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Solar and Battery

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    How big a battery does your house actually need?

    Start with what your house uses after dark. Then go a size up, because in a few years there is usually an electric car, a heat pump, or somebody home in the daytime.

    A kitchen bench and a living room at night with two lights on, a kettle and a toaster on the bench, a dishwasher indicator lit below it and a dark television beyond

    Michael Brown

    16 August 2026

    Where the power goes in a day

    About half of what your house uses goes while the sun is up, and the array covers most of that as it happens. The battery is there for the rest of it, from sunset through to the morning.

    SunriseSunset
    Surplus: all there is to charge withOvernight load: what the battery covers

    Both move month to month, so an annual figure off your bill cannot answer this. The overnight side is what the battery covers, and it grows when the gas heater comes out and a car turns up in the driveway.

    An overhead view of a house with rectangular solar panel layouts drawn onto three faces of its roof, over an aerial photograph of neighbouring houses, driveways and trees
    The roof decides how much surplus there is to store, and how much of the year the battery can fill itself off the panels alone.

    The same house in June and in October

    Same house, same array, same battery, and only the weather changes. June is the month the roof cannot keep up; October is the month it makes more than the battery can hold.

    The house, and the assumptions

    Array
    6.6 kW, north-facing, unshaded
    Battery
    13.5 kWh usable, one Powerwall 3
    Household
    Four people, ducted reverse-cycle, no electric car
    Array output, late June
    2.4 kWh per kW installed
    Array output, mid-October
    4.6 kWh per kW installed

    A day in late June

    Shortest days of the year, heaviest heating load.

    Used across the day
    26.0 kWh
    Of that, used between sunset and sunrise
    11.4 kWh
    Used while the array is producing
    14.6 kWh
    The array produces
    15.8 kWh
    Left over to charge the battery
    1.2 kWh
    Overnight load covered
    1.2 of 11.4 kWh

    A day in mid-October

    Longer days, heating off, cooling not yet on.

    Used across the day
    18.0 kWh
    Of that, used between sunset and sunrise
    7.6 kWh
    Used while the array is producing
    10.4 kWh
    The array produces
    30.4 kWh
    Left over to charge the battery
    20.0 kWh
    Overnight load covered
    All 7.6 kWh

    In June the roof spares 1.2 kWh, so the battery tops up off the grid at midday and still carries the evening. In October it fills by early afternoon and 5.9 kWh of it goes unused. That 7.6 kWh is today's overnight load, and it only goes up from here.

    Why we go a size up

    The battery goes on the wall once and stays there, and almost nothing a household does over the next few years makes it use less power after dark.

    A battery charges from the grid as well as the roof

    A Powerwall does not have to wait for your panels. It charges from the grid too, and the middle of the day is when grid power is cheapest, because every solar system in the street is exporting at once. So a June day like the one above does not strand a big battery.

    That changes the balance between panels and storage. A smaller array with a bigger battery will usually do better than a bigger array with a smaller one, and it is worth holding your three quotes up against that.

    The federal discount counts capacity in bands

    The Cheaper Home Batteries discount counts usable capacity in bands. The first band counts in full and every band above it counts for less, which is why we stop at one size up.

    100%
    60%
    15%
    13.5 kWhOne Powerwall 3counts as 13.5
    27 kWhTwo of themcounts as 21.8
    0142850kWh usable

    +61%

    more counted capacity for twice the hardware on the wall

    Rates as at 29 July 2026

    What a bigger battery buys you in a blackout

    In an outage, what holds the lights on is kilowatts and which circuits your switchboard puts behind the battery. Capacity buys you more hours on those circuits. Covering more of the house is a switchboard job, and we look at that on the visit.

    What a car or a heat pump does to your night

    Most Canberra houses we quote still burn gas for something. When that comes off, and when a car turns up, the load moves into the evening and the night.

    SunriseSunset
    Add to the house
    SurplusOvernight load

    What we collect at the visit

    We do not write a quote at your kitchen table. Somebody comes out and takes the four things below, and the quote is written afterwards, once we understand what your house is doing.

    1. Two people at a desk in an office, in front of computer monitors

      Your data

      Twelve months of it, in half-hour steps.

    2. Two people working on three wall-mounted units with their covers off, showing the wiring inside, with a stepladder and tools on the floor

      The switchboard

      What can go on backup, and how many phases.

    3. Two people lifting a solar panel up the outside wall of a house, one on the ground and one leaning down from the roof

      The roof

      How much it can spare, and when.

    4. A single-storey house at night with its interior lights on behind full-height glazing, under a clear sky

      What you want

      Backup, an evening peak, or the car that is coming.

    The side of a single-storey house at dusk, lit from inside through large windows, with a white home battery mounted on the dark blue cladding beside a glazed door

    We look at the house first

    Twelve months of your meter data, the switchboard and the roof. The quote is written after that.

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