % was enough. On a dark winter evening when the battery runs to the floor at 8 pm, a 20 % reserve means 2.7 kWh bought at the evening rate instead of the overnight rate. Multiply by the number of days a year the battery actually hits the floor, and you have the annual price of your reserve.
How to size it
A reserve that has never been used is an insurance premium paid daily. Size it like insurance: from what you would actually need, and how likely you are to need it.
What you would run in a cut
Most homes need very little during a cut. A fridge and freezer together average well under 100 W, the broadband router and a few LED lights a few tens of watts more. Call it 150 W, and 1 kWh runs those for around six hours. The things that empty a battery fast are heating, cooking and hot water: an electric shower or an oven draws several kW and turns 2.7 kWh into less than an hour. If you have a heat pump or an electric hob, decide in advance whether you would use them in a cut. If not, a small reserve goes a long way.
How long cuts last where you live
Most power cuts in Britain are short, but that varies a lot between a town centre and a rural line at the end of an overhead network. Your distribution network operator (the company that owns the wires, not your supplier) publishes information about outages in your area, and your own memory of the last few years is a fair guide. A home that has seen two brief cuts in five years has a different case from one that loses power for hours every winter storm.
Putting the two together
- Essentials only, short cuts likely: 10 % is plenty, about 1.35 kWh, most of a day of fridge, freezer and router.
- Essentials plus some comfort, or longer cuts: 20–30 %.
- Medical equipment, a well pump, or a history of long outages: 50 % or more, and accept the daily cost.
Why 0 % is not dangerous
People worry that a 0 % reserve lets the battery run itself flat and come to harm. It does not. The percentage you see is a share of the usable capacity, and the system keeps its own margin beneath that which you cannot reach from the app. Discharging to 0 % in everyday use is the design case, not an abuse of it. Whether cycling the full range every day affects long-term life is a separate question, covered in Does cycling a home battery every day wear it out?, and the answer there does not change the point here: the reserve is for you, not for the battery.
Two things a low reserve does affect. If a cut comes when the battery is at its floor, you have nothing. And Storm Watch, if you turn it on, charges the battery to full ahead of severe weather warnings regardless of the reserve, which is a good reason to leave it on if outages worry you.
Changing it by season or rule
The reserve does not have to be one number all year. Cuts cluster in stormy weather; long dark evenings are when held-back energy costs most. It can be raised in winter and lowered in summer, or changed by a rule: higher when the forecast says a grey day is coming, lower on a sunny morning when a full battery is certain by noon. Raising it when the car starts charging stops the Powerwall draining into the car. Each change is a trade between the daily cost and the insurance, made when the trade is clearest.
Without a backup gateway
Not every Powerwall can actually power the house in a cut. That depends on how it was installed and what backup hardware it has; check with your installer if you are not sure. On a system that cannot island, the reserve still applies exactly as described: everyday use stops at the line. The difference is that the held-back energy can never be used, because in a cut the system shuts down with the grid. On such a system any reserve above the minimum is pure cost, and it makes sense to set it low.
In SunHarvest
The reserve is the setting SunHarvest's rules change most. A rule can set it from a trigger and conditions: raise it to 100 % when the import price falls below a threshold and lower it when the window ends, hold 60 % when solar remaining today falls below a few kWh with the battery already low, or drop to 20 % at 06:00 on weekdays only if tomorrow's forecast is above 20 kWh. The change log on Control › Powerwall names who made each change, whether that was you, a rule, the AI planner or the Tesla app, so a reserve that keeps moving is never a mystery. Rules are free on every account; the rules guide has the details.
General information, not financial or electrical advice. Tariff terms, prices, warranties and connection rules change and differ by supplier, region and installation; check the current documents for yours. SunHarvest is not affiliated with Tesla or Octopus Energy.