Solar power in Australia used to be sold on a promise that you could cover your roof in solar panels, sell any electricity you don’t use, and watch the feed-in credits roll in. Nowadays, the home solar and battery industry looks a bit different.
Feed-in tariffs have dropped to a few cents per kilowatt hour, and any power you buy back costs 30 to 35 cents. That means the money is no longer in exporting power, but in using what you generate yourself to cut your grid usage.
When you’re looking at sizing a solar and battery backup system, the goal is no longer gaining energy credits back, but matching up to how your household actually uses the power. If you get this right, you can slash your bills without overspending on capacity you can’t use.
Here’s how to work out exactly what you need.
Start with your usage
The number that matters most is your daily energy usage in kilowatt hours. Regulator modelling suggests the average Australian home uses around 11 to 13 kWh per day from the grid, though everyone’s usage varies and the average figure in 2020 reached as high as 16 kWh. Features like air conditioning, a pool, electric hot water, and kitchen setup can cause drastic changes to this figure.
By pulling a few bills across different seasons, you’ll get a better idea of the sort of capacity you need for your specific circumstances.
After that, it’s worth figuring out how much electricity you use after dark. Nighttime usage drives your battery sizing because your panels can handle daytime load directly when the sun is up. A home that uses most of its power in the evening needs more storage than one that runs its appliances during the day.
That’s why two households that both use the same amount of electricity can require very different battery setups. If you work from home, you will generally self-consume most of your solar as it’s generated while leaning on the battery less. On the other hand, if the house sits empty during daylight hours, most of your usage might land in the evening and have to come out of battery storage.
What size battery first, how many solar panels second
It might be counterintuitive, but you’ll want to size your battery first rather than your panel capacity. Work out the storage you need for evening and overnight load, then size the solar array to fill that battery and cover daytime usage.
Solar panels are the cheap part of an installation, so many installers will suggest a bit of headroom, while the battery is the expensive part of the system and must be sized carefully to avoid overspend.
As a rough starting point, the typical 6.6 kW solar panel system can pair with a battery that lands between 8 and 13 kWh of usable capacity for an average home. This is a good starting point for the average home, but always remember that your own usage is a more useful figure than a rule of thumb.
Usable home solar and battery capacity
A battery advertised as “10 kWh” may not actually give you 10 kWh of energy each day.
Usable capacity is the amount the battery actually delivers, as a slice of the total capacity is reserved to protect the cells and hit the warranted lifespan. The usable capacity varies by manufacturer, but a typical amount is 90 to 95 percent, meaning 9 to 9.5 kWh of usable capacity on a 10 kWh battery.
Lead acid batteries like you may find in caravan solar setups run an even lower usable capacity – often as low as 50 percent – because deeper discharge can reduce the lifespan of the batteries. For small off-grid or backup systems, they remain suitable.
Another number worth considering is your backup reserve. Instead of using all your available capacity every day, you may want to hold back a portion to protect against blackouts and avoid unexpectedly running out after a period of heavy usage. A typical reserve amount is 10 to 30 percent.
Modern home batteries use lithium iron phosphate (LFP), which is a safer and longer lasting chemistry than older setups that use nickel manganese cobalt chemistry (NMC). For any modern setup, LFP is the sensible option.
What chews up the most power?
To decide on how much capacity you need, it’s worth considering how many power-hungry appliances you have and how often you use them. Two of the biggest power consumers are HVAC and electric vehicles.
Heating & Cooling
Temperature changes are big energy sinks – particularly heating. Electric hot water, resistive heaters, and that ageing spare fridge in the garage are big consumers of electricity. Swapping out an old fridge for a new one or opting for heat pump HVAC rather than older options can shave big watts off your load.
Electric Vehicles
EVs are a game changer in this arena, and not just for petrol savings. The average Australian drives around 30 to 40 km a day, which works out to about 5 to 8 kWh of EV usage. You can offset this with additional panels, but the trick is to think of topping up over the course of a week rather than fully charging the car every day. You may commute each day and leave the car plugged in over the weekend to charge directly from solar instead of every night.
