A battery does not make power. It turns a 5c export into a 30c saving

The short answer

What actually decides whether a solar battery pays off

Solar batteries are finally cheap enough to think about, and from May 2026 a federal rebate knocks thousands off the price. Yet most buyers still size a battery the wrong way. They ask how many kilowatt-hours they can fit, when the number that decides whether a battery pays is how many kilowatt-hours they actually cycle through it each day. A battery does not generate power. It moves a kilowatt-hour you would have exported for a few cents into one you use at night that would have cost you about thirty. This page explains how that arbitrage really works, and how to size a battery around it.

Here is the core takeaway in one line: a battery does not generate energy, it shifts a 5 c export into a 30 c saving, so the real return is the roughly 25 c arbitrage on each kilowatt-hour you actually cycle every day. That single fact reframes every other decision. It means the right measure is not how big the battery is, but how much of it you fill with surplus solar by day and empty into your home by night. It means oversizing rarely pays, because capacity you never cycle earns nothing. And it means the federal Cheaper Home Batteries rebate, live from 1 May 2026, changed the maths by taking thousands off the price, while its tiering quietly discourages going huge. The right question is not "how many kilowatt-hours can I fit?" but "how many will I cycle?"

The blind spot

Why most battery guides push you toward the wrong size

Most articles about solar batteries make three mistakes, and each one nudges buyers into spending more than the maths supports.

First, they treat capacity as the headline. The whole pitch becomes "more kilowatt-hours equals more savings", as if a battery were a fuel tank. It is not. A battery only earns money on the energy that flows through it. Two homes with the same nightly consumption save the same amount whether their battery is 10 kWh or 20 kWh, but the bigger one costs far more. Throughput is the number that matters, and almost no guide leads with it.

Second, they sell backup as the benefit. Sizing a battery for "days of independence" sounds reassuring, but blackouts are rare and most home batteries need extra hardware to even run during one. Paying for capacity that sits at 80% all year, waiting for an outage that may not come, is the slowest possible payback. The arbitrage job and the backup job are different, and conflating them leads to oversizing.

Third, they quote a flat rebate. Plenty of guides written before May 2026 either ignore the new federal rebate or describe it as a single per-kilowatt-hour figure. It is tiered on purpose: the government wants you to cover your evening load, not build a personal power station. A guide that misses the 14 kWh step makes a 20 kWh battery look cheaper than it is. Numbers like these belong in a dated box, which is why every figure on this page is stamped with the date it was checked.

How a battery actually earns its keep

Expert analysis: the machinery behind the payback

The arbitrage mechanism: why ~25 c a cycled kWh is the real number

Strip a battery back to its job and it is an arbitrage machine. By day your panels make surplus power. Without a battery you export it for about 5 c a kilowatt-hour. With a battery you store it and use it at night instead of buying grid power at about 30 c. The value created is the difference: roughly 25 c for every kilowatt-hour you move from midday to evening. That is the only number that matters for the saving. It is also why the saving scales with throughput, not capacity: a kilowatt-hour sitting unused in a half-empty battery has created nothing. Round-trip losses shave this slightly, which is the next point, but the headline is simple. You are buying a 25 c spread, one cycled kilowatt-hour at a time.

The federal rebate, and how its tiering reshapes sizing

From 1 May 2026 the Cheaper Home Batteries Program discounts roughly $244 per usable kilowatt-hour, applied upfront through Small-scale Technology Certificates, the same machinery as the panel rebate. The design detail that matters is the tiering: the full rate applies to the first 14 kWh of usable capacity, then drops to about 60% from 14 to 28 kWh. This is deliberate policy. The government wants households to cover their evening load, not to over-build, so the subsidy gets stingier exactly where oversizing begins. The practical effect is that a 10 kWh battery is heavily discounted, a 14 kWh battery is the sweet spot at full rate, and every kilowatt-hour beyond that costs you more out of pocket while earning the same 25 c arbitrage. To qualify you need solar (new or existing), a Clean Energy Council approved battery and inverter, at least 5 kWh usable, and a VPP-capable unit, though you need not join a VPP. One claim per address, not means-tested.

