The short answer

For a typical home, one battery of around 10–15 kWh is the sweet spot — enough to carry your evening and overnight on stored solar. A Tesla Powerwall 3 holds 13.5 kWh, which covers most households; it scales to 54 kWh with three expansion units, but few homes can use that much daily. The reason "it depends" is the real answer: storage value follows a curve that rises, levels off, then stops paying — so the right number is wherever your curve flattens.

Why solar without storage leaks money

Panels without a battery have become a worse deal than they used to be, and the cause has a name: the solar duck curve. So many homes now export at midday that wholesale prices regularly fall toward — or below — zero in the middle of the day. In New South Wales the flat feed-in tariff for 2026-27 is just 3.4–6.5 c/kWh, down again from the year before, because that's all midday solar is now worth. Yet you buy your evening power back at 30–40 c/kWh retail. Exporting cheap and re-buying dear — that spread is the money a battery recovers by shifting your own solar into the evening.

Better, enough, too much

Add the first few kilowatt-hours of storage and self-sufficiency jumps — you stop buying the early evening. Keep adding and you reach "enough": the point where the battery covers your whole evening-and-overnight gap and self-sufficiency flattens near 100%. Past that is "too much". An oversized battery can't fill from a single day's surplus and rarely completes a full cycle — and a battery's return comes per cycle, so capacity that cycles less than once a day is dead capital. The chart's red zone is where each extra kWh stops earning its keep.

The exception: peak-export arbitrage

There's one way "too much" becomes "still worth it": time-of-use export. The same duck curve that crushed midday feed-in makes evening energy scarce and valuable. NSW's regulator IPART publishes peak-export benchmarks of 17, 25, even 33 c/kWh for solar dispatched between roughly 5–8pm — while midday is near zero. Store the near-worthless midday solar and sell it back into that evening peak, and a bigger battery keeps earning the spread instead of sitting idle.

It's real, not theoretical. Wholesale-exposed retailers like Amber let owners ride price spikes — one NSW household reported a $50 evening when a heatwave pushed wholesale to $12/kWh, and a 6.6 kW + 32 kWh setup running roughly $500 a year ahead. Western Australia's DEBS scheme deliberately pays 10c at peak versus 2c off-peak to push owners toward batteries. The catch is the plan: only about four NSW retailers currently offer time-varying export, and you need one to capture the spread. The federal Cheaper Home Batteries rebate (from July 2025) cut the entry price — installs jumped from ~200 to over 1,500 a day — which is what makes upsizing for arbitrage worth running the numbers on.

So what size for you

Start at the sweet spot: enough to cover your evening and overnight use — for most homes that's a single 10–15 kWh battery, and the tool marks it on your curve. Go bigger only for a clear reason: you're on a time-of-use export plan and can sell the spread; you want longer outage backup; or you're heading toward off-grid, where the maths flips to days of autonomy. And remember the order of operations: a battery only earns if there's surplus solar to store, so if your array is small, more panels come before more storage.

The battery stops storing and starts trading

For a decade the pitch for a home battery was self-consumption: keep your own solar for the evening. The duck curve has quietly rewritten that. As midday energy approaches worthless and evenings spike, the value isn't in storing energy, it's in time-shifting it — and that's a trading position, not a backup plan. Virtual power plants and wholesale-exposed retailers are turning thousands of home batteries into a dispatchable fleet that buys low and sells high on the owner's behalf.

So size the sweet spot for the energy you actually use — but understand that a battery on the right plan is the first appliance you've owned that can earn money while you sleep. The next question is what happens when you cut the grid entirely: how much it takes to go off-grid.

Common questions

Game: battery facts

Five quick rounds on storage, the duck curve and arbitrage.

Method & sources

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