Climate & Season
It may seem obvious, but a system installed in sunny Brisbane can generate more power on average than the same system in Melbourne or Hobart. Additionally, every location changes its amount of daylight hours between summer and winter. More southern homes often need larger solar arrays to land on the same average output, but it’s worth building a buffer to guard against cloudy weeks and panel degradation, as well as additional appliances you may add later.
Where your batteries go
Home battery installs are governed by Australian standards, and the rules are pretty strict. Usually the best spot is a garage or external wall with a non-habitable room behind it that stays cool, shaded, and dry.
Batteries can’t be installed in habitable rooms like bedrooms or living areas. They need 600mm of clearance from a door or exit, including a garage door, and windows or vents to habitable rooms can’t sit within 900mm above the battery.
If the wall behind the battery has a living space within and isn’t already fire-rated, a non-combustible barrier is required, and a garage install may need bollards to safeguard against collision.
Temperature is an important consideration. The optimal room temperature for a battery is between 15 and 25 degrees. A baking garage or west-facing wall in afternoon sun can reduce the battery’s capacity and shorten its lifespan.
Several battery manufacturers require an internet connection for full warranty coverage. For example, the Tesla Powerwall drops from 10 years of warranty to just 4 if you don’t keep it connected to the app.
Rebates
Sizing can affect what you can claim back. The federal Cheaper Home Batteries Program discounts battery installs for common household systems. Eligible systems fall between 5 and 100 kWh of nominal capacity with the discount applying to the first 50 kWh of usable capacity. The battery and inverter must be Clean Energy Council approved, and it must be installed by an accredited installer alongside a solar system.
Since rule changes this year, the discount tapers by size: the discount is highest for the first 14 kWh and steps down above that, so oversizing brings diminishing returns. The system must be Virtual Power Plant (VPP) capable; however enrolment into such a program remains your choice.
Sizing your battery
Start with your daily kWh figure. You could grab this from a single bill, but the best figure is an average from several power bills across the seasons.
Estimate the share that you use after dark. A general rule of thumb is that a household out of home during the day uses a third of its power in daylight and two thirds after dark, but work-from-home households might land on a lower nighttime figure.
This nighttime number is the amount of energy the battery must be able to deliver each evening. It isn’t the capacity you shop for though: it should be lower than both the nominal figure and the usable capacity, because you hold some in reserve.
Scale that up to a nominal capacity figure and you have your starting point. As a rough guide, the nominal capacity you need is your evening power usage divided by the usable percentage of the battery (around 0.9), divided again by the share you’re willing to cycle (0.85 if you keep 15 percent in reserve).
A napkin-math example
Assuming a family that uses 20 kWh a day, is home mostly in the evenings, and 12 kWh of load falls after dark, a typical battery install might be 16 kWh of nominal capacity to comfortably cover the evening with a little reserve for blackouts.
That’s 12 kWh nighttime load divided by 0.9 for the usable capacity, then 0.85 to leave a 15 percent reserve. That gives us 15.7 kWh of nominal capacity, rounding to 16 kWh.
A solar array to feed this system could fall in the typical 6.6 kW to 10 kW range depending on roof space and orientation.
A work-from-home couple with the same total usage but consuming more during the day might get away with a smaller battery and put the savings towards extra panels to cover the direct usage instead.
Solar battery sizing: things to remember
Always look at the usable capacity rather than the nominal figure.
Ask where the battery should sit on your property, and whether it needs bollards or a fire barrier.
Ask whether the capacity quote assumes someone is home during the day or for a nighttime load.
Remember that battery installation is not a DIY job in Australia; it must be done by an accredited installer.
If you’re a renter, you may wish to consider portable options like the Bluetti FridgePower.
The right system isn’t the biggest one, but it’s the one that’s matched to your bill, your routine, and your roof. Instead of focusing on a top-line number, try to work around your evening usage, leave room for anything you may add later, and shop around for quotes.
Keeping this in mind, solar can still pay dividends, but in the power you keep rather than the power you sell.
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