Should you buy a battery and sell at the peak rate?

The best feed-in tariffs in Australia are all evening rates. Panels don't generate in the evening. So the only way to sell into that window is to have stored the power first.

Which leads somewhere that sounds clever: buy a battery, fill it at lunchtime, sell it back at the peak rate. Let the grid pay for the battery.

I thought the same thing. Then I did the arithmetic on my own house.

Stored power can do two jobs

A unit of power in a battery can do one of two things. You can sell it back to the grid, or you can use it yourself and skip buying a unit. It only ever does one: sell it, and it's gone.

So the real question isn't "how much would the grid pay me for it?" It's "how much would the grid pay me, next to what I'd have paid to buy that same unit instead?"

Best to do it on a real house, with a real battery.

What a Tesla Powerwall would do at my place

A Powerwall holds 13.5 units. On a good summer day my roof sends about 18 units of surplus out to the grid, plenty to fill it. After the roughly one part in ten lost storing and releasing, it can hand back about 13.5 in the evening.

So it's evening, I've got 13.5 units in the battery, and two choices.

Sell them. My feed-in pays 6 cents at best, so 13.5 units earns about 80 cents.

Use them. A Powerwall runs itself, and it spends its charge on the dearest hours first. At my place that's the 5–8pm peak, where a unit costs me 49.72 cents, and the battery covers about six units there. Then the 8–10pm shoulder at 33.57 cents, a couple more. Then it slips into the overnight hours at 29.92 cents for the rest. Added up, those 13.5 units save me about $4.75 I'd otherwise have handed the grid.

Same battery, same 13.5 units: selling earns 80 cents, using saves $4.75. Using them is worth about $3.90 more. Every full night.

One honest note. A Powerwall doesn't fill all the way every night — about six summer nights in ten at my place, and far fewer in winter, when the surplus isn't there. So treat $4.75 as a good-summer-night best case; on an average night it's closer to $3. And I've used my best feed-in rate on the selling side. On a big export day those units really sell for 2.5 cents, so selling comes out worse still. Neither hedge changes the direction.

The same test, city by city

My habits don't change. Tariffs do, enormously. So I took my exact house and its Powerwall and stood them in each capital, on real published plans. Each row is one full battery — 13.5 units — on a good summer evening: what you'd get selling it, against what you'd save using it on your own bill.

* Darwin sits on Jacana's Super Feed-in Tariff, which pays 18.66c for power sent to the grid between 3 and 9pm — about double the usual rate. It's the one plan in the country built to reward evening selling, which is why the Northern Territory comes closest. Even there, using the power still wins. (The flagship in this issue runs the house with no battery, so it uses Jacana's standard 9.33c, because without storage you can't reach that evening window in the first place.)

Everywhere, using beats selling. The gap is widest where power is dearest to buy (Adelaide), and narrowest in the Northern Territory, on that unusually high evening feed-in. Even there, selling still leaves you behind.

Why the gap is so wide

A retailer sells you electricity for the wholesale cost, plus network charges, plus the cost of running the business, plus a margin. When you buy a unit, you avoid all of that.

When you sell a unit, you're paid roughly what the power is worth to them — the wholesale end of that stack, and no more.

Those are not the same number, and they were never going to be.

There is exactly one place where it works

Horizon Power publishes 55.99 cents for 3pm–9pm in a handful of remote Western Australian towns. So I looked up what those same households pay to buy power back.

33.2621 cents a unit. Horizon's A2 residential tariff: flat, all day, the same wherever you live, because Western Australia subsidises regional tariffs to match the city. It is the identical rate a Perth household on Synergy pays.

So in those towns it genuinely pays to sell. Fill a battery at lunchtime, sell it at six, and you are about 23 cents a unit ahead of using it yourself. Nowhere else in this country does the sum come out that way.

Which is the loneliness principle doing its work. Those towns burn diesel and have almost nobody else exporting. The power in your battery is worth more to the grid there than it is to you, and the price says so.

So, a battery?

A battery is a good thing to own. It lets you keep power you'd otherwise have given away, and it covers the evening when your panels have stopped.

But the money in it is in what you no longer buy, not in what you sell. Which means the first question isn't which battery, it's how much of your own solar you could use without one. Shifting a load into the middle of the day costs nothing, and you can do it this afternoon. A battery then catches whatever's left over.

The battery model is one real home: a 6.6 kW array in Canberra, metered hour by hour for a full year (1 September 2025 – 8 August 2026), with a Tesla Powerwall added in simulation. Summer figures are Dec–Feb. The Powerwall's 13.5 kWh usable capacity and 89% round-trip efficiency are Tesla's own published specifications. "A good summer night" means the days the battery fills — about six nights in ten in summer at this house, fewer in winter. Each city takes the same house and habits, re-priced on that state's real 2026-27 plan. Feed-in and import rates are GST-inclusive, accessed 2–4 September 2026; Horizon A2 and DEBS band C accessed 9 September 2026. One household, modelled — not a market survey.