Round-trip efficiency and cycle warranties: the hidden derate

Two technical numbers quietly trim the headline return. The first is round-trip efficiency, about 90%, meaning roughly 10% of what you store is lost to heat and conversion. In practice you must put in about 1.1 kWh of surplus solar to get 1 kWh back, so the effective arbitrage is a little below the raw 25 c. The second is the warranty. Batteries are warranted for about 10 years or a fixed number of cycles, often one cycle a day over the period. A battery that is cycled hard, for example by a Virtual Power Plant, can hit the cycle limit before the year limit, which shortens its useful life. These are not reasons to avoid a battery, but they are reasons to size it so it cycles fully once a day, not to chase extra throughput that wears it out faster than it earns.

VPP economics: extra income against extra cycles and control

A Virtual Power Plant (VPP) lets your retailer use your battery during peak demand in exchange for payments or credits, often a few hundred dollars a year. The trade-off is real. Every VPP discharge is an extra cycle against your warranty, and you hand over some control of your stored power at the moment you might most want it on a hot evening. VPP offers also differ sharply between retailers, and they only exist where there is retail competition: Victoria, New South Wales, South Australia, Queensland and the ACT can shop offers, while Western Australia (Synergy) and the Northern Territory (Power and Water) sit outside the National Electricity Market and run any VPP through the state utility. The honest framing is that a VPP can add useful income, but it is a bonus layered on top of the self-consumption case, not the reason to buy a battery.

What this costs real households

How good batteries end up underperforming

The mechanism above is not theory. It is exactly how well-meaning buyers leave money on the table.

They buy the biggest battery the rebate allows

A household sees the federal rebate and reasons that bigger is better, so they fit 20 kWh. But their evening load is only 8 kWh, so two thirds of the battery never cycles. They paid for capacity that earns nothing, the rebate paid a lower rate on the top half, and their payback stretched out by years. A 10 kWh battery would have saved almost the same amount for far less.

They size for backup, not for arbitrage

A buyer wants to "keep the lights on in a blackout" and sizes for several days of autonomy. The battery now spends most of the year nearly full, waiting for an outage that rarely comes, while the daily arbitrage that actually pays the bills uses only a slice of it. The backup peace of mind is real, but it is being bought at the price of the slowest payback in the catalogue.

They forget the round-trip loss

A spreadsheet says the 10 kWh battery will save a tidy sum, but it assumes every stored kilowatt-hour comes back. About 10% does not. Over a year that gap is enough to push a "seven-year payback" past eight, and it is invisible until the bills arrive. Sizing and expectations should bake in the derate from the start.

They join a VPP without reading the cycle terms

A household signs up to a VPP for the headline credits, then discovers the retailer is cycling the battery twice a day in summer. The extra income is real, but the battery now approaches its warranted cycle count years early, and the family has less stored power on the evenings they wanted it most. The VPP was worth considering, but not without reading the cycle limits first.

The insider insight

The right battery covers your evening gap, not "days of backup"

Here is the part the brochures never frame this way. A home battery has one everyday job: to bridge the gap between sundown, when your panels stop, and bedtime, when your usage drops. For an average home that evening window draws around 10 kWh. That is the figure to size to, because it is the energy you will reliably cycle every single day, and cycling is what earns the 25 c arbitrage. Anything beyond that evening gap is capacity that only earns on the occasional cloudy stretch or in a blackout, which is to say almost never.

The practical consequence: work out your evening kilowatt-hours first, from sundown to bed, then buy a battery that cycles that amount fully once a day. And remember the round-trip loss, so plan to store about 1.1 kWh of surplus for every 1 kWh you want back. Size to the cycle, not to the catastrophe.

The bottom line

Why this matters right now

Two things changed at once in 2026. Feed-in tariffs collapsed toward a few cents, which widened the gap between what an exported kilowatt-hour earns and what a self-used one saves, and a federal rebate cut the price of capturing that gap. Together they make a battery worth a serious look for the first time, but only if you size it for the job it actually does. The job is to bridge your evening gap, cycling roughly your nightly load every day to earn a 25 c arbitrage on each kilowatt-hour. Buy a battery sized to that cycle, claim the rebate without straying past the 14 kWh full-rate tier, account for the round-trip loss, and treat any VPP income as a bonus. Do that and the battery pays for itself on the strength of the energy you move, not the capacity you bought.