Short answer: size a home battery around the power you use after dark, not around your total daily usage and not around the size of your solar system. Get that one number right and the rest of the decision gets much simpler.
In Iconic Energy’s own installation experience, around 90% of the household battery systems we have installed have been between 16kWh and 24kWh. That sits above the 10kWh to 13.5kWh range most national guides quote, and the gap is worth understanding before you settle on a number for your own home.
Start with what you use after the sun goes down
Solar covers your daytime load while it is generating. A battery covers what is left: the evening, overnight and early morning. So the figure that sets battery size is not your daily usage, it is the slice of that usage your panels cannot reach.
Three steps to get there:
- Find your average daily usage in kilowatt-hours. Most bills show it directly. If yours does not, divide the kWh on the bill by the number of days in the billing period.
- Work out how much of that lands outside daylight hours. Rules of thumb put it at roughly half to sixty per cent, but that is a guess about your household. If you have a smart meter, your retailer can usually provide interval data showing exactly when you draw power. That beats any rule of thumb.
- Decide how much of that evening and overnight load you actually want the battery to cover. Covering all of it, every night of the year, is a different (and larger) battery to covering most of it most of the time.
A worked example, using round numbers. A home using 24kWh a day, with 55% of that used outside daylight hours, needs about 13kWh to get from sunset to sunrise. Add a margin for the nights that run above average, and for the capacity you hold back rather than discharge, and the practical size is higher than the 13kWh the arithmetic produced. Run the same sum on your own bill and you have a starting point rather than a sales figure.
Usable capacity is the number that matters
Batteries are advertised two ways, and the difference catches people out.
- Nominal capacity is the total capacity of the cells.
- Usable capacity is what you can actually draw out, after the reserve the system holds back to protect the battery.
Your sizing sum from the last section is in usable kilowatt-hours. When you compare quotes, check which number each one is quoting, because a nominal figure will always look bigger.
It matters for the federal discount too. Eligibility for the Cheaper Home Batteries Program is set on nominal capacity, between 5kWh and 100kWh. The amount of support is calculated on usable capacity. Same battery, two different numbers doing two different jobs.
In Tasmania, winter is the test
Size a battery off a mild shoulder-season bill and it will look adequate. Tasmanian winters apply the real test: the shortest days and the lowest solar generation arrive at the same time as the heaviest heating load.
That does not mean buying a battery big enough to run a Tasmanian July on stored sunshine, because in the depths of winter there is often not enough surplus solar to fill it anyway. It means being clear-eyed about what you are buying. A battery sized on your annual average will carry you comfortably through much of the year and will lean on the grid through the worst of winter. A battery sized to survive a July evening without grid support will spend spring and autumn half empty.
This is the single biggest reason a generic national sizing calculator gives the wrong answer here.
Charging from the grid in winter
A battery does not have to be filled by your own solar. Many systems can also be set to charge from the grid, where the equipment and your tariff allow it, and that changes the winter picture.
The idea is simple: buy power when it is cheapest, store it, and use it when power is dearest. Tasmania has a residential time-of-use tariff, Aurora Energy’s Tariff 93, where power costs more during weekday peak windows, currently weekday mornings and late afternoons into the evening, and less at all other times including weekends. On that kind of tariff, charging during the off-peak window and discharging through the evening peak can be worth doing on the days your panels cannot fill the battery themselves.
That matters most through a Tasmanian winter, when generation is at its lowest and the evening heating load is at its highest. It is exactly the time of year when a solar-only battery struggles to stay full.
Three honest caveats:
- It depends on your tariff. On a flat-rate tariff there is no cheaper window to buy in, so charging from the grid simply costs you the standard rate. The saving comes from the gap between peak and off-peak prices, and those prices are reset each year.
- It depends on the equipment. Not every battery and inverter can be told to charge from the grid, and the ones that can need the schedule set up properly. It is worth raising before you choose equipment rather than after.
- It is not automatically worthwhile. Grid charging puts extra cycles through the battery for a smaller margin than storing your own solar, which is effectively free once the panels are paid for. Whether it stacks up depends on your tariff, your usage and how often winter leaves the battery empty.
Worth asking about. Not worth assuming.
What the federal battery discount does to the decision
The Australian Government’s Cheaper Home Batteries Program reduces the upfront cost of an eligible battery by around 30%. It is delivered through small-scale technology certificates, and it is tiered by usable capacity. Since 1 May 2026:
| Usable battery capacity | STC factor applied |
|---|---|
| First 14kWh | Full factor |
| 14kWh to 28kWh | 60% of the factor |
| 28kWh to 50kWh | 15% of the factor |
Read that as a sizing signal, not just a price. The first 14kWh attracts the full factor. Capacity above that still attracts support, but less of it per kilowatt-hour, and certificates are calculated on up to the first 50kWh of usable capacity. So the cost per usable kWh of a battery rises as it gets larger, which is one more reason to size from your actual evening load rather than rounding up for comfort.
