Learn how to work out the right battery reserve level for your home, what it costs, and when to change it.
A few months after you install your battery, you open the app and find a setting you didn't expect. It might be called Discharge Cut-Off SOC (state of charge), UPS Reserve, or Reserve Power Mode. The names vary, but they all mean the same thing. It's asking you to pick a number, and you may not know what to set it to.
That number is your solar battery reserve capacity, and there's no single right answer. There is a right way to choose yours, though. It depends on how often your power goes out, what in your home can't go without it, and how big your battery is.
What Your Solar Battery Reserve Capacity Actually Holds Back
Your reserve is the share of stored energy the battery won't use in everyday running. It stays in the battery, waiting for a blackout. Set a 20% reserve and the battery powers your evening until it reaches 20%, then the house draws from the grid instead.
Think of the battery backup reserve setting as a line you draw. Below it sits a second line you can't move: the manufacturer's own protection layer. When the app shows 0%, the cells aren't truly empty. That hidden buffer is part of the battery's design. It keeps the battery within its rated depth of discharge, the limit on how far it can safely run down.
In other words, your reserve is the battery’s minimum state of charge: a floor you choose, set above the manufacturer’s limit. Raising or lowering it changes how you use your energy, but it doesn't change how the battery protects itself. On VoltX Energy's line of solar batteries, those protections stay in place no matter what number you pick.
Three layers of a home battery: everyday energy at the top, your backup reserve in the middle and the manufacturer's protection buffer below the 0% shown in the app.
What Holding Energy Back Costs
Energy you hold in reserve is energy you don't use, so you buy that power from the grid instead. A reserve buys readiness, and it has a price.
Here's a worked example:
Say you have 10 kWh of usable capacity. A 20% reserve holds back 2 kWh. At 33.14 cents per kWh, a typical flat rate on the Ausgrid network under the AER’s 2026–27 Default Market Offer, buying that 2 kWh from the grid costs about 66 cents.
That’s the most it can cost you on a given day. A reserve only costs you when the battery would otherwise have dropped below it, and only for the energy it would have used. On a mild day, when the battery finishes the evening above your reserve anyway, holding energy back costs nothing.
The energy you hold back isn’t lost, either. It stays in the battery, so the next day the battery needs less sunshine to refill, and you export more of your solar power. Your real cost is the gap between what you pay for grid power and what you earn for exports. With a 5-cent feed-in tariff, that gap is about 28 cents per kWh.
Put together, here’s the most a 20% reserve on a 10 kWh battery would cost:
| Held every day of the year | about $242 at the grid rate, or about $205 once you count exports |
| Held through the storm months only | About $121 at the grid rate, or about $103 once you count exports |
To estimate your own cost, check your app for how many days the battery reached its reserve. Multiply those days by the kWh held back, then by the difference between your usage and feed-in tariff. The battery won’t always have needed the full reserve on those days, so treat the result as an upper estimate.
Should I Set a Backup Reserve at All?
Check one thing first: whether your system has blackout protection. The reserve setting works on every VoltX Energy battery, but without a blackout protection add-on, the energy it holds back can't reach your home when the grid goes down. You’d be giving up bill savings for backup you can’t use, so leave the reserve off. If you're unsure, our guide to why some solar batteries don't work in a blackout explains what to look for.
If you do have blackout protection, you'll find the reserve setting under battery settings or discharge settings. Its factory default is off, so unless your installer switched it on, nothing is being held back today.
How Much Battery to Keep for Blackouts
You can work your solar battery reserve level from three things you already know. Together, they turn a guess into a reason.
How Often Your Power Goes Out
Think back over the last year or two. Count the outages that lasted more than a few minutes, and when they happened. Your distributor (Ausgrid, Endeavour Energy or Essential Energy in NSW, Energex in Southeast Queensland, Ergon Energy in regional Queensland) shows current and recent outages on its website. If your street rarely loses power, a large reserve mostly sits unused.
What Genuinely Can't Go Without Power
List what you'd need running in a blackout: the fridge, a few lights, the Wi-Fi, a phone charger, perhaps a sump pump. Keep it to what truly can't stop, then turn the list into a number:
Essentials’ power draw × hours you want covered ÷ usable capacity = your reserve.
A fridge, a few LED lights, the Wi-Fi, and phone charging together draw about 300W on average. The fridge briefly draws more each time its compressor starts. Covering eight hours takes 2.4 kWh. Add about 10% for inverter losses, and you need roughly 2.6 kWh, which is a 26% reserve on a 10 kWh battery. For longer outages, see what your battery can do.
How Big Your Battery Is
A backup reserve percentage means different amounts on different batteries. A 10% reserve on a 30 kWh battery holds 3 kWh, more than a 20% reserve holds on a 10 kWh battery. On a large system, a small percentage can cover your essentials. That's one reason high-capacity batteries are changing the energy independence equation.
When to Raise or Lower Your Reserve
The best level today may be the wrong one in three months. Treat it as a setting you revisit.
Before Storm Season
Raise the reserve as the storm season arrives (roughly October to March in NSW and Southeast Queensland) and whenever your distributor gives notice of a planned outage. Raising it a day or two before forecast weather gives your solar time to fill it.
If the weather turns before the sun can top it up, you can also charge from the grid. Lower it again when the weather settles so the battery can start cutting your bills again. Owners who prepare before summer blackouts avoid the scramble when warnings arrive.
If Your Battery Is In a VPP
A virtual power plant (VPP) lets an energy company use a group of home batteries, including yours, to support the grid. Some programs let you set a reserve they won't draw below, and some cap how high it can go. Before you join, check how the program treats your reserve. Our guide to what a VPP-compatible battery needs explains what to look for.
If Someone at Home Relies on Medical Equipment
A reserve setting can't guarantee uninterrupted power for essential equipment. Depending on your setup, there may also be a brief gap while the system switches to backup, which some equipment can’t tolerate. Talk to your installer about what the system can and can't run, and to your healthcare provider about a backup plan.
If you, a family member, or someone you care for relies on life support equipment, register as a life support customer with your retailer or distributor. Every distributor keeps a life support register, and registration brings advance notice of planned outages. It can't protect you from unplanned ones, which is where your backup plan matters most.
Conclusion
Your reserve is a trade-off between energy kept ready for an outage and energy used to lower your bills. If you’re not sure where to start, use the formula above to work out what your essentials need, and set that as your reserve. Raise it as storm season begins, then lower it when the weather settles so the battery can get back to cutting your bills.
Talk It Through Before You Change It
If you'd like a second opinion before changing your reserve, our support team can help. Tell us what you need to keep running and how often your area loses power, and we'll explain what your system can do and where to find the setting in your app.
Frequently Asked Questions
There's no single right number. Multiply what your essentials draw by the hours you want covered, divide by your usable capacity, and use that as a starting point. Many households land somewhere between 20% and 30%, then raise it for storm season.
No. On car and 12V batteries, “reserve capacity” is a rating measured in minutes or runtime. It’s unrelated to the reserve setting in your solar battery app.
No. Your reserve sits above the manufacturer's protection buffer, which stays in place even at a 0% reserve. Daily cycling within the battery's rated limits is what it's designed for.
Not by itself. It only stops the battery from discharging below that level. If you raise it the day a storm is forecast, check the actual charge in your app, too.
It can be, if the program lets you set a reserve that suits your household. Whether joining a VPP is worth it in 2026 depends on whether payments outweigh the control you give up.
Only if your system has blackout protection and the office is on a backed-up circuit. Our guide to powering a home office through an outage explains what to plan for.