One timing point worth knowing: the discount is based on your installation date, not the date you sign, and the factor steps down over time. Full detail is on our solar rebates and incentives in Tasmania page, and current pricing is in our guide to what a home battery costs in Tasmania.
Do you want power during a blackout?
This question changes the answer, so it is worth settling early.
Not every battery keeps your lights on during an outage. Backup has to be designed in, and most systems back up selected circuits rather than the whole home. Depending on the battery, inverter and electrical configuration, backup can cover selected circuits and in some installations may extend further.
If outage protection matters to you, it affects which battery and inverter suit your home, which circuits are backed up, how the switchboard is wired, and how much capacity you want to hold in reserve rather than cycle every day. Capacity you reserve for blackouts is capacity that is not saving you money on an ordinary Tuesday, which is a reasonable trade to make deliberately and an expensive one to make by accident.
Battery size against solar size
A battery can only store solar you are not already using. If your system is small and you export very little, a large battery will sit part empty most days no matter how the sizing sum looks.
That is why the two decisions belong together. Iconic Energy generally designs residential solar systems from around 8kW upwards, depending on household usage, roof space and future loads such as an EV, and a system in that range gives a battery something to work with. If you are adding storage to an older, smaller array, the honest answer is sometimes that more panels will do more for your bill than more storage.
Worth checking before you commit either way: our solar savings calculator gives a quick estimate from your quarterly bill, and our solar panel installation page covers how we approach system design.
The common rules of thumb, and where they fall over
“Most homes need 10kWh to 13.5kWh.” A reasonable national average, and a poor fit for a household with electric heating, a heat pump hot water system or an EV. It is an average of homes, not a measurement of yours.
“A quarter of your daily usage, plus a couple of kilowatt-hours.” Quick and not unreasonable as a floor. It tends to undersize homes with a heavy evening load, which is most homes running electric heating through a Tasmanian winter.
“Match the battery to your solar system size.” This one confuses two different units. Your solar system is rated in kilowatts of output, your battery in kilowatt-hours of storage. What connects them is how much surplus you actually generate and fail to use, which depends on your roof, your habits and the season.
None of these are useless. They are just starting points, and the difference between a starting point and a decision is your own usage data.
How we size a battery
Every battery recommendation from Iconic Energy starts with a site assessment. For sizing specifically, we work from:
- How much power you use after dark, when your panels are not generating
- How much solar you currently export rather than use
- Whether you want backup power during an outage, and for which circuits
- What you are planning next, such as an EV or replacing a heating or hot water system
Oversizing can be an expensive mistake. A battery that is consistently under-used leaves capacity, and capital, sitting idle. We work from your bills and solar export data rather than guessing.
Frequently asked questions
What size battery do I need for a 6.6kW solar system?
There is no fixed pairing. The right size depends on how much of that system’s output you currently export rather than use, and on your evening load. A 6.6kW system on a house that is empty all day exports a lot and can justify meaningful storage. The same system on a house with daytime occupants may export very little, in which case the battery has less to store.
How long will a 10kWh battery run a house?
Treat it as arithmetic rather than a number of hours. Ten usable kilowatt-hours covers ten kilowatt-hours of consumption. If your evening and overnight draw is around 8kWh, a 10kWh usable battery will generally see you through the night. Add electric heating running hard on a winter evening and that same battery empties considerably faster.
Is it better to add more panels or more battery?
If you are exporting a lot of solar at the regulated minimum feed-in tariff of 9.276 cents per kilowatt hour for 2026-27, storage captures value you are currently giving away. If you are exporting very little, extra panels usually do more. We look at your export data before recommending either.
Is there a minimum battery size for the federal discount?
Yes. Eligible batteries have a nominal capacity between 5kWh and 100kWh.
Can I add a battery to my existing solar later?
Often, yes. It depends on your inverter and your switchboard, and whether a hybrid inverter or a separate battery inverter is the right approach. Our solar battery installation page covers retrofits in more detail.
Can I size a battery to go off-grid?
Leaving the grid entirely is a different design problem, with different sizing, different equipment and a much larger battery. It is worth discussing on its own terms rather than treating it as a bigger version of a grid-connected battery.
Getting a size that fits your home
Battery sizing is one of the few decisions in a solar system that is genuinely hard to change later, so it is worth spending an hour on your own numbers before you spend anything else.
Iconic Energy is a Tasmanian owned electrical and solar business based in Kingston, installing home batteries across Greater Hobart. If you want a size worked out from your bills and your export data rather than a package off a price list, get in touch and we will arrange a site assessment.